GRANITE INTRUSIONS OF THE SOUTHERN YENISEI RIDGE

Prof. Yu. A. Kuznetsov

My three years of geological investigations in the Southern Yenisei Ridge have shown that this region is composed chiefly of crystalline Precambrian schists and of a series of acidic intrusions of different ages, manifested in very different ways despite the uniformity of their granitic composition. The granite intrusions of the Southern Yenisei Ridge to a significant degree determine the entire geologic-petrologic character of this region, for with them are associated diverse, often superimposed, repeated metamorphisms that alter the entire aspect of the sedimentary and ancient metamorphic formations beyond recognition; genetically linked with them as well are fairly varied deposits of useful minerals, some of which are already being exploited. The present work is devoted to the petrology of the acidic intrusions of the Southern Yenisei Ridge. For quite understandable reasons, their petrographic description is preceded by a very brief general geological sketch and a stratigraphic scheme of the Southern Yenisei Ridge.

GENERAL GEOLOGICAL SKETCH

The Southern Yenisei Ridge, i.e., the mountain-taiga region extending along the right bank of the Yenisei from the Siberian Railway to the Angara River, is composed mainly of Archean and Lower Proterozoic crystalline schists, pierced by a series of acidic and basic intrusions of different ages. Only in the northern, Cis-Angara part of the ridge do normally sedimentary folded strata of the Middle and Upper Proterozoic appear, which is due to a rather sharply expressed northward plunge of the fold axes. Along the margins of the Southern Yenisei Ridge this entire Precambrian is transgressively overlain by relatively gently lying strata of the Lower and Middle Paleozoic, as well as by loose deposits of the Mesozoic and younger age, which in the northern—sharply lowered—part of the ridge have not yet been removed from the surface of the ancient peneplain.

The composition and structure of the Southern Yenisei Ridge are extremely complex. The age relations of the metamorphic, sedimentary, and magmatic formations that I have distinguished and described have not in all cases been precisely established; nevertheless, the data at my disposal are sufficient to propose, as a first approximation for this region, the following stratigraphic scheme (see p. 32).

As is evident from this scheme, the most ancient component of the metamorphic and sedimentary strata of the Southern Yenisei Ridge is the Kan metamorphic complex, which, among other things, is of special importance for this region because of its wide distribution—the entire southern and central ...

Scheme of the Precambrian stratigraphy of the Southern Yenisei Ridge.

Era	Age subdivision	Red-colored series
Cambrian	Cambrian.	Red-colored series.
Proterozoic	Upper Proterozoic.	Slight angular unconformity. V.—dolomite formation (Upper limestone division of S. V. Obruchev). Angular unconformity. N.—dolomite formation (Middle limestone division of S. V. Obruchev).
Proterozoic	Middle Proterozoic.	2. Angara and Belogorsk (?) granites. 1. Olivine-hypersthene diabases, norites, and norite-peridotites. Ust-Angara limestones and Uderei schists (in part?).
Proterozoic	Lower Proterozoic (to be further subdivided).	3. Nizhnekan granites (post-tectonic). 2. Posolnensk gneiss-granites and migmatites (syntectonic). 1. Ancient diabases (partly synchronous with the Murozhno-Tatar complex). Murozhno-Tatar metamorphic complex (suite of mica schists, suite of phyllites, and suite of Sukhoi Ridge of A. K. Meister).
Archean	Upper Archean.	Tarak gneiss-granites and migmatites. Yenisei metamorphic complex (?)
Archean	Lower Archean.	Charnockites. Kan metamorphic complex.

part of the ridge, is in essence composed entirely of rocks of the Kan complex. At the same time, the Kan metamorphic complex, composed of original crystalline schists of the hypersthene-gneiss facies and charnockitic intrusions, is highly interesting from the petrological point of view as well. A special work is devoted to the petrology of the Kan complex (20).

The Yenisei metamorphic complex, composed of biotite-plagioclase gneisses, amphibolites, rare interbeds of marble, and in places abundantly migmatized, has a limited distribution, being traceable as a narrow strip along the Yenisei River. The metamorphism of the Yenisei complex was caused by the injection of the Tarak and Posolnensk granites; the original material for the formation of the gneisses and amphibolites consisted partly of crystalline schists of the Kan complex, and partly of a sedimentary series younger than the latter but older than the Tarak intrusion.

The Tarak gneiss-granite intrusion forms a huge pluton of acmolithic character within the rocks of the Kan complex; apparently, small intrusions of plagiogranitic composition, as well as ancient migmatites in the Yenisei metamorphic complex, are also connected with it.

The Kan metamorphic complex and the Tarak granite-gneiss intrusion are cut by a large number of dikes of ancient, usually uralitized diabases, having a persistent NW strike. Among the rocks of the Yenisei and Murozhno-Tatar complexes, diabases of this type occur very rarely. The upper age limit of the diabase intrusion is determined by the fact that these diabases are intruded and metamorphosed (converted into amphibolites, sometimes garnet-bearing) by the Posolnensk gneiss-granites.

The Murozhno-Tatar metamorphic complex is widely distributed in the northern part of the Southern Yenisei Ridge, and its best sections are found along the Angara River between the Tatarka River and the village of Rybnaya. The complex includes: a suite of phyllites, passing in places into mica and mica-garnet schists and containing interbeds of micaceous marbles and dolomites, and a suite of quartzites (of Sukhoi Ridge), composed of alternating beds of quar-

quartzites and graphitic schists, in which I found spicules of monaxonic sponges, and the phyllite suite lying stratigraphically higher. Apparently, the phyllite suite includes actinolite schists and massive prasinite rocks formed at the expense of diabase extrusions and intrusions. The above-mentioned dikes of ancient diabases are regarded as the roots of the latter. The Murozhno-Tatar complex lies transgressively upon the eroded surface of the Kan complex and forms several large folds plunging to the north, with a general northeastern strike. Following the folding and regional metamorphism with the development of rocks of the green-schist facies, it was intruded by the Posolnaya granites and mica-bearing pegmatites, which caused a local increase in the degree of metamorphism.

The Posolnaya gneiss-granite intrusion (the Posolnaya and Piskunov plutons, and the Taseevo, Angara, and Yenisei migmatites) manifested itself chiefly in extensive migmatization of a metasomatic type (feldspathization), with the development of porphyroblastic microcline gneisses and massive migmatitic granites. Apparently, the formation of mica-bearing pegmatites (the Kan, Kondakov, and other deposits) is genetically connected with the Posolnaya intrusion; these pegmatites produced a rather strong pneumatolytic metamorphism of the enclosing strata, with the development of two-mica gneisses and schists, sometimes with garnet, disthene, staurolite, and tourmaline.

The red porphyritic microcline granites, which form a large pluton in the lower reaches of the Kan River, are connected with the same magmatic cycle as the Posolnaya intrusion, being a somewhat later formation. They are characterized by manifestation in the normal-granite facies, are distinguished by an independent internal tectonics, by sharp cross-cutting contacts, and by a very weak manifestation of injection metamorphism. The Nizhnekan granites are also accompanied by pegmatites, but chiefly written-granite and feldspathic ones.

A considerably younger formation is the suite of the Ust-Angara gray argillaceous limestones with numerous interbeds of gray argillaceous schists, which in the upper horizons of the suite completely displace the limestones. The Ust-Angara limestones and argillaceous schists are gathered into steep, small folds of general northwestern strike, plunging in the same direction. The transgressive occurrence of the limestones on the rocks of the Murozhno-Tatar complex is evident. Quite provisionally, the so-called “Uderei” argillaceous schists (25), encountered in sections along the Angara between the village of Rybnaya and Cape Greben, are combined with the Ust-Angara limestones into one sedimentary complex; petrographically they are very close to the schistose components of the Ust-Angara suite and are dislocated in the same way as the latter.

The Ust-Angara limestones, as well as all the more ancient formations, including the Nizhnekan granites, are cut for the most part by latitudinal dikes of southern olivine-hypersthene diabases. The original magma proved to be considerably differentiated, and within this diabase formation we encounter both leucocratic norites and rocks of norite-peridotite character.

The Angara gray microcline granite forms a small stock among the Ust-Angara limestones; moreover, its apophyses, in the form of dikes of pegmatitic granite, cut, according to Meister (25, p. 84), the olivine-hypersthene diabase. Apparently, the granite porphyries of Belaya Mountain (the Belogorsk intrusion) in the basin of the Kuzeeva River are of close age. The Tatar granites described by Meister are probably also close in age.

The youngest formations of the southern part of the Yenisei Ridge are developed along its eastern margin and are represented by thick strata of dolomites

and clay shales, the complete section of which was described by S. V. Obruchev (31). Here two divisions, or formations, are clearly distinguished, separated by an obvious unconformity and by a phase of folding. The lower dolomite formation (the Middle Limestone division of S. V. Obruchev) is folded into rather steep folds of NE strike, and its clayey components are invariably schistose. The upper dolomite formation (the Upper Limestone division of S. V. Obruchev) forms very gentle brachyfolds, usually exposed in the cores of Cambrian anticlines, and is not at all schistose. The variegated Lower Cambrian transgressively, but only with a slight angular unconformity, overlies the upper dolomite formation. Any traces of intrusive activity (even quartz veins) are absent in these clayey-dolomitic strata.

The above scheme of Precambrian stratigraphy of the southern part of the Yenisei Ridge does not include small intrusions of alkaline syenites, possibly of Paleozoic age, dike rocks of the Telbes volcanic cycle, which occur sporadically in the southern part of the ridge and are quite comparable with those in the environs of the city of Krasnoyarsk, as well as serpentinite intrusions, whose age has remained uncertain.

From what has been set forth above it may be seen that the Precambrian history of the Southern Yenisei Ridge naturally falls into four successive tectono-magmatic cycles, each of which began with the deposition of sedimentary strata and basic volcanism and ended with folding, metamorphism, and acid intrusions. The fifth tectonic cycle was not accompanied by volcanic activity. Traces of the manifestation of the first two tectono-magmatic cycles have been preserved in the form of thick strata of crystalline schists of the Kata and Mesozone and enormous gneiss-granite intrusions. The crystalline schists of the Kan and Yenisei metamorphic complexes, as well as the Tarak gneiss-granites, evidently are of Archean age. The third tectono-magmatic cycle produced thick strata of phyllites, quartzites, and prasinites, as well as large granite and granite-gneiss intrusions and mica-bearing pegmatites. In the graphitic schists of the suite of quartzites of Sukhoi Khrebet, sponge spicules have been found; therefore, despite the regional metamorphism of the epizone of the rocks of the Murozhno-Tatarka complex, it must already be assigned to the Proterozoic. The fourth tectono-magmatic cycle was expressed in the accumulation of thick strata of the Ust-Angara limestones and clay shales, very reminiscent in their aspect of the carbonate Middle Proterozoic of Gornaya Shoria and the Eastern Sayan, and ended with acid intrusions, with which it is customary genetically to associate the gold-bearing character of the Yenisei Ridge. With the completion of this tectono-magmatic cycle the Yenisei Ridge was consolidated, and in the Upper Proterozoic we already have the accumulation of shallow-water, platform-type deposits, replacing the typically geosynclinal strata of the preceding cycles.

I. ARCHEAN ACID INTRUSIONS

1. Charnockites of the Kan Metamorphic Complex

It has already been mentioned above that a special work (20) is devoted to the petrology of the Kan metamorphic complex, in which a detailed petrographic characterization of the charnockites is given and the question of their genesis is examined in detail. Here I shall only note that there is convincing evidence for the palingenetic origin of the charnockitic intrusions, which arose as a result of the selective melting of rocks of the Kan metamorphic complex during its general regional metamorphism.

2. The Tarak Gneiss-Granite Intrusion

In the southeastern part of the S. Yenisei Ridge, a granite intrusion is widely distributed which is exceptionally interesting, among other things, also from a practical point of view, since it is monazite-bearing, and the placers of the Tarak monazite deposit, discovered and investigated by me in 1937, were formed through the destruction of precisely these granites. The granites have been studied in greater detail over a very small area directly adjoining the Tarak deposit, within the watersheds of the Kurysh River—the Tarak River—the Kazachya River. The entire remaining part of the Tarak pluton has been crossed only by rare traverses, by which it has been established that the Tarak granites make up an enormous pluton elongated in a northwestern direction, i.e., in accordance with the enclosing sequence of crystalline schists of the Kan metamorphic complex, consisting of garnet and hypersthene gneisses with relatively rare beds of pyroxene-plagioclase crystalline schists. From the south and east it is transgressively overlain by gently lying red-colored Paleozoic rocks, which makes it impossible to judge the true dimensions of the magmatic body. But even the part now exposed from beneath the Paleozoic cover occupies an area of about 1700 km² and reaches 120 km in length.

The rocks of the Tarak intrusion are very diverse in their composition and at the same time possess a number of very characteristic features that make it possible to distinguish the Tarak granites from the other granites of the region.

The normal and most widespread type of Tarak granite is characterized by its coarse-grained character, porphyritic structure, light grayish or yellowish color, and strikingly fresh semitransparent alkali feldspar, reminiscent of moonstone. Very often the granites bear traces of crushing, and sometimes they are rather strongly cataclased, with the development of augen gneisses and even finely schistose mylonites. Microscopic study reveals in the normal type of granites an allotriomorphic texture and a characteristic mineralogical composition. The quartz of these granites is usually strongly crushed; moreover, besides the magmatic quartz, another generation is also indicated, cutting the feldspars in veinlets. The quantitative content of quartz is normal for granites, i.e., on the average about 30%. The predominant mineral is microcline, absolutely fresh, with a microcline lattice beautifully developed throughout the whole grain and with characteristic thin spindle-shaped, sometimes veinlike perthitic intergrowths. In addition, rounded inclusions of quartz and, more rarely, plagioclase are very abundant in the microcline. The plagioclase is represented by strongly sericitized albite or albite-oligoclase, judging from the extinction angle in the direction [100]—from −15 to −12°. The boundaries of the plagioclase with microcline are strongly corroded by the latter, and within the plagioclase grains irregular patches of microcline are not uncommon, indicating a clear process of metasomatic replacement of plagioclase by microcline in the last stage of crystallization of the magma. The dark-colored components are represented by biotite and muscovite. The biotite is yellow-brown, with pleochroic halos around zircon inclusions, but more often (in cataclased varieties) it is completely replaced by chlorite; along with chloritization, small grains of sphene are released. Apatite, monazite, and zircon are present as accessory minerals. Characteristic of the Tarak granites is the complete absence of magnetite, which accounts for the original composition of the heavy-mineral concentrates of the Tarak deposit, consisting of 50–70% monazite. Monazite is observed in almost every heavy-mineral concentrate of normal granite in the amount of one or two or several grains, included in plagioclase or biotite, and in the latter case

around monazite there is always observed a black, non-pleochroic halo. Monazite forms slightly brownish grains of rounded shape, somewhat turbid and often surrounded by a brown envelope that does not affect polarized light. With some practice monazite is easily distinguished from zircon by its somewhat lower refraction and birefringence, turbidity, and noticeable coloration. It forms larger rounded grains, whereas zircon forms small, elongated grains, sometimes with well-developed faces.

Two samples of granites typical of the Taraka intrusion have been analyzed (see Tables 1 and 2).

Table 1.

Chemical composition of rocks of the Taraka intrusion

Component	328/1937 Weight percent	328/1937 Mol. quantities	306/1937 Weight percent	306/1937 Mol. quantities	614-v/1937 Weight percent	614-v/1937 Mol. quantities	1044-a/1937 Weight percent	1044-a/1937 Mol. quantities
SiO₂	70.82	1080	71.20	1186	74.26	1238	64.77	1080
TiO₂	0.42	5	0.32	4	tr.	—	0.51	6
Al₂O₃	13.23	130	13.81	135	11.56	113	15.08	148
Fe₂O₃	2.60	16	1.88	12	2.42	15	5.32	33
FeO	1.73	24	1.64	23	0.99	14	2.45	34
MgO	0.72	18	0.45	11	0.34	9	1.55	39
MnO	0.06	1	tr.	—	none	—	0.13	2
CaO	0.88	16	1.94	35	1.01	18	2.02	39
K₂O	4.74	50	3.59	38	5.50	59	3.25	35
Na₂O	3.16	51	4.66	75	2.32	37	3.49	56
P₂O₅	0.08	—	0.10	—	0.11	—	0.05	—
−H₂O	0.58	—	0.43	—	0.50	—	0.56	—
Loss on ignition	0.97	—	0.37	—	0.32	—	0.87	—
Total	99.99	—	100.02	—	99.36	—	100.05	—

Sample 328/1937 (Taraka River) is a coarse-grained yellowish-gray porphyritic rock, in which a count on the Shand stage (in 4 thin sections) gave the following results: quartz—28%; microcline—37%; plagioclase—29%; biotite, accessories, and secondary minerals—6%. The microscopic picture is ordinary, and a certain deviation from the average type is expressed in the fact that the biotite is completely chloritized and partly replaced by epidote, while the plagioclase is very strongly sericitized; measurements of the extinction angle in the least altered areas in the direction \([100]\) give \(-12^\circ\), which corresponds to an anorthite content of 7–8%.

In its chemistry, sample 328/1937 most closely corresponds to the chemistry of rapakivi, differing from the latter only by a somewhat lower value of \(c\) and a higher \(fm\), and, correspondingly, by the ratio \(c/fm\). Recalculation by Zavaritskii’s method shows complete agreement of the chemical-analysis data with the average type of granite.

Sample 306/1937 (Taraka River) is a light-gray, fine-grained porphyritic granite. Gneissosity is clearly manifested, expressed in the parallel orientation of microcline segregations and biotite plates. The quantitative mineralogical composition is: quartz—31%; microcline—40%; plagioclase—24%; biotite and accessories—5%. Measurements on the Fedorov stage show that the plagioclase contains 21% of the anorthite molecule (extinction in the direction \([100] = 0\)) and is comparatively weakly sericitized. Perthitization in microcline is weakly developed. Biotite is partly chloritized. Among the accessories sphene and monazite are rather abundant. In the chemical analysis, a somewhat higher content of silica and calcium oxide is noted in comparison with the preceding sample. But these differences are not of essential importance, and recalculations by the methods of Zavaritskii and Niggli give essentially the same results.

The chemistry of sample 306/1937 comes very close to the chemistry of the average type of granite (according to Zavaritskii), differing from the latter only by an increased value of \(a\), and has a clearly expressed rapakivi character (according to Niggli), although true rapakivi are characterized by a higher value of the coefficient \(k\).

328/1937—porphyritic yellowish-gray coarse-grained granite. Taraka River, tributary of the Kurysh River. Analyst Krivets.

306/1937—porphyritic gray fine-grained granite. Taraka River, tributary of the Kurysh River. Analyst Krivets.

614-v/1937—coarse-grained yellowish aplite-granite. Taraka River, tributary of the Kurysh River. Analyst Krivets.

1044-a/1937—medium-grained hybrid garnet-cordierite granodiorite. Kan River in the Komarovo Rapid. Analyst Krivets.

Table 2.

Recalculations of chemical analyses of the Tarak granites.

	si	al	fm	c	alk	k	mg	c/fm	a	c	b	s
328/1937	366	41	23	5	31	0.50	0.24	0.21	13.4	1.1	6.7	78.8
306/1937	348	40	17	10	33	0.34	0.19	0.61	15.0	1.5	4.7	78.8
614-v/1937	442	41	19	6	34	0.61	0.17	0.32	12.7	1.1	3.6	82.6
1044-a/1937	258	35	34	9	22	0.38	0.28	0.28	12.3	2.6	11.9	73.2

The granites described seem to show gradual transitions to aplite granites, which differ, besides the complete or almost complete absence of biotite, in their white, yellowish, or pinkish coloration and in the absence of porphyritic structures. Under the microscope, these rocks reveal a sharp predominance of the same lattice-like and fresh microcline over albite, which contains 5–6% of the anorthite molecule. The allotriomorphic structure is expressed even more sharply. Magnetite has not been found at all in these aplite-granites. Monazite occurs considerably more rarely than in normal granites.

The analyzed sample of aplite-granite 614-v/1937 (see Table 1) is characterized by a coarse-grained allotriomorphic structure, yellowish coloration, and the following mineralogical composition (determined on the Push-integrator): quartz—42%, microcline—42%, plagioclase—16%; biotite and accessories—2%. The microcline in the rock is entirely fresh, but the plagioclase has begun to be sericitized and is not susceptible to precise determination. As accessories: monazite, sphene. In the features of its chemical composition this aplite-granite is very close to sample 328/1937 and likewise clearly shows signs of belonging to the rapakivi type of magmas. The differences from the chemism of the normal Tarak granite reduce to elevated values of si and alk, which is natural for a leucocratic differentiates.

Among the Tarak granites there are also occasional coarse-grained pegmatitic segregations of the same mineralogical composition and, finally, in several places quartz veins with microcline and druses of rock crystal have been encountered.

Along the eastern margin of the Tarak granite massif, approximately along the boundary with the transgressively overlying Paleozoic, mica-bearing pegmatites are widely distributed; these were studied and partly prospected by Mordasov (29), Egorov (7), Polikarpova (35), and Kazarinov (44). The pegmatites of this district are lens-shaped segregations, always occurring among reddish and pink aplite-like granites and forming perfectly gradual transitions with the latter. The pegmatitic lenses are coarsely banded, their central part being composed of white quartz, and the peripheral part of a coarse-grained quartz-microcline aggregate, which in places displays graphic texture and an admixture of packets of biotite and muscovite.

The normal type of granites described above can be traced as a broad belt along the eastern margin of the area of development of the intrusive rocks and is probably typical of the central parts of the pluton. Aplite granites and pegmatitic segregations occur within the normal granites as irregular patches, distributed without any visible regularity.

Among the granites, inclusions of xenoliths of the enclosing rocks are frequently observed. Gneissic xenoliths are very common and consist of quartz,

of intermediate plagioclase, orange biotite, and pale-green hornblende with an abundant admixture of apatite. They do not present any special interest. More interesting are the amphibolite xenoliths. The latter consist of a granoblastic aggregate of light-brown hornblende, labradorite, and a small amount of quartz; relics of diopside are sometimes preserved in them. In the case of vigorous migmatization, hornblende and pyroxene are almost completely replaced by biotite; a considerable quantity of quartz and microcline develops, the latter mineral clearly metasomatically replacing plagioclase and showing a distinct tendency toward the development of large porphyroblasts. The primary rock of such xenoliths was evidently the pyroxene-plagioclase crystalline schists of the Kan complex.

Finally, mention should be made of the hybrid rocks, widely distributed in the western endocontact of the Tarak pluton. They are characterized by highly variable composition and variable structure. Usually these are dark-gray rocks with large segregations of white or light-gray microcline, including flakes of biotite and sometimes quartz, and with a medium-grained groundmass of quartz, plagioclase, and abundant biotite, which sometimes has a gneiss-like character. Under the microscope this groundmass has an allotriomorphic, and sometimes a clearly granoblastic, structure and consists of brown biotite, quartz, and plagioclase with an anorthite content of 25—30—35%. A very characteristic feature is the extraordinary abundance of apatite. The amount of monazite in them is also higher than in normal granites. The microcline of the segregations has quite irregular boundaries and developed rather metasomatically.

Fig. 1. Variation diagram of granitoids of the Tarak intrusion.

In the case of more profound hybridization, melanocratic rocks very rich in biotite are produced; the content of microcline in them drops sharply as a result of an increase in the amount of biotite and plagioclase, the composition of which sometimes reaches andesine. It is characteristic, however, that even in strongly hybridized rocks microcline shows a tendency to form large segregations, which is why in most hybrid rocks a porphyritic structure is still observed, although microcline segregations in them are rarer than in normal granites and occur the less frequently, the more strongly the magmatic rock is contaminated by foreign material. Thus, hybridization normally manifests itself in an increased role of biotite and in an increase in the basicity of plagioclase, while at the same time the amount of apatite and monazite greatly increases. Such melanocratic, biotite-rich granitoids are very widespread in the endocontact zone of the intrusion, where the belt of their continuous development reaches several kilometers in width. Very often, however, such hybrid rocks occur as lenses also within the area of development of normal granites, marking the presence of completely assimilated xenoliths or roof pendants. It is characteristic that in the hybrid rocks the gneissosity is much better expressed than in normal granites and has a persistent NW strike with a steep dip to the NE, i.e. one-

...in a manner consistent with the orientation and gneissic character of the xenoliths and with the gneissic character of the enclosing crystalline schists of the Kan complex. It must therefore be supposed that, in the process of formation of the Tarak pluton, phenomena of assimilation of the enclosing rocks played an exceptionally important role, and that the gneissic character of the Tarak granites is to a considerable degree inherited.

Another type of hybrid rock is distributed chiefly in the exocontact of the Tarak pluton, where injection gneisses very often prove to be conformably penetrated by light-gray granitoids, generally of granodioritic composition, but distinguished by the presence of large grains of red garnet, and sometimes also of cordierite. The garnet here is evidently an unstable relic of the assimilated rock, for it is energetically replaced along cracks by green biotite. Cordierite, on the contrary, gives the impression of a mineral quite stable under the conditions of the assimilation process, although it too is in places pinitized, and sometimes replaced by a yellowish isotropic chloritic or serpentinous substance. Characteristic of the cordierite in such hybrid rocks is the weak development of twinning and of pleochroic halos.

These hybrid garnet-cordierite granodiorites usually are dark-gray medium-grained rocks in which, besides abundant gray quartz, white feldspar, and biotite, a considerable admixture of pink garnet and greenish cordierite with a greasy luster can already be distinguished with the naked eye. White microcline forms rare but large segregations—up to 2 cm long—with numerous inclusions of quartz and biotite.

The rocks possess a homogeneous texture and reveal no elements of gneissic structure. The microstructure is allotriomorphic, although the microcline is noticeably xenomorphic with respect to the other components.

One of the most typical specimens of such cordierite-garnet hybrids (No. 1044a/1937) was analyzed, and its quantitative and mineralogical composition was determined on the Push integrator and is expressed by the following figures: quartz 33%, plagioclase 24%, microcline 15%, biotite and accessories 15%, garnet 3%, cordierite 10%. All components of the rock are very fresh, and only in the cordierite is the development, along cracks, of a fibrous mineral with high birefringence observed. Judging by the extinction angle in the direction [100]—+15°—the plagioclase contains 30–32% An. Garnet and cordierite always contain inclusions of quartz, plagioclase, and biotite. In the microcline inclusions of cordierite have been noted, and in places a distinct overgrowth and, as it were, replacement of garnet by cordierite. As may be seen from the data of the mineralogical analysis, the ratios of the principal minerals—quartz and feldspars—are typical for granodioritic rocks. Unusual in general for magmatic formations are garnet and cordierite.

The chemical composition of this specimen is closer to the chemistry of quartz diorites than of granodiorites, although the deviations from this, the closest, type are very great. They are expressed mainly in increased values of the coefficients al and fm with strongly lowered c. Recalculations by Zavaritsky’s method give the same results—the chemically closest type is quartz diorite, from which our rock differs by a lowered value of the quantity c. Thus, the data of the chemical and mineralogical analyses are in contradiction, which, however, is only apparent, for our hybrids contain garnet and cordierite—minerals foreign to normal magmatic rocks—and the name has to be given without taking account of the presence of the latter.

The hybrid character of the described rocks is unquestionable. The original magma was evidently of granitic composition. Comparison of analyses of the most widespread granites and of the cordierite-garnet hybrid (see Table 1 and Fig. 1) shows that, as a result of assimilation of rocks of the Kan metamorphic complex, the granitic magma proved to be considerably enriched in iron and magnesium. The great role of magmatic (mutual) reactions is beyond doubt, and probably the appearance in the hybrid rocks not only of members of the normal reaction series, but also of such minerals as garnet and cordierite, is due to the extraordinary scale of contamination. However, the genesis of one and the other of these minerals is apparently different. Above...

it has already been mentioned that garnet is rather a relict of assimilated material, whereas cordierite gives the impression of a mineral crystallized from a melt.

The described garnet-cordierite granodiorites evidently represent an extreme case of hybridization of granitic magma. Hybridization phenomena in general are highly typical of the Tarak intrusion, and even “normal” porphyritic granites in fact always prove to be, to a greater or lesser degree, contaminated by foreign material; and the fluctuations in the chemical composition of the granites that can be seen in Tables 1 and 2, and that stand out especially sharply in the variation diagram in Fig. 1, are evidently explained by the same contamination phenomena, which in this case were only weakly expressed.

The contact zone of the Tarak pluton is well exposed in the gorge of the Kana River, in the sector from the Kazachya River to the Kurysh River (Fig. 2).

Already below the mouth of the Kazachya River, in the garnet gneisses of the Kan complex, the contact influence of the granitic intrusion is felt, expressed in recrystallization of the gneisses, in the development of cordierite and biotite at the expense of garnet, etc. Above the mouth of the Kazachya River begins a zone of injection gneisses of very complex composition and structure. In some outcrops we see massive, coarse-grained garnet hybrid granitoids, crowded with bodies of dark biotite-garnet gneisses; in others, on the contrary, the material of the enclosing rocks predominates, and we have a typical injection gneiss—arterite—with numerous concordant veinlets of quartz-garnet-feldspar material. It is interesting that the material of magmatic injection is almost always strongly hybridized and consists essentially of quartz and acid plagioclase with an admixture of biotite and large grains of garnet. Microcline is either absent or is observed in small quantity. The material of the enclosing rocks is represented in these arterites by fine- and medium-grained dark gneisses consisting of a granoblastic aggregate of quartz, medium plagioclase, biotite, large grains of cordierite, remnants of garnet, and accessory constituents. It is interesting that cordierite and biotite develop at the expense of garnet.

Fig. 2. Contact zone of the Tarak intrusion.

Legend: Granites; Hybrids; Arterites and gneisses; Precambrian limestones; Alluvium.

About two kilometers above the mouth of the Kazachya River, the injection gneisses are cut by a series of dirty-white veins of aplite-pegmatite (orthotectite), reaching 40 m in thickness. The orthotectites consist of quartz, microcline, and a very small amount of sericitized albite-oligoclase. Their contact effect is expressed in some recrystallization of the enclosing rocks, with garnet being replaced by biotite. In addition, rather intense feldspathization is observed here, with the appearance in the biotite gneiss of individual porphyroblasts of microcline or even of whole quartz-microcline veinlets. In the former case rocks are formed that perfectly recall the hybrid xenocontact rocks described above.

At Komarovsky Rapid there are exposed the strongly contaminated hybrid garnet-cordierite quartz-diorites described above, containing a large number of xenoliths of fine-grained biotite gneiss. Sometimes these granodiorites are literally crowded with parallel-oriented schlieren and bands of gneisses. Such varieties pass imperceptibly into arteritic gneisses with injection of light-colored coarse-grained pegmatite.

Higher up, for about 1 km as far as the Skakalnaya shivera, approximately the same picture is repeated, but here the gneissic components again begin to predominate; the gneisses are sometimes cut by veinlets of white coarse-grained aplite-pegmatites, and in places rather thick bodies of massive hybrid rocks appear, with large segregations of microcline and with abundant cordierite and garnet.

Farther on, as far as the Skakalny settlement, the Kana crosses a zone of especially strong injection-assimilation phenomena. Here the garnet gneisses enclosing the intrusion have been transformed either into biotite-plagioclase gneisses, or into migmatites with porphyroblastic microcline, sometimes with pink garnet and cordierite. The magmatic material proper, in the case of more powerful injection, has the character of a normal biotite granodiorite (although already

distinct from the Tarak granites, indicates contamination). In small bodies contamination is manifested more strongly, and in them pink garnet, cordierite, and abundant biotite develop. In addition, injections of completely white aplite-pegmatites are widespread; these also are sometimes filled with large crystals of garnet and contain large grains of apatite. The amount of magmatic material varies from place to place, and the contact rocks described sometimes appear as banded gneiss pierced by aplite-pegmatite veinlets, while at other times, on the contrary, they are massive, more or less strongly contaminated granodiorites.

Above the Skakalny spring there are developed gray hybrid granitoids with abundant xenoliths of fine-grained biotite gneiss, as well as with abundant melanocratic schlieren, having the form of lenses or bands, often bent and folded. In 0.5 km above the Skakalny spring, in these rocks a zone of intense mylonitization up to 150 m thick is observed.

Farther on, as far as the place where the valley of the Kana River sharply widens, which is observed two kilometers above the Skakalny spring and 4 kilometers below the mouth of the Kurysh River, normal yellowish-gray porphyritic granites already predominate; among them, however, lenses of strongly hybridized varieties are often observed, as well as schlieren of biotite and biotite-cordierite gneisses.

The total thickness of the injection-assimilation contact zone of the Tarak pluton reaches 6 km. As is evident from its description, there is no sharp contact of the intrusive rocks with the enclosing gneisses. In the outer part of the contact zone, phenomena of injection of the arterite type predominate, with simultaneous contamination of the magmatic material by the material of the injected rocks; as a result of contamination, the magmatic constituent of the arterites has, in general, acquired a plagiogranitic composition. The inner part of the contact zone is characterized by extremely widely developed phenomena of an assimilation character, with the development of various hybrids; moreover, the massive rocks of the endocontact are filled with gneissic schlieren and xenoliths. It is interesting that, in the contact zone of the Tarak intrusion, phenomena of metasomatic feldspathization with development of microcline play a relatively minor role. These phenomena were observed, but on a comparatively small scale; moreover, they prove to be considerably later than the formation of the arterites and, apparently, are contemporaneous with the injection of white aplite-pegmatites.

The vein suite of the Tarak intrusion is represented exclusively by white, rarely pink, aplites and pegmatites of orthoclase character. The aplites and pegmatites often contain garnet, and in one case an albite-microcline aplite with garnet and pale greenish sillimanite was found, forming considerable accumulations in the rock.

The tectonics of the Tarak pluton has remained almost unexplained because of the fragmentary nature of the observations. Attention is drawn to the abundance of xenoliths and schlieren in the body of the granite, always oriented quite uniformly and in accordance with the gneissosity of the exocontact injection gneisses. In the granites themselves, large tabular segregations of microcline are almost always oriented parallel to one another, likewise creating a well-expressed gneissosity; the planes of \(S\) in the area examined have a consistent northwestern strike with a dip to the northeast at an angle of 30 to 60°, i.e., in general, concordant with the overall strike of the Kan metamorphic complex. It is difficult to speak of the form and dimensions of the Tarak pluton, since our investigations have outlined only its western contacts, while the southern and eastern ones are covered by transgressively overlying Paleozoic strata; consequently, one can only conjecture the colossal dimensions of the Tarak pluton, the area of whose surface exposure in the part examined reaches 1700 km². According to the systematics of Cloos (11, 57), the Tarak massif, on the basis of the most preliminary data, may apparently be assigned to gneiss-granite areal plutons of the second or even first magnitude, with concordant contacts and conformable internal tectonics. After the formation of the intrusion, its rocks were, along certain zones, crushed

and crushed with the formation of mylonites (predominantly of the phyllonite type) and augen gneisses; moreover, weak traces of crushing, healed by recrystallization, are widespread.

The facies character of the Tarak intrusion is rather well determined by the nature of the contact zone and by a number of other features. The Tarak granites metamorphosed garnet gneisses and other rocks of the Kan complex, belonging to the deepest facies of hypersthene gneisses, with the development of new mineral associations. Characteristic is the absence, in the contact zone, of hypersthene and of pink and brown almandine-pyrope garnet—the leading minerals of the Kan metamorphic complex—which are here replaced by biotite and cordierite (instead of brown garnet there appears a little red garnet with \(n = 1.801\), i.e., of purely almandine composition). This indicates sharply changed physicochemical conditions within the contact zone during the intrusion; at the same time, study of the rocks of the latter gives the impression of a certain regressiveness of the contact metamorphism, partly lowering the degree of metamorphism of the rocks of the Kan complex. These relations indicate that the emplacement of the Tarak intrusion took place only after the Kan formation had undergone its deep metamorphism, with the acquisition of the features of the facies of hypersthene gneisses, and after the Kan metamorphic complex had been displaced into higher levels of the lithosphere. At the same time, the characteristic features of the Tarak intrusion—powerful injection-assimilation phenomena in the contact zone, far-reaching phenomena of hybridization, and remarkably fresh microcline—indicate that the formation of the Tarak granites occurred under conditions of high pressure and exceptionally slow cooling; and the Tarak intrusion probably belongs in general to one of the deepest magmatic formations, while the metamorphism caused by it has clearly expressed features of the amphibolite facies (51).

The age position of the Tarak intrusion is determined by the following relations. The Tarak gneiss-granites metamorphose the rocks of the Kan metamorphic complex and are themselves cut by numerous dikes of quartz diabases, which cut the younger Yenisei metamorphic complex but are themselves metamorphosed by the Posolnaya intrusion and, probably, are genetically connected with the bedded deposits of uralitized diabases and prasinites among the strata of Lower Proterozoic phyllites. There is also every reason to think that the orthogneisses and “ancient migmatites” described below, widely distributed among the gneisses and amphibolites of the Yenisei complex in the western part of the Southern Yenisei Ridge, are genetically connected with the same Tarak intrusion. Thus, the Tarak gneiss-granite intrusion is younger than the Kan and Yenisei metamorphic complexes, but older than the Lower Proterozoic phyllites and the Posolnaya granitogneisses. Its emplacement is evidently connected with Upper Archean tectogenesis.

The later alterations of the rocks of the Tarak intrusion impose a definite stamp upon the general aspect of this magmatic complex. Among them one may note first of all: 1) cataclasis and mylonitization of the granites, 2) silicification and epidotization of the granites.

Traces of crushing are noticeable in almost any specimen and thin section of the Tarak granites, for which, in general, the following are characteristic: blue quartz, bent plates of biotite, and gneissosity with the development of augen textures; but in some cases the crushing is very fine, and within narrow zones the granites prove to have been transformed into thin mylonitic schists. Phenomena of epidotization of the granites with the formation of massive quartz-epidote rocks are chiefly connected with the zones of crushing and mylonitization; these phenomena are evidently connected with late allometasoma-

by mineralization caused by an unknown intrusion. It is interesting to note the presence, within the body of the Tarak granites, of quartz veins clearly not genetically connected with the granite, as well as the presence in the placers of the Tarak River and its tributaries of gold, scheelite, and cinnabar, which apparently are foreign to the Tarak intrusion and rather indicate the manifestation, within this area, of comparatively young hydrothermal activity. The possibility is not excluded that this hydrothermal activity is connected with the intrusion of red porphyritic granites cutting the Tarak intrusion and transgressively overlain by the red-colored Cambrian, and which were discovered by Kazarinov in the basin of the Alega River (10).

Mineral resources genetically connected with the Tarak intrusion are not diverse. Among them, first of all, monazite should be noted, being a common rock-forming mineral of granites. Industrial concentrations of this mineral, understandably, occur only in secondary occurrence—in placers formed through the destruction of granites. As for other possible mineral resources, in particular ore deposits of pneumatolytic and hydrothermal genesis connected with the Tarak intrusion, there is no substantial basis for expecting the discovery of such deposits. The Tarak pluton, judging from all data, has been denuded very deeply, and all emanation deposits connected with it have undoubtedly long since been washed away. At one time, pegmatites forming lenses in the body of the granite aroused interest from the point of view of their mica content. However, repeated inspection and sampling of these pegmatites (Mokrushinskoe and other deposits) showed their nonindustrial character.

The Southern Yenisei Ridge, as I have established, is in general a peculiar province for which monazite proves to be a very characteristic mineral and the most common component of the heavy-mineral concentrate. In connection with the discovery in 1937 of the Tarak monazite deposit, which is now acquiring major industrial significance as the richest in the Union, the question of the genesis and patterns of distribution of this mineral assumes special importance.

As already noted, monazite is one of the most widespread minerals in the heavy-mineral concentrates of the Southern Yenisei Ridge, but the distribution of monazite in the alluvial deposits of this region is uneven. The highest contents of monazite in concentrates are observed in springs and small rivers washing the Tarak granite intrusion, as well as the rocks of the Kan metamorphic complex, chiefly in the area of development of charnockite intrusions (kuzeevites). Regionally, but with low content, monazite is distributed in rivers washing the Yenisei metamorphic complex. Monazite is practically absent from concentrates collected from the area of development of the Lower Kan intrusion, the carbonate Proterozoic, and the red-colored Paleozoic strata.

Monazite in the Southern Yenisei Ridge occurs in several varieties. The most widespread is the yellow variety, forming transparent and semitransparent, small (always less than 0.5 mm), poorly crystallized grains. Less common is reddish-brown monazite, forming larger tabular or prismatic grains up to 2 mm in size. Sometimes a certain intermediate, yellowish-brown variety is observed. Finally, only in specimens and thin sections of certain kuzeevites was greenish-brown, noticeably pleochroic monazite observed. Monazite possesses the usual optical properties, with the angle of the optical axes ordinarily very small—about 7°; sometimes, however, in unquestionable monazites, an almost regular uniaxial-crystal figure was also observed. Extinction is almost straight. From very similar zircon, with some practice, monazite is easily distinguished by its somewhat lower refractive index.

… and birefringence, with a noticeably brownish, and in charnockites—greenish-brown coloration (zircon in thin section is always colorless), and by the frequent presence of a shell of brown isotropic material, representing a product of the destruction of monazite. In the case where monazite grains are enclosed in biotite, a black, non-pleochroic halo is always observed around the latter.

In the petrographic study of the magmatic and metamorphic formations of the Southern Yenisei Ridge, I established microscopically the presence of monazite in garnet and hypersthene gneisses and charnockites of the Kan metamorphic complex, in ancient migmatites of the Yenisei metamorphic complex, and also in the rocks of the Tarak granite intrusion; moreover, already during petrographic processing it was noted that it is most abundant in melanocratic hybrid varieties, while in light aplite-granites it is practically absent. At my request, V. P. Kazarin subjected to heavy-mineral analysis 46 hand-specimen samples collected in various parts of the basins of the Taraka and Kazachya rivers and belonging to the various types of rocks encountered here. The hand specimens were crushed in a mortar, washed in a pan, and then examined under a magnifying glass. As a result of this investigation it turned out that the highest monazite content was shown by: 1) hybridized granites, followed, in decreasing order of monazite quantity, by 2) pegmatite segregations among granites, 3) aplite veins and pegmatite segregations among injection gneisses, 4) leucocratic granites, 5) injection-assimilation rocks (arterites), 6) diabases. He also established that reddish-brown monazite predominates in aplites, pegmatites, and leucocratic granites, while yellow monazite is characteristic of hybrid rocks, i.e., the same relations are observed here as were previously outlined by Matveev for the monazites of the Borshchevochny Ridge (24). In the rocks of the Kan metamorphic complex only the yellow variety of monazite is widespread.

Such relations suggest that the primary source of cerium, thorium, and the rare earths is the Kan metamorphic complex; into the Tarak intrusion they entered as a result of assimilation by the latter of enormous masses of enclosing rocks. The concentration of monazite in the exocontact hybrid varieties of the intrusion is apparently explained by the fact that monazite, analogously to apatite, generally has a tendency to accumulate in xenoliths and hybrid rocks during the process of contamination (59). This idea is partly confirmed by the fact that monazite has a regional distribution in the rocks of the Kan complex, including in those parts of the latter where no signs whatever of contact metamorphism caused by the Tarak intrusion are found. The very weak monazite content of the rocks of the Yenisei complex speaks to the same effect, although the latter likewise has been injected and metamorphosed by the Tarak intrusion. Thus the latter apparently played only the role of a secondary concentrator of monazite. Promising with respect to monazite content is the entire area of development of the Kan metamorphic complex and of the Tarak gneiss-granite intrusion, and especially reliable are the areas of development of endocontact hybrid granitoids.

II. LOWER PROTEROZOIC ACID INTRUSIONS

1. The Posolnaya gneiss-granite intrusion, its migmatites and mica-bearing pegmatites

Granites of an entirely different type form two fairly large plutons situated in the basin of the Posolnaya River and the Piskunovka River, and occurring among the crystalline schists of the Kan complex and partly among phyllites and quartzites of the Lower Proterozoic. The conditions of exposure of this

the southern Yenisei Ridge are such that even the Posolnaya pluton, despite the considerable concentration of traverses over the area of its development, could not be precisely contoured, and its boundaries shown on the geological map are entirely conventional. The same must also be said of the Piskunovka granites, which were observed only on the watershed between the Shilka and Piskunovka rivers, in road cuttings. All these granites are surrounded by broad migmatite fields and themselves have a migmatitic character. It is impossible to draw a boundary between the migmatites and the granites; therefore, on the geological map, areas of regionally developed migmatites are shown by the same symbol as granites.

The Posolnaya intrusion, in many features of the granites composing it, in the character of the metamorphism of the enclosing sequences, and in the manifestation of the aplite–pegmatite phase, differs sharply from the Tarak intrusion described above. The petrographic composition of the rocks composing the intrusion is strikingly uniform. Both massifs are composed entirely of yellowish-pink gneiss-like microcline granites, among which xenoliths of amphibolitic composition are observed only very rarely. In the marginal parts of the Posolnaya pluton, and also in small granite dikes—satellites of the main body—rocks of adamellite composition sometimes appear. The granite plutons are surrounded by a fairly broad aureole of migmatites, but not of the arterite type, as in the Tarak intrusion; rather, they formed by metasomatic feldspathization with the development of large, sometimes enormous porphyroblasts of pink and red microcline. The Posolnaya intrusion is accompanied by an abundant aplite–pegmatite phase, developed chiefly outside the pluton itself and even outside the injection zone. The aplites and pegmatites are extremely varied in the character of their occurrence, and some of the pegmatites are of industrial mica-bearing importance. It is interesting that the pegmatites of the Posolnaya intrusion produce a rather strong pneumatolytic metamorphism of the enclosing rocks and, evidently, are later in time of formation than the migmatites.

Microcline granites, as already noted, are not only the most widespread but probably the only type of rock composing both plutons. In most cases the Posolnaya granites are leucocratic, medium-grained, pinkish or pinkish-yellow rocks, only very rarely having a porphyritic texture. Almost all the granites possess a clearly expressed gneissic character, observed both in individual specimens and in entire outcrops; moreover, this gneissosity is always fully concordant with the structure of the enclosing rocks. Cataclasis phenomena, with the formation of augen gneisses in the Posolnaya granites, in contrast to the Tarak granites, were not observed at all.

Microscopic study reveals a decisive predominance of microcline, the amount of which usually reaches 50–60% of the total mass of the rock. Microcline forms large and small grains of entirely irregular outlines; moreover, the relations of microcline with other minerals show that it was not only the last to crystallize, but also, in part, developed metasomatically at the expense of plagioclase. The microcline is always completely fresh and transparent, has a beautifully expressed and very coarse lattice, developed throughout the grain, often contains perthitic intergrowths of the film-like and veinlet type, as well as inclusions of quartz and plagioclase. The latter is usually very scarce in the Posolnaya granites (10–15%). It is represented by oligoclase with an anorthite content of 18–20% and is distinguished by an almost complete absence of twinning and by straight extinction in sections perpendicular to (010), owing to which it can easily be mistaken for orthoclase. It forms small, sometimes idiomorphic, but more often irregular grains as a result of corrosion by microcline. Very often it contains myrmekitic intergrowths of quartz, and almost always serici-

tized, and sometimes replaced by plates of muscovite. Quartz is present in granites in normal quantity; it occurs as irregular grains with the usual cloudy extinction, but without noticeable cataclasis. In many cases the quartz content is noticeably reduced, and then the rock assumes a granosyenitic composition. The dark-colored components are represented by biotite, present in an amount of about 5% of the rock mass. The biotite is ordinary, yellow-brown, but often chloritized. In some varieties muscovite appears, developing at the expense of plagioclase or biotite. Apatite, zircon, magnetite, ilmenite, and sphene are present as accessories, and the amount of ore accessories is rather considerable.

The structure of the granites is indistinctly hypidiomorphic, and the idiomorphism of the plagioclase is usually completely obscured by processes of its replacement by microcline. In most cases a gneiss-like texture is observed, the gneissic character being due either to the parallel arrangement of biotite plates, or to a certain banding expressed in the appearance of indistinctly bounded parallel dark fine-grained bands enriched in biotite, or else in the appearance of, as it were, parallel veinlets of a coarser-grained quartz–microcline aggregate. The latter types of gneiss-like granites are observed chiefly in the contact zones and evidently already have a migmatitic character.

Among the granites, small irregular coarse-grained areas are occasionally observed, consisting of quartz, microcline, and strongly enriched in magnetite. The body of the Posolnaya pluton is accompanied by numerous small satellites and cutting dikes of the same granitic or adamellitic composition. These dike granites are distinguished by their fine grain size, as well as by the increased role of plagioclase and muscovite in comparison with the normal type of granites. The amount of plagioclase here is usually approximately equal to the amount of microcline, and the plagioclase is sometimes represented by zonal andesine with a content of 35–38% anorthite. In these same rocks there is always a rather considerable amount of muscovite, forming large bundles and sometimes intergrown with biotite.

Chemical composition of the granites and migmatites of the Posolnaya intrusion.

Table 3.

Component	1216/38, wt. %	1216/38, mol. qty.	573-a/39, wt. %	573-a/39, mol. qty.	I, wt. %	I, mol. qty.	II, wt. %	II, mol. qty.	III, wt. %	III, mol. qty.	IV, wt. %	IV, mol. qty.
SiO₂	71,07	1184	65,47	1091	72,60	1210	58,77	979	69,90	1165	71,95	1200
TiO₂	0,22	3	0,44	5	0,07	1	1,11	14	0,35	4	—	—
Al₂O₃	13,71	134	16,56	163	12,63	124	17,62	173	15,43	151	14,90	146
Fe₂O₃	2,52	16	2,62	16	1,50	9	3,40	21	0,51	3	2,32	15
FeO	1,03	14	1,48	21	1,68	23	2,37	33	3,22	45	0,54	8
MgO	0,37	9	0,49	12	0,11	3	1,76	44	0,54	14	0,58	15
MnO	—	—	—	—	tr.	—	0,12	2	—	—	—	—
CaO	1,28	23	1,97	35	0,86	15	1,28	23	1,79	32	0,87	16
Na₂O	3,79	61	4,33	69	5,16	83	7,69	124	2,51	40	3,65	59
K₂O	5,11	54	5,60	60	4,21	45	4,19	45	6,10	65	4,10	44
−H₂O	0,06	—	0,20	—	—	—	—	—	—	—	—	—
Loss on ignition	0,41	—	0,37	—	0,28	—	0,42	—	0,20	—	0,93	—
[[unclear: additional loss row]]	—	—	—	—	0,70	—	—	—	—	—	—	—
Total	100,05	—	100,07	—	100,10	—	98,73	—	100,55	—	99,84	—

The chemical composition of a granite fairly typical of the Posolnaya pluton is shown in Table 3.

1216/1938—yellowish, medium-grained granite. Posolnaya R. Analyst I. P. Onufrienok.
573-a/1939—yellowish, medium-grained migmatitic gneiss-granite. Posolnaya R. near the village of Greben. Analyst I. P. Onufrienok.
I—“Rudikovka granite.” Rudikovka R. Meister (25, p. 233).
II—“Umptekite.” Rudikovka R. Meister (25, p. 223).
III—“Angara gneiss.” Angara R. opposite Rybnoe village. Meister (25, p. 269).
IV—“Yenisei gneiss.” Yenisei R. between the villages of Abalakova and Rudikovka. Meister (25, p. 269).

Table 4.

Recalculations of analyses of granites and migmatites of the Posolnaya intrusion.

Sample	si	al	fm	c	alk	k	mg	c/fm	a	c	b	s
	by Niggli	by Niggli	by Niggli	by Niggli	by Niggli	by Niggli	by Niggli	by Niggli	by Zavaritskii	by Zavaritskii	by Zavaritskii	by Zavaritskii
1216/38	361	41	17	7	35	0,47	0,16	0,41	15,4	1,3	3,9	79,4
573-a/39	288	42	16	9	33	0,47	0,18	0,56	17,7	2,3	4,6	75,4
I	389	40	14	5	41	0,35	0,07	0,34	16,2	0	4,4	79,4
II	201	35	25	5	35	0,27	0,37	0,19	22,9	0,3	9,5	67,3
III	330	43	18	9	30	0,62	0,22	0,49	14,0	2,1	6,2	77,7
IV	377	46	17	5	32	0,43	0,28	0,30	13,5	1,0	7,0	78,5

The analyzed specimen (1216/1938) is a yellowish, medium-grained rock with rare porphyritic segregations of microcline and with a noticeable gneissose character. Quantitative mineralogical composition: quartz—26%; microcline—38%; plagioclase—30%; biotite, muscovite, and accessory minerals—6%. The texture is rather allotriomorphic, and only small inclusions of plagioclase in microcline possess their own outlines. The microcline is very fresh, with a coarse lattice; it contains no perthitic intergrowths; it quite clearly replaces plagioclase and is apparently one of the latest formations. The plagioclase is strongly sericitized and has the composition albite-oligoclase (An content = 12%), judging by extinction in the direction [100] = −10°. Along with brown biotite there is a little muscovite. Accessory minerals are magnetite, apatite, zircon, and sphene.

Recalculation by the method of Zavaritskii shows an intermediate position of our rock between the alkaline and intermediate granite of the latter’s classification. In the ratio of the coefficients calculated by Niggli’s method, the analyzed specimen is very close to the chemistry of rapakivi; here, too, attention is drawn to the increased alkalinity, expressed in the high value of the coefficient alk.

The other specimen (573-a/1939, lower course of the Posolnaya R. near Greben village) represents an example of a typical migmatitic granite or, more precisely, granosyenite. The rock shows a clear gneissose character, caused by the parallel arrangement and concentration along certain surfaces of the dark-colored components. The texture is allotriomorphic, or even granoblastic, with a sharp manifestation of traces of replacement of plagioclase by microcline. Quantitative mineralogical composition: quartz—19%; microcline—36%; plagioclase—35%; biotite—7%; hornblende—2%; titanomagnetite, sphene, apatite—1%. The microcline is very fresh, with a coarse lattice, and forms irregular, greenish grains, often with inclusions of quartz and plagioclase. The plagioclase, judging by the extinction angle in the direction [100]—7°, contains about 18% An. It is strongly sericitized; myrmekitic intergrowths of quartz, located along boundaries with microcline, are widely developed. Quartz forms rounded grains, accumulating in individual areas or forming inclusions in microcline and plagioclase, and gives the impression of a relic of a replaced (feldspathized) rock. Among the dark-colored minerals there are brown biotite and green common hornblende. The biotite is partly chloritized, and is rarely altered to muscovite. Accessory minerals are titanomagnetite and apatite. Secondary minerals are sphene and epidote, observed, however, in very small quantities.

The chemistry of this rock is very close to the chemistry of Niggli’s granosyenitic magmas and is quite similar to the chemistry of the specimen 1216/38 just described, from which this rock differs only in its lower quartz content and somewhat more basic plagioclase and, accordingly, in the values of the coefficients si and c.

Rocks of a composition different from the granites and granosyenites described occur within the body of the Posolnaya pluton only extremely rarely. I shall note melanocratic schlieren in the granosyenites near Greben village, having the composition of gabbro-diorite and consisting of predominant green hornblende, residual colorless monoclinic pyroxene, and biotite replacing ...

hornblende and fresh, slightly zoned plagioclase. As accessory minerals there are abundant apatite, sphene, and ilmenite. Apparently, these melanocratic schliers are products of incomplete reworking of the pyroxene-plagioclase rocks of the Kan complex.

The contact zone of the Posolnaya intrusion is very wide and is expressed in a highly varied manner, being represented by various migmatites, amphibolites, biotite and biotite-amphibole gneisses, developed both at the expense of ancient crystalline schists of the Kan and Yenisei metamorphic complexes and at the expense of phyllites and quartzites of the Lower Proterozoic. The Posolnaya and Piskunovka plutons, apparently, lie wholly within rocks of the Kan metamorphic complex, consisting (outside the zones affected by the acid intrusions) of garnet and hypersthene gneisses, charnockites, and pyroxene-plagioclase crystalline schists, which, within the limits of a rather broad contact zone, underwent recrystallization with the development of biotite-plagioclase gneisses and amphibolites of various types, and in some areas proved to be more or less strongly feldspathized and transformed into diverse migmatites. In the northern part of the Southern Yenisei Ridge, already within the area of development of phyllites and quartzites of the Lower Proterozoic, in sections along the Yenisei below the mouth of the Angara, as well as along the Taseeva above the Kondakov mine and on the Angara opposite Rybny Island, only broad zones of migmatization are observed; within these, among the migmatites, numerous bands of gneisses and amphibolites occur, but the granites that caused the migmatization were not observed in outcrops.

Migmatites surround the Posolnaya and Piskunovka plutons in a rather broad, though not continuous, aureole; they are especially strikingly developed along the strike of the tapering granite body.

For example, about 6 kilometers below the mouth of the Shilka River, on the right bank of the Yenisei River, original, slightly mylonitized migmatites with a sharply expressed porphyroblastic structure are exposed for almost 1 km. Their porphyroblasts consist of fresh pink or red cross-hatched microcline; the microcline single crystals have either a prismatic or a rounded elliptical form and reach 2–3 cm in diameter. It is interesting that the rounded form of the porphyroblasts is the primary form, which arose while they were still growing. This is indicated by the presence in many porphyroblasts of inclusions of biotite and quartz, the distribution of which completely repeats the outlines of the porphyroblast. Very often, however, the porphyroblasts prove to be partly or completely crushed and recrystallized into a fine-grained granoblastic aggregate. In the first case the porphyroblast is slightly deformed and only its periphery is granulated, or else zones of granulation are visible also within the single crystal; in the second, the porphyroblast assumes a lenticular form and is already wholly recrystallized into a granoblastic, aplite-like aggregate of microcline.

The main mass of such porphyroblastic migmatites is fine-grained and schistose. It consists of a granoblastic aggregate of quartz, plagioclase containing 33% An, a small amount of microcline, brown biotite, and densely pleochroic bluish-green riebeckitic hornblende. Accessory minerals are apatite and zircon. Incidentally, under the microscope inclusions of quartz, biotite, and riebeckite are observed in almost all the microcline porphyroblasts.

The initial stages in the formation of the migmatites described can be observed in outcrops on the right bank of the Yenisei immediately below the mouth of the Shilka River, where the pyroxene-plagioclase crystalline schists of the Kan complex are relatively weakly migmatized. Bands of non-migmatized pyroxene-plagioclase rocks have been transformed into garnet amphibolites. In those areas—

…areas where only individual porphyroblasts of microcline are observed; under the microscope it can be seen that the common hornblende, characteristic of garnet amphibolites, has been completely replaced by biotite; in the case of vigorous feldspathization the garnet also disappears, likewise passing into biotite, while at the same time the main mass of the porphyroblastic migmatite is enriched in quartz, microcline, and ribekite, and thus a porphyroblastic migmatite is formed, indistinguishable from those described above.

Migmatites of the same type are observed in the western marginal contact of the Posolnaya pluton in sections along the Posolnaya River. It is entirely natural that here—in the marginal contact—the zone of migmatization is manifested less impressively. Here, too, porphyroblastic migmatites are widely developed, in which all transitional varieties may be observed, beginning with a schistose dark gneiss with rare porphyroblasts of microcline and ending with completely massive granitoid rocks, in which only bands and irregular accumulations of a fine-grained quartz–biotite–plagioclase aggregate reveal their migmatitic character. The porphyroblastic migmatites in this contact zone alternate with thick units of completely unmigmatized biotite–plagioclase gneisses and are cut by a series of pegmatite veins and small granite dikes.

Migmatites of approximately the same type, but finer-grained, are distributed in the area of the Kondakovo mica mine and on the Angara River opposite the village of Rybnaya, as well as in sections along the Yenisei below the village of Abalakova and in the lower courses of the Rudikovka and Kamenka rivers. These migmatites were described in detail by Meister (25) under the names of Taseevo, Angara, and Yenisei orthogneisses and Rudikovka granites, and more recently by Yakzhin (52). Within these two zones, various feldspathized rocks and metasomatic migmatites are developed, interbedded with biotite–plagioclase and biotite–amphibole gneisses and plagioclase amphibolites. Along the Rudikovka River, granitoid migmatites predominate, very reminiscent of the Posolnaya migmatitic granites, distinguished by clearly expressed traces of protoclase. There are grounds to suppose that in these areas the schists of the Murozhno-Tatar complex, rather than the massive rocks of the Kan complex, underwent migmatization; this accounts for the sharply expressed schistosity of the migmatites, caused by the parallel orientation of tabular microcline porphyroblasts and the enveloping arrangement of the mica flakes.

Chemical analyses of the Angara and Yenisei migmatites, borrowed from Meister (25, p. 219), are presented in Table 3 (analyses III and IV). Their chemistry, as can be seen from Table 4, is very close to the chemistry of the Posolnaya migmatites and migmatitic granites and likewise has a clearly expressed rapakivi character, although this feature is naturally somewhat obscured here.

In Meister’s work (pp. 218 and 223), moreover, chemical analyses are given of a gneissic alaskite granite and of umtekitе from the lower course of the Rudikovka River. The chemistry of the “alaskite granite” (see Tables 25–26, analysis 1) corresponds exactly to the average type of alkaline granite of Zavaritsky and to the normal alkaline-granitic type of magma of Niggli. The chemistry of Meister’s “umtekite” is in fact closest to the chemistry of the umtekitic type of magma of Niggli and to the average type of umtekite of Zavaritsky. Thus, both rocks have a sharply expressed alkaline character and, according to Niggli’s classification, belong to the sodium series, whereas all the other migmatites and migmatitic granites associated with the Posolnaya intrusion are characterized by a more or less sharp predominance of potassium over sodium. What accounts for this peculiarity of the chemistry of the Rudikovka rocks is as yet difficult to say, but the possibility is not excluded…

the possibility that in this area there are manifestations of still younger intrusive activity. It is curious, incidentally, that in the Rudikovka granite-gneisses fluorite is an almost constant accessory mineral, although it is completely uncharacteristic of the Posolnaya intrusion. However, an attempt to relate the chemical analyses of rocks of the Posolnaya intrusion gives very smooth variation curves (Fig. 4), which would seem to indicate the presence of genetic links among all the analyzed rocks. The only exceptions are the migmatites (analyses III and especially IV), distinguished by a sharply elevated value of $mg$ and a lowered $alk$, which is natural for this type of rock. Incidentally, the high value of the coefficient $mg$ indicates the formation of migmatite at the expense of magnesium-rich rocks. For analysis III (the Angara gneiss of Meister) this is also confirmed by geological observations. The Angara migmatites undoubtedly formed by feldspathization of Lower Proterozoic granulites. A similar conclusion is quite natural also for the migmatites of the Kostylnikovo and Abalakovo area, whence comes the sample analyzed by Meister (analysis IV).

Contact gneisses and amphibolites. The Posolnaya pluton intruded into rocks of the Kan metamorphic complex, characterized by mineral associations of the hypersthene-gneiss facies, i.e., by the parageneses plagioclase–diopside–hypersthene and plagioclase–hypersthene–almandine, as well as by the absence of primary amphiboles, sphene, and minerals of the epidote group. These rocks, in areas that underwent contact action from the Posolnaya intrusion, experienced retrogressive metamorphism with the acquisition of associations characteristic of the amphibolite facies (51). Here there are developed beds of biotite-plagioclase gneisses, consisting of quartz, oligoclase, and biotite, sometimes

Fig. 3. Variation diagram of granites and migmatites of the Posolnaya intrusion.

with an admixture of garnet, sillimanite, and epidote, interbedded with beds of plagioclase amphibolites, which are characterized by ordinary hornblende and intermediate plagioclase; in some varieties abundant red garnet appears, in others sphene and epidote. Judging from a number of transitional varieties and relict structures, these amphibolites form at the expense of pyroxene-plagioclase rocks of the Kan complex. The biotite-plagioclase gneisses have an entirely identical chemical composition with the garnet gneisses of the Kan complex and apparently formed at the expense of the latter. Thus, repeated metamorphism here was expressed above all in the replacement of garnet by biotite, and pyroxenes by amphiboles; in some cases, at the same time, as a result of a reaction between orthopyroxene and plagioclase, red garnet develops. Apparently, a characteristic feature of this metamorphism is also a decrease in the basicity of plagioclase; the liberated anorthite molecule goes to form epidote, and simultaneously sphene forms at the expense of ilmenite. In the zones of the described injections these rocks are metasomatically enriched to a greater or lesser degree

by developing microcline; at the same time amphibole is replaced by biotite, and sometimes alkaline hornblende develops. Around pegmatite veins the formation of muscovite is observed, and occasionally of tourmaline and sillimanite.

The aplite-pegmatite phase of the Posolnaya intrusion is extremely widespread; characteristically, the pegmatites occur chiefly outside the granite massifs and the migmatite zones surrounding them. Very often the pegmatites prove to be mica-bearing; some of their deposits have been prospected and studied and described in greater or lesser detail, which makes it possible for me to give only a very concise characterization of them.

The most widespread type of pegmatite consists of graphic granites and various orthotectites. The latter, in mineral composition, hardly differ from granites, but possess a variable structure. For the most part these are uneven-grained yellowish-pink rocks with segregations of coarse-grained material having a graphic texture. Abundant muscovite is characteristic; in the endocontact zones, an admixture of tourmaline and garnet is sometimes observed. The graphic granites form predominantly thin, sheet-like veinlets (up to 2 m), are light pink in color, and, besides quartz, microcline, and albite-oligoclase, they already contain a more considerable admixture of black tourmaline and greenish muscovite, although the size and content of the latter nowhere reach any significant magnitude.

It is noteworthy that the orthotectites and graphic granites cause noticeable metamorphism in the enclosing gneisses, expressed in more or less vigorous muscovitization of the latter with the formation of two-mica gneisses or even schists; moreover, the intensity of this metamorphism depends on the thickness of the pegmatite body and is approximately proportional to the amount of muscovite in the pegmatite vein. Muscovitization is always stronger in the hanging wall of the pegmatite vein. For example, below the mouth of the Bobrovka, on the left bank of the Yenisei, biotite-plagioclase gneisses are cut by a series of veins of pink orthotectite with schlieren of graphic granite and fairly abundant muscovite. In the footwall of the vein, even at the very contact, the gneiss is almost unchanged. Pneumatolysis is expressed only in the appearance of fairly large grains of apatite, sphene, and orthite with an epidote rim, and also in the development along cracks of a small amount of muscovite and chlorite. In the hanging wall, however, the gneisses are strongly pneumatolytized at a distance of 12–15 m from the contact, with the formation of two-mica gneisses, which differ from the ordinary type of biotite-plagioclase gneiss by the abundance of large flakes of muscovite, evidently developing at the expense of plagioclase, the content of which in such two-mica gneisses is always sharply reduced. It is interesting that the garnet in the two-mica gneisses is perfectly fresh and evidently quite stable under the conditions of the muscovitization process. Among the two-mica gneisses, here strongly foliated, thin quartz-muscovite veinlets are rather often observed, consisting of quartz, muscovite, and rare grains of completely decomposed plagioclase. Around them, naturally, pneumatolysis is especially strong.

Pegmatites of pegmatoid structure, in particular with developed pneumatolytic phases, naturally occur much more rarely, but they have been studied considerably better, for in some cases they prove to be industrially mica-bearing, and sometimes contain significant concentrations of beryl. Alongside orthotectites and graphic granites, pegmatites of this type were observed in the basins of the Posolnaya and Shilka rivers, where, among other things, a new muscovite deposit (Aleksandrovskoye) was recently discovered and put into operation, as well as in the long-known Kondakovskoye and Barginskoye (Kanskoye) mica deposits.

The study of the last two deposits introduces substantial corrections into the existing notions concerning the nature of the relationships between the pegmatite melt and the country rocks, and concerning the origin of the crystalline schists enclosing the pegmatites. It is therefore necessary to dwell in somewhat greater detail on the characteristics of the Kondakov and Barginsky (Kansky) deposits.

The pegmatites of the Kondakov mica deposit have been studied and described in detail by Yakzhin (51). This deposit is situated on the right bank of the Taseeva River, 18 km from its mouth, i.e., in the extreme NW corner of the area investigated by us.

In the structure of the district, the chief role is played by various crystalline schists, forming a belt of NW strike about 10 km wide. In the southern part of the belt there occur migmatized gneisses and amphibolites. In its main part, however, this belt is composed of two-mica and mica-garnet schists and of rarer mica schists with staurolite, kyanite, and sillimanite. Among these varieties of mica schists there occur interbeds of garnet-hornblende, epidote-biotite schists, micaceous quartzite, and marble.

The pegmatites form bedded veins and lenses up to 18–20 m thick and 300–350 m long. Sporadically there occur stock-like and nest-like bodies, irregular or oval in plan. The pegmatite veins vary rather strongly in their composition and structures; alongside simple graphic granites, sharply layered pegmatites are widely developed, with distinct: an albitic selvage, a zone of graphic granite, a coarse-grained pegmatoid zone, and a quartz belt in the center. Albitic pegmatites with a sharply expressed quartz-beryl phase are encountered sporadically.

The metamorphism of the entire belt that includes the mica-bearing pegmatites of the Kondakov deposit is to a considerable degree due to the injection of pegmatites and is essentially pneumatolytic in character. It is true that some of the crystalline schists occurring here—for example, amphibolites and biotite-plagioclase gneisses and migmatites, developed in the SW part of the pegmatite field—were evidently formed even before the intrusion of the pegmatite melts. But the massive injection of the latter caused an almost regional pneumatolysis of the entire pegmatite field; moreover, both gneisses and amphibolites, as well as phyllites and quartzites, underwent this secondary pneumatolytic metamorphism; and the products of this pneumatolysis were various mica schists, sometimes containing garnet, kyanite, staurolite, and tourmaline.

The genetic connection of these mica schists with the pegmatites is proved first of all by the fact that, apart from zones of mass development of pegmatites, mica schists of this type have nowhere been encountered in the Southern Yenisei Ridge. The initial stages of analogous pneumatolysis, expressed in the muscovitization of biotite-plagioclase gneisses in contact with orthotectic veins of the Bobrovka River and the Posolnaya basin, have been described above. In the Kondakov deposit, owing to the abundance of closely spaced pegmatite veins and the wide development of pegmatites with developed pneumatolytic phases, similar muscovitization is developed regionally within the limits of the entire pegmatite field. Apparently such minerals as garnet,* sillimanite, kyanite, and staurolite also owe their origin to pneumatolysis. Of interest in this respect are observations on the conditions of appearance of staurolite in the schist sequence.

One and a half kilometers below the mine settlement, on the right bank of the Taseeva River, there is exposed a sequence of rather uniform, finely scaly mica-garnet schists, consisting of quartz, biotite, muscovite, and garnet, which forms porphyroblasts up to 2–3 mm in diameter. The schist sequence here is penetrated by a number of thin (10–30 cm) concordant quartz and quartz-feldspar veinlets. It is characteristic that only in places where such veins are concentrated and in the selvages of …

these veins the schists acquire coarse grain, and large porphyroblasts of garnet and staurolite develop in them. Especially clearly the essentially pneumatolytic origin of the staurolite is indicated by the presence of quartz veins, on the selvages of which rims 2–3 cm thick of coarse-grained staurolite are developed, forming crystals up to 2–3 cm long.

The initial stages of the process of formation of mica-garnet schist can be observed in sections along the Angara River on the northwestern continuation of the Kondakovo pegmatite field. Near the village of Smetanina, and especially on the right bank of the Angara River, one can see a monotonous sequence of gray-green phyllites consisting essentially of quartz, chlorite, and sericite. In these phyllites, only in places where thin concordant veinlets and lenses of gray transparent quartz are developed, is there an appearance of garnet grains and flakes of biotite and muscovite; moreover, the sizes of the grains of these minerals are in direct dependence on the density of the quartz injection, and also on the distance from individual quartz veinlets.

For disthene and sillimanite there is no such clear evidence of their genetic connection with pegmatites, but it should be noted that disthene in the Southern Yenisei Ridge has been encountered only in the mica schists of the pegmatite fields of the Aleksandrovskoye and Kondakovo deposits, while sillimanite is most common at contacts with pegmatites.

The pegmatites of the Barginskoye (Kansk) mica deposit have been excellently described by Altgauzen (1).

The enclosing rocks are garnet-feldspar and pyroxene-plagioclase rocks of the Kansk complex; moreover, the pegmatite veins are always enveloped, as though by a sheath, in schistose biotite and hornblende gneisses, a circumstance which Altgauzen especially emphasizes: “All pegmatite veins are confined only to schistose rocks—biotite and partly hornblende gneisses” (1, p. 164); moreover, “the connection between the form of the pegmatite body and the external form of the enclosing gneisses is quite clearly outlined. This connection is expressed in the fact that where persistent ‘beds’ of gneisses occur, the pegmatite veins are persistent along strike and have a sheet-like or pancake-like form; in those cases, however, where the gneisses possess irregular block-like contours, the pegmatite veins also have irregular forms, sometimes repeating the bends of the contours of the gneisses” (1, p. 165). A large part of the pegmatites has the character of sheet veins striking in NW rhumbs; in some cases the veins dip more steeply than the enclosing rocks, the angles of dip ranging from 15 to 60°. The form of the veins is predominantly plate-like. They are well maintained along strike for 200–400 m, with a thickness of 1–2 m. In one case the length reaches 2000 m, with a thickness from 10 to 50 m. On the other hand, completely irregular bodies of lenticular form are also encountered.

The structure of the veins is varied. Sometimes the entire pegmatite vein is filled with a uniform mass of pegmatitic material; more often, however, banded veins are observed, with the usual structural scheme, i.e., with an aplitic selvage on the periphery and a quartz belt in the axial part, while the main part of the vein is composed of a pegmatitic, rarely graphic, aggregate of quartz and feldspars, with a considerable admixture of muscovite, tourmaline, beryl, etc.

In examining the question of the genesis of the pegmatites, Altgauzen makes a significant error of fundamental importance. He believes that the biotite gneisses enveloping the pegmatite veins are the primary component of a metamorphic sequence which, having fallen into conditions of the katazone, underwent strong metamorphism; moreover, part of the material underwent complete softening and subsequently yielded massive pyroxene-plagioclase and garnet-feldspar rocks, while another part (clayey rocks) proved more refractory and, under the same conditions, preserved its schistose texture. The pegmatites of the Barginskoye deposit, in Altgauzen’s opinion, formed mainly earlier than the massive rocks had completely solidified. “In the presence of a softened mass, among which …”

were sheet-like and, more rarely, dyke-like bodies of schistose and fissured biotite gneiss; it is quite obvious that the easily mobile pegmatitic melt, in seeking the most convenient paths for penetration into higher zones of the earth’s crust, found in the gneisses precisely those mechanical conditions that met these mechanical tendencies. Thus, the preserved areas of gneisses were a kind of “diversion channels of magmatic extracts” (p. 235).

Fig. 4. Schematic section of the Kansk pegmatite deposit (after Altgauzen).

Scale 1:10000.

Conventional signs: garnet–feldspar rocks; pyroxene–plagioclase rocks; gabbroid rocks; biotite gneisses; pegmatites; faults.

Altgauzen’s ideas on this question are striking in their artificiality. In reality the relationships between the pegmatites and the enclosing mass of garnet–feldspar and pyroxene–plagioclase rocks of the Kansk complex are much simpler. The pegmatitic melt intruded into the rocks of this formation not only when its principal metamorphism had been completed, but also after it had been brought into higher horizons of the lithosphere and had undergone strong compression—perhaps more than once—which was relieved by disjunctives, along which the rocks were naturally strongly mylonitized and schistose. It is entirely natural that the pegmatitic melt intruded along these disjunctives, which coincide with the predominant strike of the formation but dip somewhat more steeply than the latter. The biotite gneisses enclosing the pegmatitic veins, however, are the product of pneumatolytic metamorphism of the rocks of the Kansk complex, caused by the intrusion of the pegmatitic melt. This is proved not only by the constant association of pegmatite and biotite gneiss, and by the cross-cutting occurrence of pegmatites within the mass of crystalline schists, which is well emphasized on Altgauzen’s map and section (Fig. 4), but also by the dependence between the thickness of the pegmatite vein and the thickness of the selvage of biotite gneiss, and by the presence of transitions from biotite—

garnet gneisses into garnet–feldspar rocks, the constantly observed phenomena of biotitization of garnet, with the development of complete pseudomorphs, about which, incidentally, Altgauzen himself also mentions in various places in his work. Finally, it should be noted that the transformation of garnet and hypersthene rocks into biotite gneisses is, in general, in our area a characteristic contact-metamorphic process, observed near all acid intrusions and, especially, pegmatite veins.

Comparison of the numerous chemical analyses of massive garnet–feldspar, pyroxene–plagioclase rocks and contact biotite gneisses, especially the study of the variation diagram given on p. 225 of Altgauzen’s work (1), shows that the biotite gneisses, while having the same silica content as the massive rocks, differ from the latter in general by a sharply increased content of constitutional water, potassium oxide and partly alumina, and by decreased contents of calcium oxide and magnesium oxide; the remaining components remain almost unchanged. It may therefore be thought that the pneumatolysis of the massive pyroxene–plagioclase and garnet–feldspar crystalline schists, caused by pegmatite melts, was accompanied by the introduction of considerable amounts of water and potassium. It is possible that calcium and magnesium were simultaneously removed from the replaced rock, which is entirely possible in a metasomatic process.

After their formation, the Barginsk pegmatites underwent noticeable crushing. The pegmatites are not infrequently mylonitized, with the development of augen textures. These mechanical effects, among other things, greatly impoverished the deposit, since the muscovite in them is strongly deformed.

It must be said that the pegmatites of the Barginsk deposit differ somewhat from the Kondakov and Shilkinsk pegmatites in the weaker manifestation of pegmatoid phases, as well as in the character of the contact effect on the country rocks, which is expressed essentially in the biotitization of garnet and the amphibolization of pyroxenes. Muscovite, tourmaline and other pneumatolytic minerals are only weakly developed here. It is now difficult to say what accounts for these differences. Possibly the Kondakov and Shilkinsk pegmatites were generally richer in volatiles than the Barginsk ones, although the broad development of exo- and endocontact pneumatolysis in these areas could have been caused, or at any rate intensified, by the lesser depth of formation of the Kondakov pegmatites. Finally, the possibility is not excluded that the pegmatites of the Barginsk deposit are in general genetically connected not with the Posolnensk, but with an older, for example Taraksk, granite intrusion; this is partly confirmed by the strong cataclasis and mylonitization of the Barginsk pegmatites, quite comparable with that of the Taraksk granites.

Facies and conditions of formation of the intrusion. As indicated above, the Posolnensk intrusion caused a complex metamorphism, expressed in the formation, at the expense of very diverse initial material, of biotite–plagioclase gneisses, plagioclase amphibolites and migmatites; moreover, the mineral associations of the contact rocks of the Posolnensk intrusion are characteristic of the amphibolite facies (Eskola), i.e. they are characterized in general by the same features as the rocks of the exocontact of the Taraksk intrusion. A substantial difference of the exocontacts of the Posolnensk granites is the complete absence of cordierite, which is probably explained by the wide manifestation here of alkaline metasomatism, with which the formation of migmatites and, apparently, of the granites themselves is connected. Migmatitic formations are widely distributed and very typical of the Posolnensk intrusion; moreover, in the character of their manifestation they differ strongly from the Taraksk migmatites, where we saw

a sharp predominance of layered injection with the formation of apterites with far-reaching assimilation phenomena. The Posolnaya intrusion is characterized by the predominance of metasomatic migmatization (feldspathization), with the development of porphyroblastic microcline and, accordingly, a minor role of injections of the arterite type. Judging from the fact that the minerals clearly introduced into the migmatites are microcline and also, apparently, biotite and riebeckite, the migmatizing solutions had a distinctly alkaline character. In the field it is impossible to draw a boundary between the migmatites and the granites proper of the Posolnaya intrusion; indeed, this boundary probably does not exist, and in any case a significant part of the Posolnaya granites has a distinctly migmatitic character, as indicated by their inherited gneissoid character and banding, allotriomorphic and even granoblastic structures, phenomena of replacement of plagioclase by microcline, etc. Also noteworthy is the exceptionally weak manifestation of assimilation phenomena proper, which accounts for the striking uniformity of the granites composing the Posolnaya and Piskunov plutons. Finally, a very characteristic feature of the Posolnaya intrusion is the extremely broad development of pegmatites, which cut the structures of the migmatites and are evidently the product of an additional injection, accompanied, among other things, by its own substantial pneumatolytic metamorphism, expressed in the development of biotite, muscovite, tourmaline, garnet, and sometimes staurolite, disthene, and orthite.

The foregoing makes it possible to envisage the process of formation of the Posolnaya granites and migmatites as follows. The observed linear arrangement of the zones of migmatization and of the gneiss-granite plutons, as well as the constant gneissoid character of the rocks composing them, coinciding with this main direction, permits one to think that the intrusion was oriented along certain tectonic zones, manifested both in the Lower Proterozoic and in the Archean basement in the form of schistosity. Apparently there was no emplacement here of a truly liquid magma, accompanied by displacement of the walls, uplift or collapse of the roof, or, finally, engulfment—assimilation of the enclosing rocks. The process was reduced to metasomatic migmatization and granitization, caused by the rise along zones of schistosity of extremely mobile alkaline solutions; moreover, the amount of introduced material was apparently not very great, as may be judged from a comparison of the data of chemical analyses of a typical migmatitic granite (1216), a migmatitic granosyenite (573a), the most widespread rock of the Kan complex—garnet gneiss (1042), and the product of metamorphism of the latter—biotite-plagioclase gneiss (333a).

Table 5.

Chemical analyses of migmatitic granite, biotite-plagioclase and garnet gneiss.

	1042 Weight percent	333a Weight percent	1216 Weight percent	1216 Difference from 1042	1216 Difference from 333a	573a Weight percent	573a Difference from 1042	573a Difference from 333a
SiO₂	69.46	69.89	71.07	+1.61	+0.18	65.47	−3.99	−4.42
TiO₂	0.87	0.65	0.22	−0.65	−0.43	0.44	−0.43	−0.21
Al₂O₃	13.19	13.20	13.71	+0.52	+0.51	16.56	+3.37	+3.36
Fe₂O₃	0.59	1.21	2.52	+1.93	+1.31	2.62	+2.03	+1.41
FeO	4.77	4.46	1.03	−3.74	−3.43	1.48	−3.29	−2.98
MgO	2.69	2.16	0.37	−2.32	−1.73	0.49	−2.20	−1.68
CaO	3.40	2.44	1.28	−2.12	−1.16	1.97	−1.43	−0.47
Na₂O	2.22	2.58	3.79	+1.57	+1.21	4.33	+2.11	+1.75
K₂O	1.68	2.36	5.11	+3.43	+2.75	5.60	+3.92	+3.24
Loss on ignition	1.22	1.09	0.41	−0.84	−0.68	0.37	−0.85	−0.72

Table 5 shows that in the process of migmatization, apparently only alkalis—especially potassium—and alumina were added to the material of the primary rock. Study of the macro- and microstructures of the migmatites makes it possible to assert that the process of feldspathization is essentially metasomatic, i.e., without a substantial increase in volume; therefore we must also allow for the removal by solutions of some amount of the oxides of magnesium, calcium, and iron from the rock being replaced, with part of the ferrous iron simultaneously passing into ferric iron. The same structural features of the migmatites indicate the long duration of the process of feldspathization and granitization, and the differential movements within the mass of the rocks being replaced that did not cease during this process.

It was noted above that dikes of two-mica granites, orthotectites, and micaceous pegmatites cut the migmatites, sometimes across their strike. Thus, the only reliably present magmatic formations are the latest in this magmatic complex and were intruded when the process of migmatization had for the most part ended. These relations compel one to doubt the generally accepted ideas on the formation of migmatites (injection gneisses) by the introduction into the country rocks of “residual” melts. Apparently, in many cases the main phase of migmatization precedes the intrusion of liquid magma.

The age relations of the Posolnaya intrusion with other metamorphic, sedimentary, and magmatic formations are established with sufficient precision. The Posolnaya and Piskunov gneiss-granite plutons and the pegmatites associated with them cut and metamorphose the Kan, Yenisei, and Murozhno-Tatarka metamorphic complexes, in particular the quartzites of the Sukhoi Range, as well as a formation of ancient diabases, which intersect the Tarak gneiss-granite intrusion and the gneisses of the Kan complex with abundant dikes (Fig. 5). The Lower Kan granites described below locally include enormous xenoliths of migmatites, entirely similar to the Posolnaya ones. Judging from the complete absence of gneiss-granite intrusions analogous to the Posolnaya, as well as of micaceous pegmatites, in the normal-sedimentary limestone–shale strata of Middle Proterozoic age, it must be thought that the intrusion of the Posolnaya gneiss-granites and migmatites—or, more precisely, the process of granitization—took place after the Lower Proterozoic sedimentary strata had been penetrated by diabase sills, folded, and schistosed, but before the intrusion of the Lower Kan granites, and still more before the deposition of the carbonate Middle Proterozoic.

Fig. 5.

The mineral resources of the Posolnaya intrusion are connected only with its pegmatites, which have been briefly described above. Of the various minerals of the pegmatite veins, practical importance has so far been acquired by muscovite, the exploitation of which on a fairly large scale is proceeding in the Kondakov—

... in the [[unclear: ending of place name]] and Aleksandrovsk deposits. The Barginskoye deposit was exploited, but at present is under preservation. The use of feldspar and quartz from many pegmatite veins is quite possible. Finally, in the Kondakovsk and Barginskoye deposits I have established, for certain veins, a high content and considerable reserves of beryl. It is interesting that in the Posolnaya granites monazite is rare, and industrial concentrations of monazite in the placers of the basin of the Posolnaya River have not yet been encountered. This is evidently explained by the practically complete absence of assimilation of the originally monazite-bearing gneisses of the Kan complex, owing to which not only did no secondary concentration of the rare earths take place, as we had in the Tarak intrusion, but these rare earths, apparently, were even partly removed in the process of migmatization and entered into the composition of the orthite, in which the contact gneisses are sometimes fairly rich.

2. Lower Kan Granite Intrusion.

In its lower course, for a distance of about 80 km, the Kan River cuts through an enormous granite pluton and its contact zone, penetrated by a series of satellite bodies. Beginning at the Bolshoy Rapid (15 km from the mouth of the Kan) to the village of Berezovoye, i.e., for a distance of about 25 km, along the banks of the Kan River there are exposed essentially only hornfelses and injection gneisses, cut by a large number of concordant and cross-cutting pegmatite and aplite veins and by several small (lenticular bodies) masses composed of rocks of syenitic and dioritic composition, which are clearly hybrid in character. From the village of Berezovoye to the village of Rostovtsevo—for 40 km—the Kan exposes the main granite pluton, composed of red and pink porphyritic granites. Near the northern contact the granites contain a considerable quantity of schlieren masses of dioritic composition. Closer to the eastern contact, the granites are in places filled with enormous inclusions of migmatites of the Posolnaya type, and also sometimes contain xenoliths of amphibolitic composition. The width of the eastern exocontact zone is small, and at a distance of 1 km from the contact the influence of the granite intrusion is already almost imperceptible. The Lower Kan granite pluton is accompanied by aplites, pegmatites, and lamprophyres. Granite plutons of smaller size have been mapped by M. P. Nagorsky (30) along the right bank of the Yenisei above the mouth of the Kan River.

The decidedly predominant type of rocks composing the main pluton is red and pink microcline porphyritic granites. The predominant component in them is red or pink microcline, forming segregations sometimes up to 4 cm long, though more often smaller. The boundaries of the segregations with the rest of the rock mass are indistinct, and in the marginal parts of the microcline grain there are always many inclusions of plagioclase and quartz. The microcline lattice is developed unevenly, in patches, mainly around inclusions, and sometimes is not developed at all. Constant perthitization with patchy or veinlike perthite is characteristic. Plagioclase makes up no more than 20–25% of the mass of the rock. It has the composition of albite-oligoclase, more rarely oligoclase, is sometimes weakly zoned, and sericitized; it forms grains appreciably smaller than microcline, having in the specimen a whitish color. Transparent gray or even black quartz is present in the amount normal for granite—25–30%, and only rarely does its amount fall to 10–15%. The dark minerals are represented only by brown biotite, sometimes chloritized, present in small quantity. Porphyritic granites entirely without biotite also occur. Muscovite is encountered rather often. As accessory minerals: apatite, zircon, and magnetite,

whereas the latter is a constant constituent part of the granites, by which, among other things, the Nizhnekan granites—together with the coloration and character of the microcline—differ sharply from the Tarak granites. Cataclastic phenomena, in the form of cloudy extinction of quartz, are observed in almost every specimen, but at times—especially in the contact zones—the granites are noticeably gneissose. In addition, as an essential textural element there is a tendency toward a parallel arrangement of tabular crystals of microcline, which is already an expression of prototectonics.

Table 6 gives two analyses of granites typical of the Nizhnekan pluton.

Table 6.

Chemical composition of rocks of the Nizhnekan intrusion.

Component	1072/1937, wt. %	1072/1937, molecular amounts	871/1938, wt. %	871/1938, molecular amounts	846/1938, wt. %	846/1938, molecular amounts
SiO₂	73,42	1223	68,03	1133	59,63	994
TiO₂	0,07	1	0,30	4	0,85	11
Al₂O₃	13,25	129	15,08	148	16,87	166
Fe₂O₃	2,09	13	2,07	13	3,45	22
FeO	0,80	11	1,65	23	3,68	51
MgO	0,49	12	0,63	16	3,12	78
MnO	—	—	—	—	0,26	4
CaO	1,49	27	2,30	41	6,74	120
Na₂O	3,36	54	4,26	69	2,84	46
K₂O	4,00	42	3,71	39	1,40	15
P₂O₅	—	—	—	—	0,26	2
− H₂O	0,15	—	0,20	—	0,70	—
Loss on ignition	0,36	—	1,80	—	0,38	—
Total	99,85		100,36		100,18

Table 7.

Recalculations of analyses of rocks of the Nizhnekan intrusion.

No.	si	al	fm	c	alk	k	mg	c/fm	a	c	b	S
1072	406	43	16	9	32	0,44	0,25	0,56	12,8	1,8	3,7	81,7
871	313	41	18	11	30	0,36	0,25	0,61	14,8	2,7	4,5	78,0
2001	190	32	34	23	11	0,25	0,44	0,68	8,6	7,4	13,5	70,5

1072—pink porphyritic granite. Kan River, Kosoi rapid. Analyst—I. P. Onufrienko.

871—red coarse-grained granite. Kan River. Analyst—I. P. Onufrienko.

2001—tonalite, hybrid. Rybnaya River, tributary of the Kan River. Analyst—Krivets.

The analyzed specimen 1072/1937 is a yellowish-pink coarse-grained rock with large—up to 3 cm long—porphyritic segregations of microcline. Mineralogical composition: quartz, microcline, plagioclase, biotite, muscovite, sphene, zircon, magnetite, apatite. The coarse grain of the rock makes determination of the quantitative mineralogical composition on the Push integrator unreliable; therefore I give the results of calculating the normative composition, computed by the CUPW method: quartz—34.9%; orthoclase—23.4%; albite—28.3%; anorthite—7.5%; in addition, the calculation gives normative corundum, enstatite, magnetite, and hematite, which evidently actually enter into the composition of biotite, muscovite, and sphene together with the corresponding amounts of potassium and calcium. The rock is indeed rich in characteristic smoky quartz, which is, among other things, idiomorphic with respect to microcline. The microcline is very fresh, with weak development of vein-like perthite. Microclino-

lattice appears only in isolated patches, chiefly near perthite intergrowths; Carlsbad twins are common; it is sharply xenomorphic with respect to quartz and plagioclase. Plagioclase forms idiomorphic tabular grains, is sharply repeatedly zoned, the composition of individual zones, judging from the extinction angles in the direction \([100]\), varying from \(+11^\circ\) to \(-12^\circ\), fluctuating from \(27\% An\) to \(10\% An\). The small unzoned grains usually have, in this section, an almost straight extinction. The dark-colored minerals are represented by biotite with pleochroism from straw-yellow to brownish black. As accessory minerals: rather abundant magnetite, apatite, zircon, sphene. Secondary products—chlorite after biotite, sericite and large flakes of muscovite after plagioclase.

Sample 871/38 represents a coarse and equigranular red granite with an average grain size of 0.5–1.0 cm. It consists of the same components as the preceding one. The difference is reduced to a noticeably smaller content of quartz in the red granite, which is reflected both in the chemical analysis and in the features of the structure. The normative composition of the principal components, calculated by CUPW: quartz—22.9%; orthoclase—21.7%; albite—36.2%; anorthite—11.1%; and, in addition, recalculation gives magnetite, ilmenite, hypersthene, of which only abundant magnetite is actually present in the rock, while the others are in reality replaced by biotite and sphene. Plagioclase is slightly zoned, the central and larger part of the grain giving extinction in the direction \([100]\) of \(+5^\circ\), while a narrow band on the periphery of the grain gives \(0^\circ\), i.e., the composition changes only within the limits from \(23\) to \(21\% An\). A sharp idiomorphism of plagioclase with respect to microcline is characteristic, owing to which the microstructure in some parts of the thin section assumes a monzonitic character. Microcline with a distinct lattice and very weakly perthitized is xenomorphic with respect to plagioclase, but sharply idiomorphic with respect to quartz. The latter forms large irregular grains with weakly expressed cloudy extinction. Biotite is strongly chloritized, forms small plates gathered in bunches and situated together with quartz and microcline in the interstices between plagioclase grains; it is noticeably xenomorphic with respect to the latter. As accessory minerals: abundant magnetite, sphene, apatite, zircon. Secondary minerals—sericite developing as a very fine dusting over plagioclase, chlorite after biotite, and a little muscovite.

Fig. 6. Variation diagram of granitoids of the Lower Kana intrusion. Analyses 573-a and 1026 represent the chemistry of the Posolnensky granites and are plotted on the diagram for comparison.

In the features of their chemistry the two rocks are very close to each other, and the differences between them are, in essence, determined only by a somewhat different silica content. Calculation of the numerical characteristic according to Zavaritsky shows that the analyzed granites are very close to the “average granite,” the deviations in sample 1072 being directed toward an increase in the value \(s\) and a decrease in \(b\). In sample 871 the deviations are in the other direction: in comparison with the “average granite” the value \(a\) is increased and \(s\) is decreased. Recalculation by the Niggli method reveals a clear belonging of both analyzed granite samples to the rapakivi type of magmas. Attention is also drawn to the chemical closeness of the Lower Kana granites to the Posolnensky gneiss-granites, which likewise possess a strongly expressed rapakivi chemistry; this makes the existence of a genetic connection between the former and the latter very probable. On the variation diagram (Fig. 6) of the Lower Kana granitoids, for comparison, two analyses of the most typical granites of the Posolnensky intrusion are also plotted. True, the number of analyses is insufficient for any confident conclusions to be drawn, but it is nevertheless noteworthy that analysis of Posolnensky granite No. 1216 fits quite well on the variation curves of the Lower Kana granitoids. Analysis 573-a deviates considerably from these curves, but this analysis represents the chemistry of a granitoid rock whose migmatitic origin is beyond doubt. The deviations in the chemistry of the Posolnensky granites are most sharply expressed in their increased alkalinity, which seems quite natural if the mode of their formation is taken into account.

In the stretch of the Kana River from the mouth of the Rybnaya to the eastern boundary of the pluton, enormous inclusions—xenoliths of injection gneisses—and smaller ones—of black amphibolites—are frequently observed in the granites.

Injection gneisses of the inclusions are very diverse in their external appearance. They are always gneiss-like layered rocks of gray, yellowish-gray, or red color, depending on the amount of biotite and microcline. The layering is determined by the alternation of biotite and feldspar or quartz-feldspar interlayers or lens-

...whereas, in the case of intense migmatization, the biotite films seem to melt into the mass of quartz–feldspathic material.

These injection gneisses form enormous inclusions in the granites, up to 1/2 km across; these may simply be projections of the roof. In places they are cut by numerous concordant bodies of medium- and coarse-grained veined adamellite, which forms many small intrusions along the foliation of the injection gneiss; moreover, the contacts of the adamellite with the latter are always perfectly sharp, and at the very contact there is usually a selvage of fine-grained aplite.

Within this same zone of distribution of gneiss inclusions, smaller xenoliths of amphibolites are encountered rather frequently in the granites and granodiorites. In most cases the amphibolites are black, fine-grained, schistose rocks composed of a granoblastic aggregate of green hornblende and basic plagioclase (andesine–labradorite), judging from extinction at an angle of 25° in the direction [100], sometimes with an admixture of quartz and biotite.

The described gneisses and amphibolites of the inclusions are evidently foreign to the intrusion of the Lower Kan granites, representing fragments of an older metamorphic series captured by the granitic magma. This is proved by the presence of sharp contacts between the injection gneisses and the granites and by the discordance of elements of the internal tectonics of the granites with the structure of the gneisses. The Lower Kan granites, as we shall see below, generally form discordant bodies with sharp contacts and a very weak manifestation of injection metamorphism. Apparently, the injection gneisses and amphibolites of the inclusions are essentially mechanically captured fragments of rocks of the Kan complex and migmatites of the older Posolnaya intrusion.

In the endocontact zones of the massif, the Lower Kan granites often contain schlieren of quartz-dioritic and tonalitic composition, reaching 30 m across; these rocks, while undoubtedly hybrid formations, have the character of normal magmatic rocks, differing perhaps only in the abundance of apatite. In addition to dioritic schlieren, schlieren of granodioritic composition are often encountered, showing an inherited gneissosity. Within the northern part of the granite massif there occur small schlieren that have a lamprophyric character and are characterized by an abundance of green hornblende in idiomorphic grains together with weakly zoned plagioclase. Here, finally, small xenoliths of fine-grained biotite gneiss are widely distributed; their arrangement, together with the parallel textures of the granite itself, makes it possible to judge the internal tectonics of the massif.

Table 6 gives a chemical analysis of a hybrid tonalite (spec. 2001/38), forming small schlieren in the granite of the southern contact zone (Rybnaia River). Mineralogical composition: quartz—27%; plagioclase—58%; biotite—9%; hornblende—7%; sphene, apatite, magnetite—2%. The plagioclase is fresh, finely zoned; its composition changes in individual zones from 42% An in the center to 21% An at the periphery. Light-green hornblende forms elongated idiomorphic prisms. The biotite is also very light, with pleochroism from pale yellow to greenish brown. Sphene forms large grains and gives the impression of an essential constituent. As accessories there are magnetite and apatite, associated chiefly with the hornblende.

As a result of recalculations of the chemical-analysis data, a clearly expressed tonalitic, or more precisely peleitic, type of chemistry of the analyzed specimen is revealed (see Table 7). The hybrid character of the rock is unquestionable, although it is not apparent in the features of the mineralogical composition and structure of this specimen.

North of the main granite pluton, near the mouth of the Nemkina River, the Kan River exposes three more small intrusive bodies composed of strongly hybridized rocks.

For example, a small intrusive body, exposed immediately above the mouth of the Nemkina River, is composed entirely of dark-gray diorite, over-...

by fine-grained melanocratic schliers, these schliers reaching several tens of meters in cross-section. The diorite itself also sometimes shows inherited banding, which emphasizes its hybrid character. Microscopic study gives no new signs of hybridization, except for an increased content of amphibole and biotite, which occur in clusters.

The larger body, exposed by the Kan River near the mouth of the N. Atat River, is composed already of yellowish-gray or light-gray granodiorites and tonalites. In the northern contact zone of this pluton there is at first intensive injection of light-gray granodioritic material into the enclosing rocks, and at the contact itself an alternation of dark-gray fine-grained arteritic gneisses with intrusive breccia composed of fragments of biotite-amphibole paragneiss immersed in hybrid granodiorite.

The vein suite of the Nizhnekan intrusion is represented by abundant injections of aplites, pegmatites, and very rare lamprophyres. Aplites and pegmatites are especially widely distributed in the northern contact zone, where they form partly lit-par-lit injection, and partly cutting veins of northwestern strike, the latter having the same strike both in the intrusive rocks and in the enclosing sequence. In the eastern contact of the main pluton they occur much more rarely. Here, too, there are both lit-par-lit injection and cutting veins, but the strike of the latter here is already latitudinal. Lit-par-lit veins of flesh-red pegmatite are fairly widely distributed in the SW contacts—in the basin of the Rybnaya River, where they are worked for feldspar. In the central part of the main pluton aplites and pegmatites occur very rarely; moreover, intersections of steeply dipping aplitic veins by almost horizontal pegmatitic ones were observed. In this distribution of aplitic and pegmatitic cutting veins there seems to be a certain regularity. They are concentrated along the margins of the main pluton and are situated predominantly perpendicular to the direction of the contact.

The aplites of the Nizhnekan intrusion have a pink or red color, are medium- or fine-grained, and consist of quartz, albite-oligoclase, and reticulate microcline, with a minor admixture of biotite and quartz.

The pegmatite veins likewise consist of gray quartz and red or pink feldspar, sometimes with an admixture of biotite and silvery muscovite. Almost always a sharply expressed, more or less coarse graphic texture is observed; moreover, weak layering is often seen—the peripheral parts of the vein are composed of aplite, while the central part is composed of graphic granite, or else of feldspar alone. The microscope shows that both microcline and albite-oligoclase take part in the formation of the graphic granite; the quartz rods pass, without changing, from one feldspar into the other. Structural relations show that microcline crystallized later than plagioclase, and partly replaced the latter metasomatically. No appreciable manifestation of pegmatoid phases was observed in the pegmatites described, and they are very weakly mineralized, although muscovite is a common component of them.

Lamprophyres are little distributed. We encountered only one dike of spessartite of porphyritic structure, with phenocrysts of acicular hornblende and with a panidiomorphic groundmass consisting of zoned plagioclase, biotite, hornblende, and a small amount of quartz.

Contact metamorphism of the Nizhnekan intrusion is manifested somewhat differently in the northern and eastern contacts.

In its northern part, the Nizhne-Kan pluton is in contact with certain gneisses, whose primary features of mineralogical composition and structure are completely obscured by superimposed contact metamorphism. The width of the contact aureole here is very great, which is explained by the presence of a series of satellites, as well as by a mass of concordant and cross-cutting aplite-pegmatite injections, the contact effects of which merge with one another. Thus, here we have a superposition upon the more ancient regional metamorphism of the Yenisei (?) complex of a series of metamorphisms caused by the Nizhne-Kan intrusion, among which one may distinguish recrystallization, injection, and pneumatolytic-hydrothermal metasomatism.

Within the contact zone described, black schistose, slightly layered biotite gneisses are rather widespread; they differ scarcely at all from the corresponding rocks of the Yenisei complex. Sometimes such biotite gneisses undergo distinct migmatization, expressed in the metasomatic development of red microcline at the expense of plagioclase, whereby the entire rock acquires a distinct reddish tint; such red migmatites develop in areas especially densely penetrated by concordant veinlets of aplites and pegmatites. In other cases, among the dark biotite gneiss there are simply observed lighter and coarser-grained interlayers, composed, like the whole gneiss, essentially of quartz, oligoclase, and biotite. The only new mineral in them is muscovite. Evidently, in these cases there occurred a concordant penetration of solutions that brought about recrystallization and the development of muscovite.

In some cases there is observed, conditioned by concordant pegmatite injection, a considerable subsequent pneumatohydrothermal reworking of the described contact gneisses, as a result of which all components of the rock, except quartz, are replaced by a felt of finely scaly muscovite and green biotite.

The eastern contact of the Nizhne-Kan pluton has been studied much less thoroughly. Here the granites are in contact with garnet gneisses of the Kan complex. At the contact itself, the latter undergo changes of approximately the same character as the rocks of the northern contact. They are transformed here into clearly migmatized gneisses, and instead of garnet there develops an aggregate of green biotite and quartz; in addition, abundant muscovite develops, affecting chiefly the plagioclases. Farther from the contact, the effect of the granite intrusion is expressed only in a more or less complete replacement of garnet by green biotite, with the formation of distinct pseudomorphs, especially clearly visible in the hand specimen. At 1.5 km from the contact, the influence of the intrusion is no longer perceptible. Together with the rocks of the Kan complex, the diabases that cut them are also subjected to contact metamorphism. For example, a diabase occurring as a dike among garnet gneiss somewhat above the village of Rostovtsevo and cut, among other things, by a wide vein of red aplite-pegmatite, proves to be strongly altered; the plagioclase is partly recrystallized with the development of a granoblastic structure, while the pyroxene is completely replaced by a granoblastic aggregate of ordinary hornblende.

Tectonics. For judging the form of the Nizhne-Kan pluton, I possess too small an amount of data, since this pluton was crossed by only a few traverses. Observations on the elements of prototectonics (schlieren structure, parallel texture of the granite) show that the eastern contact of the pluton dips at an angle of 40–50° to the east; in the central part of the massif, over large areas, horizontally disposed schlieren were observed; observations in the northern contact are contra-

lenticular, but at the contact itself the parallel texture of the granite dips northward at an angle of 70°.

Judging by the abundance of xenoliths and schlieren, the pluton is not very deeply denuded. The relatively weak injection metamorphism, the occurrence of aplite–pegmatite formations only within a rather narrow contact zone, the manifestation of pneumatohydrothermal metasomatism, etc., make it possible to suppose that the Nizhnekansk pluton formed at considerably shallower depths than the Tarak and Posolnaya plutons and belongs to an entirely different type—discordant massifs with disharmonic relations to the structures of the enclosing rocks, with a quite independent internal tectonics and a normal-granite facies of rocks.

The age position of the Nizhnekansk intrusion is established by the following relations. It cuts and metamorphoses the Kan and Yenisei (?) metamorphic complexes and, most importantly, metamorphoses the formation of ancient diabases that cut the latter by abundant dikes. The upper age boundary is determined by another diabase formation (olivine–hypersthene diabases, norites, and norite-peridotites), which, as is established rather precisely, is of Middle Proterozoic age and cuts the Nizhnekansk granites with rare dikes. The question of the relations between the Nizhnekansk intrusion and the Posolnaya intrusion described above is resolved somewhat more complicatedly. First of all it should be noted that within the body of the Nizhnekansk pluton there are large xenoliths or roof pendants composed of migmatitic gneisses very similar to the Posolnaya gneisses and, judging by their sharp contacts with the granites, representing formations foreign to the latter. This alone suggests that the intrusion of the Nizhnekansk granites took place after the formation of the Posolnaya migmatites and in a substantially different tectonic setting. The relations of the aplites and pegmatites of the Nizhnekansk intrusion with the mica-bearing pegmatites of the Bargin deposit indicate the same thing. The latter have a consistent NW strike and show rather strong cataclasis, but in their immediate vicinity (somewhat below the mouth of the Bogunaeva River) there are almost latitudinal veins of aplite-pegmatites, unaffected by schistosity and evidently connected with the Nizhnekansk intrusion. Finally, the much younger age of the Nizhnekansk granites in comparison with the Posolnaya granites is also partly indicated by the entirely different tectonics of the Nizhnekansk pluton as compared with the Posolnaya intrusion. The Nizhnekansk granites apparently intruded along major faults into an already fully consolidated sequence and formed, judging by the weak development of the aplite-pegmatite phase and of injection metamorphism, at a considerably shallower depth than the Posolnaya migmatites and granites. Thus, the younger age of the Nizhnekansk intrusion in comparison with the Posolnaya one may be considered proven. At the same time, the Posolnaya and Nizhnekansk granites show great similarity in the features of their mineralogical and chemical composition and are not greatly separated in time. For both intrusions the lower age boundary is determined by the fact of intersection and metamorphism of dikes of the ancient diabase formation, which according to some data are of Lower Proterozoic age, and the upper boundary by their relation to the carbonate Middle Proterozoic and to conformable dikes of a young diabase formation occurring within the latter. Evidently, both intrusions are of Lower Proterozoic age. The Posolnaya intrusion, as established above, formed essentially by metasomatic granitization in a setting of continuing tectonic movements and in this sense is syntectonic. The Nizhnekansk intrusion, however, being a derivative of the same magmatic source that originated as a result of post–Lower Proterozoic tectogenesis, represents a somewhat later emplacement

viscous, partly recrystallized magma in a tectonically calm setting of the extensional phase. The time gap between these intrusions, however, was large enough that by the time the Nizhnekan granites were emplaced, kilometers of strata had already been removed by denudation; beneath their cover the Posolnensk migmatites had formed.

In information of a preliminary character published in 1938 (17), I provisionally synchronized the Nizhnekan granites with the Tatar, Ayakhta, and Angara granites of the Southern Yenisei taiga. I have now become convinced that this supposition was erroneous. The Angara granite cuts the dikes of the young diabase formation, which in turn cut the Nizhnekan granites.

The Nizhnekan intrusion is not rich in useful minerals. In the pegmatites and other rocks of the northern contact zone, disseminations of chalcopyrite are sometimes observed. In the central part of the massif, thin quartz veins containing traces of gold were observed. Heavy-mineral concentrates collected along the streams that erode the massif consist predominantly of magnetite and contain, in addition: sphene, ilmenite, amphibole, epidote, biotite, zircon, apatite, rutile, tourmaline, and, very rarely, monazite.

Thus, this intrusion as yet yields nothing of interest with regard to metalliferous potential. The pegmatites of the Nizhnekan intrusion are being exploited in the basin of the Rybnaia River for feldspar.

III. MIDDLE PROTEROZOIC AND YOUNGER ACID INTRUSIONS.

In addition to the granite and gneiss-granite intrusions described above, which form large plutons and migmatite fields, within the Southern Yenisei Ridge there are, rarely, small intrusions of a more or less clearly expressed hypabyssal character, occurring among the crystalline schists of the Precambrian in the form of small stocks and dikes. Among them, only for the Angara granite is a Middle Proterozoic age proved more or less precisely. The age of the others remains uncertain, and only certain indirect considerations make it possible to judge the time of their emplacement.

Angara Granite.

The carbonate sequence of the Middle Proterozoic in the area of the mouth of the Angara River is penetrated by a comparatively small stock of gray, coarse-grained granite, as well as by a suite of associated orthotectite and pegmatite veins. All these formations were described in detail at the time by Meister (25, pp. 181–215).

The granites of the stock, which, among other things, make up the well-known Strelkovsky rapid on the Angara River, are distinguished in fresh specimens by a light-gray color, coarse grain, and noticeable porphyritic character. On weathering they acquire a yellowish or slightly pinkish color. Very often, especially in the near-contact parts, they contain nests of tourmaline, and in places are filled with rounded inclusions—xenoliths—of white granite porphyry. It is characteristic that even at the very contact with limestone the granite does not change its coarse-grained structure and almost does not change its composition. Microscopic study of a number of granite specimens taken from different parts of the stock shows one and the same picture. The granites always possess a normal hypidiomorphic texture. The sharply predominant component of the rock is microcline, forming large, porphyritically prominent grains with a well-expressed lattice. Strong metasomatic perthitization is characteristic, expressed especially strongly along the margins of microcline grains. Plagioclase forms sharply idiomorphic small prisms, has the composition of albite or albite-oligoclase, and is usually strongly serici-

tized and is present in small (up to 10–15%) amounts. Quartz—in the amount normal for granite (about 30%)—is xenomorphic relative to microcline. Biotite is usually decolored and chloritized. In all specimens there is a fair amount of muscovite, which partly intergrows with biotite and partly develops after feldspar, with a little quartz being released at the same time. Tourmaline is occasionally observed in thin section. The complete absence of ore admixtures is characteristic. In specimens especially rich in tourmaline, a micropegmatitic intergrowth of quartz and orthoclase is observed.

In Meister’s work (25, pp. 181–215) a number of analyses of Angara granites are given; of these, however, only one belongs to the rocks of the main stock. Analysis 1 of Table 8 gives an idea of the peculiar—

Table 8

Chemical analyses of Angara granites (after Meister).

Component	Granite I: wt. %	Granite I: mol. qty.	Muscovite vein granite II: wt. %	Muscovite vein granite II: mol. qty.	Muscovite vein granite III: wt. %	Muscovite vein granite III: mol. qty.	Muscovite vein granite IV: wt. %	Muscovite vein granite IV: mol. qty.	Muscovite vein granite V: wt. %	Muscovite vein granite V: mol. qty.	Muscovite vein granite VI: wt. %	Muscovite vein granite VI: mol. qty.	Albite vein VII: wt. %	Albite vein VII: mol. qty.
SiO₂	70,96	1183	73,96	1233	72,25	1204	75,41	1257	70,46	1174	72,68	1211	70,12	1169
TiO₂	—	—	tr.	—	tr.	—	tr.	—	—	—	—	—	—	—
Al₂O₃	15,65	153	13,64	134	13,97	137	13,81	135	14,91	146	15,59	153	16,65	163
Fe₂O₃	0,81	5	0,78	5	0,76	5	0,34	2	1,17	8	0,45	3	0,37	2
FeO	1,93	27	1,72	24	1,90	26	1,22	17	2,66	37	0,06	1	0,0	—
MnO	0,13	2	—	—	—	—	—	—	—	—	—	—	—	—
MgO	0,86	22	tr.	—	tr.	—	tr.	—	0,25	6	tr.	—	0,0	—
CaO	0,84	15	tr.	—	0,63	11	0,67	12	0,84	15	1,02	18	1,21	22
Na₂O	3,28	53	2,82	45	2,77	45	1,20	19	1,04	17	5,15	83	4,91	79
K₂O	5,04	54	6,52	69	6,58	70	6,42	68	9,32	99	4,13	44	4,41	47
Li₂O	—	—	0,10	3	tr.	—	0,21	7	—	—	—	—	—	—
H₂O	0,48	—	0,64	—	0,31	—	—	—	0,55	—	0,39	—	1,21	—
Total	99,98	—	100,18	—	99,21	—	—	—	100,20	—	99,47	—	98,88	—

I — Granite of the main stock. Angara River, Strelkovskii rapid. Meister (1, 23, p. 182).
II — Muscovite granite, central part of a six-meter dike. Angara River opposite the village of Strelki. Meister (1, 23, p. 193).
III — The same, from the footwall. Ibid.
IV — The same, from the hanging wall. Ibid.
V — Muscovite granite, central part of a thin 75-centimeter vein. Angara River opposite the village of Strelki. Meister (1, 23, p. 196).
VI — The same, peripheral part. Meister (1, 23, p. 199).
VII — Albite. Angara River opposite the village of Strelki. Meister (1, 23, p. 205).

Table 9.

Recalculations of analyses of Angara granites

No.	si	al	fm	c	alk	k	mg	c/fm	a	c	b	s
I	352	45	18	5	32	0,50	0,36	0,25	14,0	1,0	8,0	77,0
II	437	48	12	0	40	0,59	0	0	15,0	0,0	4,8	80,2
III	403	46	12	4	38	0,61	0	0,31	15,3	0,7	3,9	80,1
IV	480	51	8	5	36	0,72	0	0,62	12,2	0,8	5,2	81,8
V	349	43,5	17,5	4,5	34,5	0,85	0,1	0,26	15,4	1,0	5,9	77,7
VI	397	50	2	6	42	0,35	0	2,57	16,9	1,2	1,5	80,4
VII	371	52	1	7	40	0,37	0	5,38	17,1	1,5	2,3	79,1

of the chemistry of the freshest and most widespread variety of granite. The ratio of the coefficients, calculated by Niggli’s method, shows that the analyzed rock belongs to the group of leucogranitic magmas, among which the engadinite-granite type is the closest.

Coarse-grained porphyritic granites contain, in places, rounded inclusions with sharp contacts, consisting of white or yellowish fine-grained microgranite or granite-porphyry, which, in comparison with the enclosing granite, are always relatively enriched in biotite and quartz. Quartz makes up as much as 50% of these inclusions; there is almost no plagioclase; the rest is microcline, muscovite, and biotite. The development of poikilitic structures is characteristic—quartz forms numerous inclusions not only in microcline, but also in plagioclase, and also forms larger, porphyritically distinguished grains. The hybrid character of these inclusions is quite evident; moreover, these inclusions apparently were brought up from great depth, since their composition indicates assimilation of siliceous rocks, whereas the stock itself lies entirely within limestones, and in the endocontacts traces of contamination are almost absent.

I was unable to find a direct contact of the granite with limestone either on the right or on the left bank of the Angara, but at several meters from the contact the granite is similar in composition and structure to the granite of the central parts of the stock, and the only difference is a noticeable crumpling and shattering, as well as a higher content of strongly sericitized plagioclase.

Thus, the endocontact changes of the granites are very weak; they are felt only at a distance of a few meters from the contact and are expressed in an increase in the amount of plagioclase, i.e., in a very weak assimilation of limestones, and not in the formation of an aplite marginal facies, as Meister asserts.

Dike granites and pegmatites. Downstream along the Angara, approximately opposite the village of Strelka, the limestones are cut by a number of crosscutting veins of coarse- and fine-grained granite, undoubtedly genetically connected with the stock of the Strelka Rapids.

For example, at the mouth of the Sogrensky spring, the limestones are cut by a vein-like body of coarse-grained light-colored granite with nests of black tourmaline and pegmatitic coarse-grained segregations. This granite is very similar to the granite of the Strelka stock. Plagioclase is represented by idiomorphic prisms of albite. Microcline, for the most part without twinning structure, but very strongly perthitized and sometimes even having an antiperthitic character, quantitatively predominates over albite. Quartz is present in normal amount. There is a little light-colored biotite, muscovite, and large grains of topaz, which together with muscovite fill miarolitic cavities.

Fine-grained muscovite granite, forming a dike 6 m thick, oriented parallel to the bank of the Angara, 0.5 km below the Sogrensky spring, is described in detail by Meister (25, p. 188), who, among other things, in his work gives analyses of three granite samples from this dike, taken from its central part (Table 8, analysis II), the footwall (Table 8, analysis III), and the hanging wall (Table 8, analysis IV). The granite of this dike is distinguished by an allotriomorphic, in places micropegmatitic structure and consists of albite, K-feldspar and quartz, abundant muscovite and topaz, and, along fractures in the rock, a little violet fluorite. Moreover, the muscovite, even under the microscope, has a pinkish color and weak pleochroism, which indicates an admixture of lithium. The granite of the central part of the dike (analysis II) shows a clear affiliation with the type of alkaline granites. In the peripheral parts

Fig. 7. Variation diagram of the granites and pegmatites of the Angara intrusion.

(analyses III and IV) a little calcium appears, which is evidently due to assimilation of some amount of the enclosing limestones. A curious chemical feature of the Angara granites, revealed in the analyses of the granite of the main stock (I), the granite of the described 6-meter dike (II, III, IV), and also in the analyses of the granite of the thin dike (V, VI), is the exceptionally low content of magnesium and calcium (the coefficient \(mg = 0\) or close to it, and \(c\) is very small), the absence of titanium, the presence of lithium, the predominance of potassium over sodium, which is reflected in high values of \(k\), and high values of the coefficient \(al\). All this is evidently a reflection of the pegmatitic character of the magma of the Angara intrusion, whose rocks, even in the main stock, are characterized by the presence of lithium-bearing muscovite, topaz, and tourmaline. The variation diagram (Fig. 8) reveals a completely smooth course of the curves and, consequently, the presence of genetic connections among all the rocks analyzed. Significant deviations from these curves are shown only by analyses V, VI, and VII, which relate to muscovite granites and albitites; moreover, the most substantial are the fluctuations of the coefficients \(k\), \(alk\), and \(al\), i.e., they are caused by an uneven distribution of alkalies, which is evidently already connected with postmagmatic processes—muscovitization, etc.

True pegmatites were encountered opposite the confluence of the Yenisei and the Angara. Here, in the limestones, there was observed an already well-expressed pegmatite vein of pegmatoid structure, composed in the central part of yellowish microcline, quartz, greenish mica with an admixture of fluorite and apatite, and in the peripheral part—fine-grained bluish albite. Here, too, there are fairly numerous thin, swelling and pinching-out veinlets of aplitic composition, consisting of variable amounts of K-feldspar, albite, and quartz. Especially widespread and characteristic are veinlets of bluish, medium-grained albite. Analysis VII, Table 8, apparently also belongs to such aplites, consisting of albite, K-feldspar, and quartz.

Contact metamorphism caused by the Angara granite and the accompanying dikes is, in general, very weak. At the contact with the Strelkovsky Rapids stock, pure limestones are only bleached and slightly marmorized, while impure ones are transformed into very fine-grained banded diopside hornfelses; moreover, at 300 meters from the contact the metamorphism is no longer perceptible. In direct contact with one of the veins of muscovite granite, Meister (25, p. 488) describes the transformation of limestone into a fine-grained hornfels consisting of pyroxene, garnet, vesuvianite, and fluorite. Farther from the contact, radial aggregates and sheaves of tremolite may be observed in the limestones. At the contact with the pegmatite vein described above, the limestone proved to be almost entirely replaced by albite, as well as by colorless garnet, pale wine-yellow tourmaline, fluorite, and muscovite; the thickness of the zone of such alterations is no more than 10 cm. Thus, the metamorphism caused by the granites of the Strelkovsky Rapids was expressed in a weak recrystallization of the enclosing rocks, while the dike granites and pegmatites also produce a certain pneumatolysis, in particular the formation of tourmaline, muscovite, and fluorite in the country rocks; but this pneumatolysis is, in general, very weakly manifested, which seems strange if one considers the probable richness of the granitic magma in volatiles (fluorine, boron, water), manifested in the development of muscovite, tourmaline, topaz, and fluorite in the granites and especially in their dike varieties, which already have a partly pegmatitic character. This contradiction should be explained mainly by the vertical contacts of the granite stock. In the general character of contact metamorphism, the Angara granites differ very sharply, for example, from the Nizhnekansky or Taraksky granites, and evidently formed in a substantially different facial setting, probably under conditions of medium and even shallow depths.

The age position of the Angara granites is determined only within very broad limits. The lower age boundary is determined by the fact that the Angara granite and the dikes associated with it cut the carbonate Middle Proterozoic, as well as the bedded bodies of olivine-hypersthene diabases occurring in the limestones.¹ The upper boundary remains indeterminate—

and only the complete absence of any traces of magmatic activity in the dolomitic Upper Proterozoic suggests that the emplacement of the granites took place before the accumulation of the latter sediments. The approximately identical stratigraphic position makes it possible to compare the Angara granite with the Tatar and Ayakhta granites of the Southern Yenisei taiga, although the latter are apparently manifested in deeper facies.

No useful minerals associated with the Angara granite are yet known. The small polymetallic deposits, in particular the Ust-Angara or Strelkovskoe deposits, developed in this area, can hardly be genetically related to it. Industrial accumulations of fluorite and cassiterite deposits of the Pitkaranta type are possible, although the searches carried out for fluorite deposits have so far yielded no positive results.

2. Belogorsk Granite-Porphyry Intrusion

In the upper course of the Shumikha River and on its watershed with the B. Kuzeeva there are widespread rocks of original granitic composition, which I have distinguished under the name of the Belogorsk intrusion. Completely analogous rocks have been encountered in the upper reaches of the Posolnaya River. The Belogorsk intrusion attracted special attention even in the field by a conspicuous manifestation of autometasomatic greisenization, by the gold-bearing character of the springs and streams that erode it, and by indications of the presence of scheelite in the heavy-mineral concentrates of these streams. Therefore, within the area of development of the intrusive rocks the prospecting traverses were greatly condensed, and this area was examined with particular care. Nevertheless, all these traverses yielded almost nothing for understanding the structure and form of the pluton, the relationships of the various types of intrusive rocks, etc. The reason is that, owing to the unusually loose composition of most representatives of this intrusion, they produce absolutely no bedrock outcrops. Along all the traverses that crossed the Belogorsk intrusion, only two small outcrops of granite-porphyries were recorded; the remaining rock material was collected from talus and channel deposits. The unusual looseness of most rocks of the intrusion was also reflected in the relief. The area of development of the Belogorsk intrusion is a clearly expressed basin, surrounded on all sides by higher mountains. Only in the central part of this basin does a small rounded hill rise in isolation—Belaya Gora, composed of denser fine-grained aplites. Thus, I do not even have definite data for deciding whether there is one large pluton in the Shumikha basin, in which several rock types are somehow manifested, or whether the latter in general form many independent small intrusions; therefore the depiction of the Belogorsk intrusion on the map is entirely conventional.

The most typical rocks of the Belogorsk intrusion are the following.

Granite-porphyries are dirty-pink, miarolitic rocks with abundant corroded phenocrysts of gray quartz, reaching 1 cm in length, and with rarer phenocrysts of pink microcline. The groundmass is fine-grained or medium-grained, of microgranitic texture, and consists of quartz, sericitized albite, microcline, biotite, and muscovite. The quartz phenocrysts are corroded and cut by small cracks, along which sericitized albite has penetrated, and in some cases also well-formed crystals of sphene. Magnetite and a rather large amount of zircon are present as accessory minerals.

Aplite granites (alaskitic granites) are yellowish, medium-grained rocks of hypidiomorphic texture, consisting of quartz, albite-oligoclase, and microcline, with a very small admixture of biotite. The fabric of the rock is very friable.

Aplite-pegmatites are the most widespread and most characteristic type. Usually these are yellowish-white, fine- and uneven-grained rocks consisting of quartz and feldspar, with a very insignificant admixture of thin plates of biotite. A miarolitic texture is characteristic; moreover, around the miarolitic cavities the rock becomes coarser-grained, and it is here that yellowish microcline is chiefly concentrated. The texture is either allotriomorphic or micropegmatitic; both textures are observed simultaneously not only in a single specimen, but sometimes even in one thin section. The principal components of these rocks are: quartz, usually without signs of undulatory extinction; microcline; and acid plagioclase, always predominating over microcline. Its composition varies from albite to oligoclase in different specimens (extinction angles in the direction [100] from 12 to 0°, more often −7–8°). Brown biotite, sphene, magnetite, and muscovite are present in very small amounts. Rocks of this character, even in small hand specimens, are often cut by thin veinlets of white, fine-grained aplite.

Quartz porphyry, a yellowish-white felsitic rock with rare and small phenocrysts of quartz and feldspar, is also one of the components of the Belogorsk intrusion. Under the microscope it reveals a very fine microgranitic, almost microfelsitic groundmass and phenocrysts of quartz, albite-oligoclase, and microcline.

Thus, a characteristic feature of the Belogorsk intrusion is the broad development of successive injections of aplite-pegmatitic composition, concentrated within a small pluton; the predominance of porphyritic and fine-grained varieties; a sharply expressed miarolitic texture; and intense autometasomatism, almost reaching the degree of greisenization. All these features indicate formation of the intrusion in a hypabyssal setting, under conditions of the stock facies (60).

The character of the contact metamorphism of the Belogorsk intrusion can be judged only from individual specimens. Garnet gneiss of the Kan complex at the contact is transformed into biotite gneiss, the chief alteration of the original rock being the replacement of garnet by green biotite. A pyroxene-plagioclase rock at the contact is transformed into amphibolite, consisting of a granoblastic aggregate of plagioclase and pseudomorphs of blue-green uralite after pyroxene. In some cases ore metasomatism was observed, as a result of which the pyroxene-plagioclase rock was transformed into an aggregate of saussuritized plagioclase and fibrous uralite, penetrated by a mass of branching veinlets of magnetite. Ancient diabases, under the influence of the Belogorsk intrusion, are also amphibolitized, with preservation of the relict ophitic structure. The contact metamorphism of the Belogorsk intrusion, therefore, is expressed in some recrystallization accompanied by strong metasomatic changes.

The age of the Belogorsk intrusion is determined only by the fact that it cuts and metamorphoses both Archean metamorphic sequences of the area and the ancient diabases. The upper age limit remains undefined. Its sharply expressed hypabyssal character does not allow it to be compared, for example, with the Nizhne-Kan or Posolnaya intrusions. More probable is its genetic connection with the Angara granites, although it may also represent an entirely independent formation.

With respect to possible ore potential, the Belogorsk intrusion is of somewhat greater interest than the Angara granite, and it is possible that the gold and tungsten mineralization of the district is connected precisely with it. This is partly demonstrated by the coincidence of the spatial distribution of the small stocks of the Belogorsk intrusion and the gold-bearing placers of the Posolno-Kuzeevsky district; moreover, in recent years, in the immediate vicinity of the largest body of the Belogorsk intrusion, a primary gold-tungsten (M. Kuzeevskoye) deposit has also been discovered, representing an entire system of quartz veins occurring in rocks of the Kan complex. The veins have been exposed by prospecting workings only in the oxidation zone. The ore minerals are abundant galena, sphalerite, pyrite, chalcopyrite, ochers, gold, and wolframite, present in some veins in large quantities together with abundant visible gold. The Bogunaevskoye gold-ore deposit, already exploited for several years, is very similar in the morphology of its veins and in the character of its mineralization, but tungsten minerals have not yet been found in it. Incidentally, at the Bogunaevskoye deposit the time of its formation is well established. The quartz gold-ore veins cut dikes of norites belonging to the Early diabase formation and are themselves cut by thin veinlets of microsyenites of Paleozoic (Telbess) age. Thus this deposit may be genetically connected either with the norites or with a younger Middle Proterozoic intrusion, for example the Belogorsk intrusion. This question, however, cannot be considered definitively resolved.

In the heavy-mineral concentrates of the streams eroding the Belogorsk intrusion, the following have been found: magnetite, ilmenite, martite, sphene, hornblende, biotite, zircon, apatite, rutile, scheelite, and gold.

3. Alkaline systems

Alkaline syenites, in the form of comparatively small dike-like intrusions, occur here and there in the middle and northern part of the Southern Yenisei Ridge. The largest body of alkaline syenites has been discovered on the Yagodka River, between the Nesmelovskaya and Borisovskaya dikes, where this stream, which generally flows among very gentle slopes, for a distance of about 0.5 km, cutting into the syenite massif, forms an almost impassable gorge.

The alkaline syenites composing this massif are very uniform in composition throughout the entire thickness of the body. Normally these are coarse-grained pea-yellow rocks, consisting 90–95% of very fresh, translucent alkali feldspar in the form of thick, irregularly arranged tablets. Pyroxene, hornblende, and biotite are admixed with the feldspar; a taxitic arrangement is characteristic of them. In all cases the rocks are exceptionally fresh and reveal no signs of cataclasis. Microscopic examination shows that the feldspar is represented by antiperthite, usually in Carlsbad twins, in which albite forms branching and mutually connected veinlets oriented approximately parallel to [010], the number of which is sometimes so great that only lenticular islands remain of the K-feldspar. Characteristically, in the albite component of the antiperthite no twin structure is ever manifested, whereas in the potassic feldspar a fine microcline grid is almost always visible. A comparatively small amount of aegirine-augite, riebeckite, and biotite is admixed with the antiperthite. The aegirine-augite has a rather deep zonal green coloration, an angle CNg up to 50°, and forms idiomorphic grains usually included in feldspar, sometimes fringed by riebeckite. Riebeckite—with pleochroism from yellow-green to blue-black and very low interference colors—also forms independent-

grains, in particular acicular inclusions in feldspar. Biotite has very dense pleochroism in reddish-brown tones and is lepidomelane in character. The taxitic arrangement is most sharply expressed in the case of biotite, which forms chiefly rosette-like intergrowths located in the interstices between feldspar grains. Sphene, zircon, and ilmenite were observed as accessory minerals.

Along with such coarse-grained alkaline syenites, medium- and fine-grained varieties of them are also observed in places; these do not differ essentially from those described in mineralogical composition. In some cases, between large feldspar grains there was observed a fine-grained panidiomorphic aggregate of albite, also partly replacing antiperthite.

Among the alkaline syenites described, in one place a small dike of kersantite lamprophyre was found. This lamprophyre is a fine-grained black rock consisting, as is revealed under the microscope, mainly of brown and green biotite, to which are added colorless augite, laths of plagioclase, blue-green hornblende, and rather abundant sphene. In the fine-grained aggregate of panidiomorphic structure that forms the main mass of the rock, rare and small porphyritic segregations of augite are observed. Lamprophyres of this type have not been encountered anywhere else, and apparently it is genetically connected with the intrusion of alkaline syenite described.

The contacts of the alkaline syenites with the enclosing rocks have not been exposed, and only from the position of the alkaline-syenite pluton in the area of development of rocks of the Murozhno-Tatarka complex may one suppose a post-Lower Proterozoic age for this intrusion.

Incidentally, the alkaline syenites described in many features resemble the quartz alkaline syenites of the environs of Krasnoyarsk (the Stolbovaya intrusion), which, as has now been precisely established, are Devonian in age (16 and 39).

Apparently, derivatives of the same magma are the fine-grained yellowish-gray syenite porphyries observed as small dike intrusions and as float in the basin of the Berezovka River (left tributary of the Yagodka), along the M. Yagodka, and in the upper reaches of the B. Ostrovnaya River (Posolnaya system). These syenite porphyries, as porphyritic segregations, contain irregular grains of perthitized orthoclase, while the groundmass is an allotriomorphic aggregate of orthoclase and albite or oligoclase with a significant admixture of taxitically arranged riebeckite. These rocks are very often cataclased and even bear traces of recrystallization. Rocks very close in composition to our syenite porphyries were described by Meister under the name of sölvsbergites (25). Apparently, the alkaline syenites and syenite porphyries described were called by Skabichevsky “Caledonian granites of the second phase” (41 and 33).

CONCLUSION.

A characteristic feature of the Southern Yenisei Ridge, impressing a definite stamp upon its entire geological aspect, is the wide development of acid, predominantly granitic intrusions, with a very weak distribution of basic ones. The acid magmatic rocks in the Southern Yenisei Ridge are composed chiefly of large abyssal intrusions and occupy an area of several thousand square kilometers, whereas the basic rocks occur exclusively in dike and sill facies and form bodies of such small dimensions that they cannot be shown on a geological map at the scale of the survey. It is also characteristic that the principal acid intrusions are granitic in composition. Other types of igneous

rocks occur comparatively rarely; moreover, as detailed study of intrusive complexes shows, almost all the diversity of igneous rocks composing one pluton or another is due to assimilation phenomena, and there are no indications of in situ differentiation. It goes without saying that in a number of cases the main phase of intrusion is accompanied by later dike formations, usually of aplite and pegmatite, and, very rarely, lamprophyre character.

An analysis of the stratigraphic and geological relationships between igneous and sedimentary rocks shows that in the Southern Yenisei Ridge acid and basic intrusions alternate in time; moreover, the basic, predominantly diabase, intrusions are, in general, synchronous with the accumulation of sediments or were emplaced at the beginning of folding, with the formation of sills (and perhaps extrusions) in the sedimentary complex and dikes (supply channels) in the crystalline basement, whereas the acid intrusions follow the main phase of folding, after the development of schistosity and regional metamorphism of the sedimentary complex, and thus complete the given tectonomagmatic cycle, which normally consists of the accumulation of sediments, injection of basic magma, folding, and injection of acid magma. The stratigraphic scheme of the Southern Yenisei Ridge presented above shows that there were at least four such tectonomagmatic cycles, ending with acid intrusions, in the Precambrian. It goes without saying that within one cycle or another there could have been interruptions in the accumulation of sediments and minor folding, perhaps accompanied by small intrusions; but all these small fluctuations cannot obscure the traces of the major, powerful movements and the general scheme, revealing a definite rhythmicity of geological processes and their development along a spiral, perhaps complicated by spirals of the second order.

The most ancient acid intrusions—charnockitic ones—bear clear traces of palingenic origin. They are characterized by the same mineral associations as the enclosing sequence of regionally metamorphosed crystalline schists; they produce in the latter no traces of contact metamorphism, and are evidently products of selective melting of certain portions of the Kan metamorphic complex in the process of general regional plutonometamorphism. The Southern Yenisei Ridge is a most interesting area where, thanks to exceptionally deep denudation, we can observe, in a preserved, so to speak, form, the initial stages of formation of palingenic magmas.

The oldest of the true “juvenile” granite intrusions—the Tarak and Posolbenskaya—are characterized by strong development of migmatization phenomena. But the character of this process in the two intrusions is substantially different. The Tarak intrusion is strongly hybridized and gives an injection of arterite type. The Posolbenskaya intrusion shows very weak traces of hybridization, is accompanied by a broad field of feldspathized rocks and metasomatic migmatites, and the granites themselves apparently have a migmatitic character. Both are accompanied by pegmatites, but their manifestation in the two intrusions is also different. The Tarak intrusion gives a bed-by-bed injection of weakly mineralized pegmatites, more or less contemporaneous with the arteritic migmatites and connected with the latter by transitional varieties. The Posolbenskaya intrusion is accompanied by abundant, structure-cutting, strongly mineralized pegmatites, producing a peculiar pneumatolysis of the enclosing rocks. This difference in the manifestation of the two abyssal granite intrusions is apparently explained by the different saturation of the rising magma with volatiles, by its consistency, and in part perhaps also by its composition. The magma of the Tarak granites evidently had, on the whole, a pegmatitic character and was very capable

toward assimilation and fine lit-par-lit injection, but nevertheless this was liquid magma. The Posolnaya granites have a clearly expressed migmatitic character, and the migmatites accompanying them formed by metasomatic feldspathization (par imbibition). Apparently, in the process of formation of the Posolnaya intrusion—at least of those parts of it that are now exposed by denudation—actual liquid magma took almost no part, and there was no intrusion involving the pushing apart of the walls or collapse of the roof, etc. What occurred was the advance, as a continuous front, of alkaline solutions or fluids that impregnated the gneisses and schists and caused metasomatic replacement of the latter by K-feldspar; moreover, a definite tendency is always observed toward the development of large porphyroblasts of microcline and the formation of rocks quite reminiscent of porphyritic granite. Consequently, metasomatic migmatization and granitization are the principal processes in the formation of the Posolnaya intrusion. True magmatic rocks—granites of cutting dikes and pegmatites—appeared later, when the process of migmatization had ended and the possibility arose for the formation of open fractures in the consolidated mass. An even later formation, connected with an independent episode of tectogenesis but apparently having common roots with the Posolnaya intrusion, is represented by the Nizhnekan granites, which appeared already in the normal granite—middle-depth—facies and are characterized by weak assimilation phenomena in the endocontact, normal contact metamorphism (hornfelsing), and a very weak manifestation of injection phenomena. Thus, apparently in many cases migmatization is caused not by residual pegmatitic melts but, on the contrary, precedes the emplacement of true granites.

Of interest is the relation of these intrusions to the tectonics of the enclosing rocks. For the Tarak granites, a very persistent orientation with a NW strike and steep dip to the E is characteristic of the elements of the prototectonics of the granites, of the orientation of xenoliths, of the inherited gneissosity of hybrid varieties and of arterites. In the Kan complex, by contrast, the folding and the banding of the metamorphic rocks that repeats it—in which, incidentally, crystallization schistosity is weakly expressed—although they have a predominantly NW strike, are much less persistent, and very often gentle folds and even horizontal bedding are observed. Evidently, the emplacement of the magma of the Tarak granites took place along some younger tectonic zone (a zone of lamination), generally coinciding with the principal structures of the Kan and Yenisei metamorphic complexes; moreover, the structures of the intrusion, judging from its acmolithic character, were controlled not by the structure of the enclosing Kan complex but by the structure of this tectonic zone. The same can be said also with respect to the Posolnaya intrusion. Here zones of migmatization are clearly outlined, traceable for hundreds of kilometers. Evidently these are tectonic zones passing from the Lower Proterozoic schist sequences into the Archean basement and oriented partly along the contacts of diverse formations of different ages. The structures of the migmatites show that during migmatization movements within the limits of this tectonic zone did not cease; this is especially sharply noticeable in the migmatites of the Lower Proterozoic schists. A completely different picture is seen in the Nizhnekan intrusion, which forms discordant bodies oriented almost across the principal direction of the structures of the enclosing rocks and formed in a tectonically quiet setting.

The younger—Middle Proterozoic and presumably Paleozoic—intrusions are of very limited distribution in the Southern Yenisei Ridge; they form small bodies of dike and stock form and are sharply hypabyssal formations. The magma of the Angara granites

had a pegmatitic character and was appreciably enriched in boron and fluorine. The Belogorsk intrusion crystallized under nearly surface conditions and is partly represented by quartz porphyries. The youngest intrusions have a sharply expressed alkaline character, which in general is typical of late intrusions of crystalline shields.

Acid intrusions of different ages are of enormous importance in the metamorphism of the Southern Yenisei Ridge. Around the Tarak and Posolnaya intrusions we observe a broad contact aureole, represented by migmatites and by various contact gneisses and amphibolites. These intrusions heated and impregnated the surrounding rocks with their emanations and created in them their own specific physicochemical environment. It is interesting that all rocks that entered the zone of contact influence—the sedimentary rocks of the Lower Proterozoic, the ancient crystalline schists of the hypersthene-gneiss facies, and dikes of ancient diabases, etc.—all underwent transformation into crystalline schists (gneisses and amphibolites) with mineral associations typical of the amphibolite sequence. This metamorphism, evidently, is progressive for the sedimentary sequences, and for the crystalline schists of the Kan complex it is superimposed and, in general, regressive. The Tarak and Posolnaya intrusions are accompanied by almost identical contact gneisses and amphibolites. The differences consist, essentially, only in the fact that cordierite and garnet are common in the Tarak contact gneisses, whereas in the Posolnaya gneisses there is no cordierite at all, and garnet is rare. This difference, apparently, is again caused by the different content of volatile components and by the sharply alkaline character of the migmatizing fluids of the Posolnaya intrusion. But all this injection-contact metamorphism of both intrusions, despite its wide distribution, is only local and is superimposed upon more ancient metamorphisms. A peculiar pneumatolytic metamorphism is associated with the pegmatites of the Posolnaya intrusion, expressed in the transformation of the enclosing rocks into muscovite and two-mica schists with tourmaline, garnet, disthene, and staurolite. The Lower Kan intrusion caused a considerably weaker superimposed metamorphism of the ancient crystalline schists. The most characteristic change in the contact zone is the replacement of garnet, pyroxenes, and amphiboles of the primary rocks by biotite, and their metamorphism is a process of a peculiar hornfelsic recrystallization.

Each of the acid intrusions is associated with its own complex of useful minerals. The Tarak intrusion yields industrial placers of monazite; the possibility is indicated of using the garnets distributed in its exocontact zone. Genetically associated with the Posolnaya intrusion are pegmatites with muscovite and beryl. With the Lower Kan intrusion—ceramic pegmatites. With Middle Proterozoic acid intrusions there may be genetically associated deposits of gold and tungsten and possible tin-stone mineralization.

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1. I myself did not have occasion to see the mutual relations of the granites and diabases, and in this case I refer to Meister (25, p. 84), whose observations are evidently entirely correct, since the diabases here form only bedded bodies bent into folds together with the limestones, whereas the granites produce cross-cutting dikes. ↩