MAIN FEATURES OF THE GEOLOGICAL STRUCTURE OF THE MAINA PLAGIOGRANITE INTRUSION

(WESTERN SAYAN)

V. N. Smyshlyaev

(Presented by Professor, Doctor Yu. A. Kuznetsov)

In the course of two years of thematic work in the northern part of the Western Sayan, the author had the opportunity to become fairly thoroughly acquainted with the geological structure of one of the largest intrusions in this region, known in the literature as the “Maina” intrusion (I. K. Bazhenov, 1923, 1924, 1925; N. A. Batov, 1931; A. G. Sivov, 1948, etc.). At present, as a result of geological mapping, more than two dozen bodies of this intrusion of varying size have been identified, and their number, apparently, will increase still further, since the plagiogranite plutons of the southern margin of the Western Sayan (A. P. Lebedev and I. V. Ginzburg, 1953), as it seems to us, are very close analogues of the Maina plagiogranites.

In the present article we shall attempt to characterize the Maina intrusion using the example of the Yenisei pluton, in which all those characteristic features inherent in this intrusion as a whole are especially clearly manifested.

Geological Structure of the Area of the Maina Intrusion

The geological structure of the area includes rocks of the most diverse origin, mainly of Cambrian age.

Effusive-sedimentary rocks of the Lower Monok Formation are most widely developed; they border the Yenisei pluton from the south (Fig. 1). Along the northern boundary of this intrusive body, sedimentary deposits and effusives of the Devonian of the Minusinsk Basin appear here and there in separate areas; they are almost everywhere overlain by Quaternary accumulations. Only in the very southern part of the area (south of the area of development of the rocks of the Lower Monok Formation), metamorphic rocks of the Lower Cambrian, assigned to the Chingin Formation, extend from west to east for many tens of kilometers.

In studying the geological structure of the area, two circumstances attract attention, namely:

1) the linear elongation of the metamorphic and effusive-sedimentary formations, as well as of the Yenisei pluton itself, which form a structure, persistent along strike, of sublatitudinal direction, bordering the Western Sayan from the north;

2) the joint spatial development of rocks of the intrusion and of the spilite–keratophyre series of the Middle Cambrian; moreover, plutons of the Maina intrusion are almost never encountered outside the fields of development of the latter.

The oldest of the geological formations described in the region is the Chinga Formation, whose age, according to A. G. Sivov (1953), is determined as \(Cm^2_1\).

Almost everywhere, the rocks composing it have a constant east–northeast strike, with dips to the south and southeast at angles of 70–85°. Intense cleavage, coinciding in strike with the bedding, has the same direction of dip, although the dip angles are somewhat gentler—50–55°.

The petrographic composition of this formation is rather varied and is characterized by interbedding of members of green and greenish-gray, sometimes graphitized, metamorphic schists. In the upper part of the formation there appear banded gray limestones, dolomites, and, more rarely, gray microquartzites. Interbeds of finely cleaved argillites and siltstones are observed quite often.

The faunistically characterized rocks of the Lower Monok Formation, assigned to \(Cm^3_1—Cm^1_2\) (A. G. Sivov, 1948, 1953), are represented by effusives occurring in a diagenetized or greenstone facies state, which are accompanied by tuffaceous material interbedded with thin horizons of clastic and organogenic rocks.

The most typical representatives of the extrusive rocks of this formation are quartz and quartz-free albitophyres and keratophyres, as well as spilites. Diabase and quartz porphyrites are encountered somewhat more rarely.

The metamorphic rocks of the Chinga Formation at the contact with the Lower Monok effusives bear traces of clearly expressed contact metamorphism, manifested in silicification and in the formation of thin zones of hornfels with abundant development of epidote–actinolite–carbonate veinlets. Xenoliths of metamorphic schists of the Lower Cambrian are occasionally encountered in lava flows and sheets.

In the northern part of the region, along the line of the Sayan thrust fault (I. K. Bazhenov, 1924; A. G. Sivov and A. A. Lyubertsev, 1941), at the boundary with the Yenisei pluton, red-colored deposits of the Lower Devonian occur in separate areas; these are represented by gently dipping bedded argillites, siltstones, and conglomeratic sandstones, at times interbedded with gray organogenic limestones (middle course of the M. Shush River). Only at a few points are the sedimentary rocks of the Lower Devonian interbedded with tuff lavas and amygdaloidal effusives.

Lower and Middle Paleozoic rocks are in many places overlain by Quaternary accumulations, which sometimes considerably hamper geological observations.

Yenisei Pluton

The Yenisei pluton, representing in plan a linearly elongated body, is situated in the zone of junction of the Western Sayan with the Minusinsk basin and can be traced continuously for almost one hundred kilometers from the Sr. Shush River in the east to the Sos River in the west. However, despite its considerable length, the width of the massif nowhere exceeds 10–11 km, narrowing in places to 2–3 km. Such sharp fluctuations in width are due to the fact that the intrusion has not yet been completely exposed by denudation processes and bears numerous remnants

Fig. 1. Structural-geological map of the Yenisei pluton of the Maina intrusion. (Author: V. N. Smyshlyaev). Scale: 0–2–4–6 km. Orientation: N–S.

1. Alluvial deposits.
2. Sandstones and siltstones.
3. Acid effusives.
4. Gabbro-norites.
5. Granodiorites, plagiogranites, plagiogranite-porphyries.
6. Biotite plagiogranites.
7. Albitites and alkaline granites.
8. Dikes of microgranites and pegmatites.
9. Effusives of the spilite-keratophyre series.
10. Metamorphic sedimentary rocks.
11. Boundary of alluvial deposits.
12. Normal stratigraphic contact.
13. Near-contact zones of hornfelsing.
14. Line of the North Sayan reverse fault.
15. Disjunctive faults.
16. Inclined flow lines.
17. Horizontal flow lines.
18. Bedding attitudes in sedimentary rocks.
19. Planar-parallel textures.
20. Schistosity in metamorphic schists.
21. Attitudes of contacts.

effusive roof. The area of some of them amounts to several square kilometers.

In addition to the main body of the Maina intrusion—the Yenisei pluton—the Lower Monok Formation in other parts of the Western Sayan is cut by smaller bodies which, in petrographic composition and other petrological features, represent analogous formations and are apparently apophyses of the main intrusive massif, but which, in the process of emplacement, received incomplete or one-sided development.

The rocks composing the Maina intrusion are most fully represented in the Yenisei pluton and are characterized by the presence of both basic varieties (gabbro, gabbro-norites, hornblende gabbros, hornblendites) and acidic varieties (granodiorites, plagiogranites, plagiogranite-porphyries, etc.). Varieties of dioritic or similar composition are apparently not characteristic in general. At least within the field of development of the rocks of the Maina intrusion, we nowhere encountered them as independent bodies; and where they do occur, their hybrid origin is beyond doubt.

Observations during geological mapping showed that the pluton has a complex structure. Study of the relationships among the rocks, of the peculiarities of their composition, and of their structural features makes it possible to state with a certain confidence that the emplacement of the intrusion as a whole took place in at least 5 stages.

1) Gabbro-norites, gabbros, hornblende gabbros, hornblendites.
2) Granodiorites, plagiogranites, plagiogranite-porphyries.
3) Biotite gray plagiogranites.
4) Red alkaline granites and albitites.
5) Microgranites and pegmatites.

However, despite the considerable diversity of the constituent rocks, the principal role belongs to the varieties of the second stage, i.e., to granodiorites and plagiogranites. Gabbroids are clearly subordinate in significance and make up about 10% of the entire area of the massif.

Biotite gray plagiogranites were found in the form of a single small, oval, stock-like body cutting the plagiogranites.

As for the medium-grained red granites, their place in the process of formation of the intrusion is still not entirely clear, since poor exposure does not permit an indisputable conclusion as to whether they are cross-cutting in relation to the plagiogranites or are connected with them by gradual transitions.

Microgranites and pegmatites form comparatively few dikes and small stocks cutting the rocks of all the preceding stages.

Below is a brief characterization of the rocks mentioned above.

Rocks of Stage 1

The basic rocks, although very characteristic of the Maina intrusion as a whole, play a modest role in the Yenisei pluton. Initially they composed a dike-like body extending for tens of kilometers, and possibly also a chain of separate dikes controlled by a latitudinal structure. However, the thickness of such a “dike” was unequal in different places, reaching 1.5–2 km in the interfluve of the Uta and Beya rivers and narrowing in other, especially eastern, sectors to 100–150 m. Naturally, tectonic movements that preceded the intrusion of the acidic magma completely shattered the sections of the dike of small thickness, while the intruding acidic magma dragged apart these fragments, as a result of which, over a distance of many kilometers

the “dike” ceased to exist as a single geological body. At the divide between the Beya and Uty rivers, as well as in a number of other places, the gabbroids underwent fragmentation only in the marginal parts, while on the whole retaining a massive character.

In mineralogical composition, among the group of basic rocks one may distinguish: gabbro-norites, gabbro, hornblendites, and hornblende gabbro. The varieties listed are connected with one another by gradual transitions.

Normal and hornblende gabbro and gabbro-norites are the most widespread. Macroscopically these are black-green or almost black coarse-grained rocks, consisting of prismatic-tabular crystals of rhombic and monoclinic pyroxenes, hornblende, and tabular grains of plagioclase. Light-colored and colored components are present in approximately equal quantities, although in some cases the mafic minerals may predominate over plagioclase.

The pyroxenes are represented by idiomorphic grains of almost pure diopside in the gabbro and, respectively, bronzite-hypersthene and diopside in the gabbro-norites. Among the amphiboles there is both green magmatic hornblende and bluish-green actinolite, replacing the pyroxenes and primary amphibole. Plagioclase is represented by fresh polysynthetic-twinned grains of bytownite No. 82–85.

Hornblendites occur in the form of schlieren, nests, and bulges. In most cases they have a coarse-grained pegmatitic structure and consist almost entirely of prismatic crystals of hornblende and 8–10% basic plagioclase. Sometimes a small amount of quartz may be present as an admixture.

Thus, for the rocks of the first stage, very characteristic features are their melanocratic appearance, coarse-grained structure, and composition that is everywhere consistent.

Rocks of the 2nd stage

Among the rocks of the second stage we distinguish granodiorites, plagiogranites, and plagiogranite-porphyries. The latter constitute a marginal facies and are connected with the other varieties by gradual transitions. Of course, this does not exhaust all the diversity of rocks formed from the second portion of magma, since adamellites are also present; but in the structure of the massif they play such an insignificant role that, without particular harm to the understanding of the geological structure of the massif, they may for the time being be left unmentioned altogether.

Without dwelling separately on the characterization of each of the above-mentioned varieties, we shall note only the general features specific to this group of rocks.

1. An abundance of idiomorphic bipyramidal or hypidiomorphic gray quartz, the content of which at times reaches 50–55%. Regardless of the part of the intrusive from which the sample is taken, the amount of quartz always remains consistently high.

2. A low content of potassium feldspars, which often may be entirely absent. At best their content does not rise above 20–25%. Among the plagioclases, albite and oligoclase are predominant, whereas andesine occurs relatively rarely.

3. A low content of mafic minerals, represented by hornblende and, much more rarely, biotite, gives the rocks a leucocratic appearance and a light-colored tone.

1. An almost ubiquitous occurrence of micropegmatitic and granophyric structures, expressed in the regular intergrowth of quartz with plagioclase. For the most part such granophyric intergrowths appear as radially fibrous or spherulitic formations, at the center of which there is a xenomorphic grain of quartz or plagioclase.

2. Finally, it is necessary to note the presence of porphyroid structures, manifested in the fact that phenocrysts of quartz are much larger than the grains of the surrounding mineral aggregate. In the apical part of the massif, the porphyroid structures are replaced by porphyritic ones, and rocks of an effusive aspect appear, represented by plagiogranite porphyries.

Rocks of Stage 3

Biotite plagiogranites have been encountered on the right bank of the Yenisei River, in the basin of the Shuner River, where they form a stock measuring \(1 \times 1.5\) km. In the endocontact zone there are xenoliths of hornblende plagiogranites and granodiorites, many of which have been assimilated to a considerable degree and transformed into schlieren.

The granites composing the stock are gray, grayish, or yellowish-white medium-grained massive rocks consisting of yellowish quartz, white grains of plagioclase, and rare plates of potassium feldspar. The mafic minerals are represented by small scales and flakes of black biotite. Porphyroid and granophyric structures are entirely absent.

Rocks of Stage 4

On the watershed between the right and left tributaries of the Shuner River, outcrops of alkaline granites and albitites are observed. They have a somewhat larger area of distribution than the biotite plagiogranites, and in plan the body composed of them has the form of an elongated ellipse with a section of \(1.5 \times 3.5\) km.

Red hues are very characteristic of these rocks; they are due to the presence of significant amounts of alkaline feldspar. Against this background, grains of gray xenomorphic quartz are clearly visible. Under the microscope, micropegmatitic intergrowths of albite and, more rarely, microcline with quartz are clearly seen.

In the near-contact part, the granites change from medium-grained to fine- and very fine-grained, and in the enclosing rocks one can often find thin pink veinlets consisting of alkaline feldspar and quartz.

Dike Rocks of Stage 5

The dike series of microgranites and pegmatites is the final link in the formation of the intrusion. It may be said that, on the whole, the Yenisei pluton is relatively poor in vein formations; however, certain areas, owing to favorable tectonic conditions, are literally saturated with dikes. In particular, such saturation is observed on the right bank of the Yenisei River, where microgranites and pegmatites form series of subparallel cutting bodies, ranging in thickness from several centimeters to \(1\)—\(2.5\) meters. Their extent is usually 50—100 m, although in some cases dikes up to 250—500 m long are observed.

The strike direction and dip of the dikes vary within wide limits, but bodies with northwestern and north-

with an easterly orientation and steep dips to the northeast and northwest.

True, in the eastern part of the massif, near the Shuner River, series of dikes are observed with gentle northwestern dip angles (20–32°).

If the eastern part of the massif is relatively rich in rocks of the final stage, the same cannot be said of its western part, where they occur rarely.

Among the vein formations, chiefly on the basis of structural features, microgranites and pegmatites may be distinguished. In some cases, however, it is very difficult to draw a sharp boundary between them, since gradual transitions from one variety to another are often observed.

The contacts of the dikes are observed both as sharp and as poorly expressed. In the first case, the composition of the rock is more or less constant from selvage to selvage; in the second case, a zonal structure is observed. Plagiogranites pass quite gradually into microgranites, then acquire a graphic structure, and in the central parts the dike passes into pegmatites with a pegmatoid structure.

In addition to the principal minerals of the dike rocks—microcline, albite, and quartz—a number of accessory minerals also take part in their structure, such as biotite, muscovite, garnet, and, very rarely, tourmaline.

Autometamorphic phenomena in the rocks of the pluton

Alterations in the rocks of the Yenisei pluton associated with postmagmatic activity are manifested quite distinctly. Thus, the basic rocks underwent amphibolization and more weakly expressed talcification. The plagiogranites, for their part, experienced strong sericitization and epidotization. Finally, all the rocks of the pluton underwent considerable albitization.

Talcification was manifested to a slight degree only in gabbro and gabbro-norites, where colorless, fine-scaly aggregates of talc develop very readily after pyroxenes.

Amphibolization, like talcification, is characteristic of the rocks of the basic series and was the result of the postmagmatic activity of the gabbroids themselves. If amphibolization were connected with the plagiogranites, it would naturally be expected that it would also be manifested in them themselves, and that new formation of actinolite would occur, or replacement by it of primary hornblende and biotite. However, nothing of the kind occurs in the plagiogranites. Therefore we believe that the processes leading to the formation of secondary amphibole (actinolite) are specific to the rocks of the initial stage and, under favorable conditions, lead to the formation of amphibolized gabbro. Many facts could be adduced in support of what has been said, but we shall note only a few.

Study of the crushed effusive roof in those places where gabbroids are in direct contact with it showed that apophyses of microgabbro branch off from the body of the gabbroids along fine fractures; these then pass into thin actinolite veinlets developing in the effusives, and often also among the gabbros themselves.

Microscopic study gives an idea that amphibolization proceeds in two directions: by way of the new formation of actinolite and by way of replacement by it of pyroxenes and green hornblende.

Newly formed actinolite, pleochroic in bluish-green tones, which apparently may serve as an indication of the appearance of sodium in its composition, develops along the boundaries of plagioclase grains, and also—

along cleavage and cracks in it, forming an intricately branched network.

Such an actinolite network usually develops in plagioclases as a single crystal, since all its segments have exactly the same optical orientation.

Sericitization is observed only in the acidic rocks, and it affects mainly plagioclases and, to a minor degree, potassium feldspar.

Chloritization apparently proceeded almost simultaneously with sericitization. As a result, many of the amphibole and biotite grains are partly or completely replaced by a scaly aggregate or by individual flakes of prochlorite and ferruginous prochlorite.

The processes of albitization apparently affected all the rocks of the pluton, but they were not manifested everywhere in the same way. Thus, in the gabbroic rocks with intense microfracturing, rims of fresh oligoclase, devoid of any products of destruction, are observed around plagioclase grains.

In plagiogranites this process is expressed more distinctly. Porphyritic crystals of plagioclase almost always have an albite rim in which twinning may be absent. The finer plagioclase grains are often completely albitized and contain 3–10% of the anorthite molecule. Apparently, the formation of chessboard albite should also be assigned to the same kind of postmagmatic alteration.

In pegmatites, albitization was manifested extremely sharply and led to the complete replacement of microcline by albite in the central parts of the dikes. Migration of potassium into the rocks enclosing the pegmatites led to the development in them of microcline porphyroblasts and to the biotitization of hornblende. As a result, at the boundary with pegmatites, plagiogranites are transformed into granodiorites and adamellites with a micropegmatitic structure, which in this case arose metasomatically. With increasing distance from the pegmatite body, the micropegmatitic structures gradually become blurred and then disappear completely. The rocks acquire a normal granitic structure.

Epidotization affected mainly the acidic rocks of the pluton, but the causes that led to its occurrence are of two kinds. It is necessary to distinguish the formation of epidote as a result of the de-anorthitization of plagioclases during their replacement by albite, and epidotization manifested as an independent postmagmatic process. Under especially favorable conditions, the epidote aggregate completely replaces all rock-forming minerals except quartz, and quartz epidotites arise.

Assimilation and Contamination

The formation of the pluton under conditions of shallow depth left a distinctive imprint both on the nature of the interaction of the magma with the enclosing rocks and on the interaction of the plagiogranites with earlier gabbroids. In particular, it should be noted that hybrid rocks are almost completely absent in the inner contact zones of the pluton. The effusive roof, both in the apical part of the massif and along its southern steep contact, is intensely fragmented with the formation of breccias, the fragments of which are cemented by plagiogranite magma. Fragments of effusives, representing xenoliths, form, in the endocontact zone, a belt 100–150 m wide, where they occur most frequently. Their sizes and shapes are highly varied; the most common are fragments

with angular outlines, ranging in size from 2 to 30 cm. In the immediate vicinity of the contact, the outlines of the xenoliths are very sharp, and macroscopically the rocks show no changes, except that in the marginal parts there appear rims of hornfelsing several millimeters thick.

As the distance from the contact increases, the number of xenoliths decreases, but they are more strongly altered, and at their boundary the plagiogranites are somewhat enriched in hornblende. The captured fragments have lightened zones in their peripheral parts, which are enriched in quartz and plagioclase. In all probability, exchange reactions in this case took place in the presence of certain amounts of interstitial liquid, as is indicated by the idiomorphism of the newly formed plagioclases and amphibole.

The weak effect of the acidic magma on the enclosing rocks and their xenoliths is explained by the fact that, under the conditions of a strongly fractured roof, there occurred a sharp drop in pressure and temperature. As a result, rapid crystallization of the magma took place, with the formation of rocks of porphyritic or porphyry aspect (plagiogranite-porphyries). Naturally, the volatiles that might have promoted the reworking of foreign material also separated rapidly (at least in the apical part of the massif), owing to the easy permeability of the rocks of the roof. As long as the thermal regime of the heated rocks favored exchange reactions, their metasomatic reworking occurred, with the filling of fractures by products of metasomatism.

An entirely different picture is observed in the interaction of acidic magma with rocks of gabbroic composition.

It has already been noted above that the basic rocks occupy the central part of the massif and, owing to the insignificant thickness of the linear body formed by them, in many places were crushed with the formation of an enormous number of xenoliths. In places where the massif has a deeper denudation section, the gabbroids are less crushed, become massive, and, correspondingly, the number of their xenoliths decreases. In these deeper parts of the pluton, in the marginal portions of the gabbroid body, the processes of assimilation manifested themselves most sharply, leading to pauloposthumous or cognate hybridism (A. N. Zavaritskii, 1937). As a result, hybrid rocks of gabbro-diorite, diorite, and tonalite composition arise. Analogous cases of the effect of acidic magma on basic rocks of the same intrusion have been described for the Caucasus by M. A. Favorskaia (1947), V. D. Morkovkina (1950), and others. The rocks mentioned above are characterized by a number of features typical of hybrid formations. Let us note the most important:

1) the presence of taxitic textures (schlieren, spotted, banded, thread-like, etc.);

2) the presence of numerous xenoliths of gabbroids, having contacts that are sometimes sharp and sometimes diffuse;

3) the melanocratic aspect of the rocks, which is due to the presence of anomalous amounts of actinolitic and ordinary hornblende (55–60%);

4) the simultaneous presence of quartz and basic plagioclase (bytownite No. 75–78), with reverse zoning observed in the plagioclases in areas composed of more acidic material;

5) the occurrence of relict pyroxene (both monoclinic and rhombic) in quartz diorites and tonalites;

6) the presence in the hybrid rocks of small amounts of fibrous jadeite.

The formation of hybrid rocks represents an entirely natural event in the development of the intrusion, resulting from the interaction (in this case) of a solid phase of basic composition with an acid magma. The processes that took place were directed toward the creation of a more equilibrated and, consequently, more stable system. The fragments of gabbroids, being at a considerable distance from the roof, were intensely heated, and the presence of significant amounts of volatiles (as indicated by the high amphibole content) promoted exchange reactions, creating a more mobile medium. Under these conditions of high temperatures, the presence of mineralizers, and relatively high hydrostatic and lateral pressures, the xenoliths acquired a state close to plastic. This is quite clearly attested by their flattened, planar forms, with the development in gradually tapering terminal parts of filamentous textures, which represent dissolved portions of xenoliths and stand out distinctly by their melanocratic appearance.

Thus, the facts cited above indicate that gabbro-diorites, diorites, and tonalites are the product of the interaction of granitic magma with basic rocks and possess all the features testifying to their hybrid nature.

Contact Metamorphism

Contact metamorphism was manifested in partial recrystallization and subsequent metasomatic hornfelsing of the enclosing rocks. The width of the hornfelsing zone is not everywhere the same and varies from 1–5 m to 15–20 m at the boundary with the metamorphic schists of the Lower Cambrian, and from 10 to 100–150 mm at the contact with the effusives of the Lower Monok Formation. In the latter case, the greater thickness of the hornfelsing zone is observed in the rocks of the roof. In the area of the steep contact of the intrusion, the width of this zone decreases sharply.

In the metamorphic schists, in the immediate vicinity of the pluton, dark-green massive epidote–quartz–feldspar hornfelses arise; these are cut by actinolite–epidote veinlets and by numerous apophyses of leucocratic plagiogranite.

The effusives of the roof, under the thermal influence of the intrusion, recrystallized their groundmass and acquired a dark-gray and gray color and a denser structure. Subsequently they underwent metasomatism, with the formation of a microgranular quartz–albite–actinolite and quartz–epidote aggregate.

Facies Character of the Intrusion and Its Age

In the published works of I. K. Bazhenov (1924), M. A. Usov (1932), and others, there is an indication that the Maina intrusion formed under conditions of shallow depths. The factual material at our disposal is wholly consistent with the stated proposition.

Weak contact metamorphism, the almost complete absence of assimilation phenomena in the endocontact zone, the presence of porphyritic, porphyry, and granophyric structures, the character of postmagmatic activity (sericitization, epidotization, albitization, etc.), and the weak development of dike rocks—all this indicates that the formation of the Maina intrusion in general, and of the Yenisei pluton in particular, took place in a sharply hypabyssal setting. In the shape of the body and in a number of individual features, the Yenisei pluton apparently may be assigned to the stock subfacies (Yu. A. Kuznetsov, 1955).

The absolute depth of emplacement of the pluton can probably be determined as 1–1.5 km, if one takes into account that the thickness of the effusive-sedimentary cover at the time of formation of the pluton was 1.5–2.5 km.

On the basis of the fact that the intrusion cuts through the faunistically characterized deposits of the Lower Monok Formation \((\mathrm{Cm}_{1}^{3}—\mathrm{Cm}_{2}^{1})\), its lower age boundary is determined as the lower part of the Middle Cambrian. The upper age boundary is dated to the middle of the Middle Cambrian, since boulders and pebbles of the intrusion rocks occur in conglomerates of the Upper Monok Formation \((\mathrm{Cm}_{2}^{1}—\mathrm{Cm}_{2}^{2})\).

Structural Analysis of the Pluton

The Yenisei pluton, as already stated above, is composed of rocks of varied composition, ranging from basic rocks to acid derivatives of the residual magma (pegmatites). Rock complexes that arose in a definite sequence at different stages occupy a quite definite place in the general structure of the massif. Owing to a number of circumstances, the formation of rocks of different stages took place in different settings, as a result of which the primary structures in them were manifested very unevenly. They are observed most distinctly in rocks of the second and first stages; the granites of the two following stages, however, are macroscopically structureless.

The elements of prototectonics are represented both by flow textures and by primary fractures.

Flow Textures

Among the textures of the first group one may distinguish:

1) plane-parallel textures;
2) oriented schlieren and xenoliths;
3) linear textures, or flow lines.

1) Plane-Parallel Textures

These textures are most characteristic of rocks of the gabbro series and much less common in biotite granites.

Among the basic rocks there are areas of considerable extent in which crystals of feldspars and mafic minerals are arranged subparallel, without, however, forming distinct bands. The lighter phenocrysts of plagioclase stand out sharply against the general dark background, with the formation of a trachytoid texture. In planes parallel to the trachytoid texture, no regular orientation of minerals is observed.

The plane-parallel textures are oriented in an east-northeast direction, with dips in the northern part to the southwest at an angle of 74–80°. Toward the southern contact the dip becomes somewhat gentler (58–60°).

In the northern part of the intrusive body, near the village of Sabinka, pink granites are exposed in a quarry, in which numerous large, nearly isometric biotite plates are arranged in the plane of northwestern strike, dipping to the southwest at an angle of 60°. This planar spatial orientation of the biotite, which stands out well against the general light background, creates the impression of a “spotted” texture.

2) Oriented Xenoliths and Schlieren

The overwhelming majority of xenoliths of the enclosing rocks and gabbroids are observed in the plagiogranites of the second stage. The intruding

magma dragged them along with it, as a result of which, in the near-contact zones and at the boundary with the gabbroids, they acquired a subparallel orientation. It is characteristic that the xenoliths and schlieren are not spatially isolated bodies, but form zones of varying width, extending for several kilometers. In essence, such a subparallel orientation is only a variety of plane-parallel texture, with the sole difference that, instead of crystals, the leading role in its formation belongs to flat xenoliths and segregations of minerals.

The predominant direction in the orientation of the xenoliths is northeastern, sometimes latitudinal, and somewhat more rarely northern. In the northern part of the intrusion, the dip, with rare exceptions, is southwestern and southern, with an inclination of 70–80°. In the central parts the dip becomes almost vertical. At the southern contact, the roof xenoliths, having a latitudinal strike, are inclined southward at angles of 62–56°, while in the apical part of the pluton their orientation becomes chaotic.

3) Flow lines

Linear textures are observed especially distinctly owing to the elongation of prismatic crystals of hornblende and phenocrysts of quartz. The crystals of the latter, perhaps, emphasize the linear direction most sharply, since they are considerably larger than the rest of the mineral aggregate and have a spindle-shaped or ellipsoidal form. This is in the acidic rocks. In the gabbroids, however, the flow lines are best recorded by the orientation of the plagioclases.

In relation to plane-parallel textures, the flow lines are in some cases arranged almost perpendicular to them, or they may coincide with them, which, ultimately, depends both on the character of the screening surface and on the direction of movement of the intruding masses.

Primary fractures

The study of primary fractures is associated with difficulties, which consist in the fact that the exposure of the pluton does not always allow it to be carried out. The disjunctive disturbances that arose at a later time have considerably obscured and complicated the already complex fracture system. Therefore, one can speak with greater confidence only of two fracture systems that may without special doubt be assigned to primary ones. These are: 1) transverse and 2) longitudinal fractures.

1) Transverse fractures

As the name itself indicates, the fractures are transverse to the flow lines and almost always form with them (not only in strike, but also in dip) an angle close to a right angle.

Of exceptional interest in this respect is the area in the middle course of the Shuner River, on the right bank of the Yenisei River, where the plagiogranites are cut by a system of fractures of almost latitudinal strike with a gentle dip (15–32°) to the north-northwest. Most of these fractures were filled with microgranites and pegmatites, which form a series of dikes ranging in thickness from several centimeters to 2.5 m. The extent of such fractures, taking into account the small transverse dimensions of the massif, reaches the impressive value of 1000–1200 m, although the length of the dikes usually does not exceed 200–300 m. In rare cases, dikes up to 1–1.2 km long were observed.

2) Longitudinal fractures

This type of fracturing is less widespread, and the primary fractures have a much smaller extent (50–100 m). However, in this case as well, some of the fractures provided a convenient site for the localization of dikes of acidic composition, or of quartz and quartz–epidote veins of varying thickness. The edges of the fractures are uneven, sometimes sinuous. The dip is northwestern and northeastern, at angles of 70–76°.

Structural relations of the pluton with the enclosing rocks

In its southeastern part, the Yenisei pluton is in direct contact with metamorphic schists of the Lower Cambrian; the schistosity in them, having predominantly a northwestern strike, forms an acute angle with the contact line, in places lying parallel to it.

In all other sectors the massif borders on Middle Cambrian effusive rocks. Study of the orientation of xenoliths has shown that the pluton plunges beneath the effusive rocks, and that its southern contact has a steep dip at an angle of 60–64°. The steep dip of the contact is also indicated by the insignificant thickness of the zone of hornfelsed rocks. However, as the contact approaches the present-day surface, it becomes gentle, and in certain places within the pluton the effusive roof is preserved in the form of numerous remnants, ranging in cross-section up to several hundred meters and even several kilometers.

Relations among the rocks composing the pluton

Within the variegated complex of rocks participating in the structure of the pluton, each of the groups that unites varieties similar in composition is characterized by both definite structural features and spatial distribution.

Thus, the basic rocks form an elongate body trending latitudinally, representing the central core, which has its own structure, distinct from that of the plagiogranites (a different orientation of flow lines, trachytoid character, etc.). In turn, the plagiogranite magma, in flowing around this body, acquired autonomous structural features in this part. This was manifested in the fact that, from sector to sector, the platy-parallel textures and oriented xenoliths repeat the sinuosity of the contact with the gabbroids. Correspondingly, the flow lines also have a steep plunge consistent with the inclination of this contact. All this, quite apart from the cross-cutting relationships, undoubtedly testifies to a difference in age between the formation of the rocks of the basic and acidic series.

If the relations of the plagiogranites with the basic rocks and with the biotite granites are clear (the latter cut the rocks of the second stage and contain rare xenoliths of them, as was already mentioned earlier), then the place of the red hornblende granites in the general process of intrusion was to some extent uncertain, and they were provisionally distinguished as having formed in the fourth stage. Study of the flow lines in the plagiogranites showed that their orientation remains constant on all sides of the stock composed of the aforementioned granites, and that no deviations in direction are observed as it is approached. At the same time, the granites themselves appear to be structureless. This apparently indicates that the red granites formed somewhat later than the plagiogranites and granodiorites.

However, the question of the relative age of the biotite plagiogranites and the normal granites still remains open, since...

they form spatially isolated bodies, and no relationships between them have been observed anywhere.

The position of the dike rocks is quite definite, and their youngest age apparently leaves no doubt, since they cut the rocks of all the first four phases without exception.

Thus, taking into account the compositional and structural features of the rocks, their mutual relationships, and their spatial distribution, there is no doubt that the Yenisei massif belongs to multiphase complex plutons.

Mechanism of intrusion, form, and type of pluton

It has already been emphasized above that a characteristic feature of the geological structure of the area is the close spatial and age relationship of the Maina intrusive complex with Middle Cambrian effusives, a large part of which are typical representatives of the spilite–keratophyre series. Such an association is not accidental and represents a completely regular phenomenon connected with the prolonged development of the North Sayan deep fault zone (V. A. Kuznetsov, 1954), which apparently originated somewhere at the beginning of the Cambrian or at the end of the Upper Proterozoic, i.e., at a time corresponding to the early stages in the development of the Sayan geosyncline (Yu. A. Bilibin, 1948; V. I. Serpukhov, 1955).

The formation of the effusive–intrusive magmatic complex began, properly speaking, with fissure eruptions of acidic and basic effusives, which formed a belt of latitudinal extent, fixing the trend of the tectonic suture. Measurements of fluidality in the effusives make it possible to say that the main direction of movement of the erupted masses was northward, at times with deviations to the northeast and northwest, which is apparently connected with the character of the surface of the ancient relief.

Tectonic movements, which repeatedly renewed this mobile zone, created conditions for the penetration into the upper horizons of the lithosphere of basic magma, which formed a dike-like body of gabbroids. Measurements of the primary structures show that the magma rose almost vertically upward (the plunge of the flow lines is westward at angles of 70–76°). The formation of the cavity occurred as a result of the opening of the fissure from west to east and was almost simultaneous with its filling by magma, since the flow lines in the overwhelming majority have a latitudinal easterly strike, that is, they show the direction of greatest extension of the magmatic masses (N. A. Eliseev, 1953). Another proof of this, it seems to us, may be the greater transverse dimensions of the chamber swellings in the western part, which played the role of peculiar reservoirs of magma, with gradual narrowing to the east. In the swellings of the gabbroid body, the flow lines turn to the northeast and north, thereby showing that in these sectors there was also meridional spreading of the chamber walls.

Between the formation of the plagiogranites and the gabbroids there was a considerable interval, and the latter had time to cool completely, having first undergone autometamorphic changes (amphibolization, talcization). Apparently, this time was quite sufficient for processes of differentiation in the deep magmatic source to lead to the emergence of a large quantity of acidic magma of essentially sodium specialization.

The occurrence of strong vertical movements of latitudinal trend led not only to the fragmentation of the gabbroids, but to a considerable

to some extent also affected the effusive-sedimentary formations of the Lower Monok suite. Acid magma intruded into this weakened zone, giving rise to the main mass of rocks composing the massif (granodiorites, adamellites, plagiogranites, etc.). Subsequently, under quieter conditions, local emplacement of biotite plagiogranites and granites, as well as dike rocks, took place. The latter arose already from residual magma enriched in potassium, since the bulk of the sodium had entered the albite molecule of the plagioclases of the earlier-formed rocks.

Observations of primary structures in the plagiogranites showed that the principal direction of movement of the intruding magma was north-northeast, with the flow lines dipping in the southern part of the massif to the south and southwest at angles of 60–68°. In the northern part of the pluton the flow lines retain the same direction, but become considerably steeper (70–74°), and only in the apical part do they become gentle (20–34°).

Planar-parallel structures emphasize the steep character of the southern contact and repeat the outlines of the gabbroid body, forming local swirls in some areas. In the northern part of the pluton, planar structures are observed in it in the immediate vicinity of the boundary of the massif with the Devonian rocks and have a northeastern strike with a dip to the southwest at angles of 60–80°. Thus, an impression is created of an incomplete asymmetric structure of the intrusive. In our view, this is explained by the following circumstances.

It is well known that the Minusinsk depression is joined to the Western Sayan along the line of the North Sayan fault (V. A. Kuznetsov, 1954) or the Sayan thrust-fault (A. G. Sivov and A. A. Lyubertsév, 1941, and others), which has a linear, regionally expressed character. In the zone of this fault, Devonian accumulations of the Minusinsk depression adjoin the crushed rocks of the Yenisei pluton, “cutting them off”; these deposits have an almost horizontal attitude and bear no traces of the influence of the intrusion. Thus, it is evident that the northern boundary of the pluton is tectonic. It is natural to suppose that, during the release of stresses that led to the formation of the North Sayan thrust, the Yenisei pluton was split into two unequal parts, one of which, together with the Lower Paleozoic formations, at present constitutes part of the basement of the Minusinsk depression. Therefore it is now difficult to judge what the true width of the Yenisei massif was. In any case, apparently, it would not be a major error to consider that the pluton was originally twice as wide as the part of it observed in the present denudation section.

From all that has been said above it follows that the Yenisei intrusive is a complex, multiphase, vertical pluton of fissure type, syntectonic with the formation of faults in the Cambrian rocks. The rocks of the pluton belong to the granite facies (N. A. Eliseev, 1953), with sufficiently clearly expressed primary textures. The internal tectonics of the massif is semi-autonomous, since one part of the planar-parallel textures repeats the direction and inclination of the contact of the intrusive, while the other part of these same textures emphasizes the features of the internal structure and the multiphase character of the pluton.

Conclusion

Study of the Maina intrusion, using the Yenisei pluton as an example, makes it possible to identify the most characteristic features of its structure and formation. In many respects these features are due to the close connection of Cambrian magmatism with the tectonic development of the northern part of the Wes-

of the Western Sayan and, in particular, with the development of the North Sayan deep fault zone.

It has already been stated above that the initial stages of the magmatic cycle were manifested in the formation of thick effusions, part of which took place under submarine conditions with the formation of spilites, diabases, and albitophyres. The further development of this mobile zone was accompanied by successive intrusions, which led to the formation of gabbroids, granodiorites, normal granites, and rocks of the dike series.

The distinctive features of the material composition of the pluton were expressed in the predominance of gabbroids and plagiogranites over the rocks of all subsequent stages and are characterized by a sharp enrichment of these rocks in sodium. The presence of acid differentiates of magma with potassium specialization is not characteristic of the Maina intrusion, and its rocks are typical representatives of the plagiogranite formation in the sense of Yu. A. Kuznetsov.

Analogous gabbro-plagiogranite complexes, differing in age but similar in geological setting, are known in many places in the Soviet Union and abroad. For the Northwestern Caucasus, the Urushten complex of pre-Devonian age has been described in detail (G. D. Afanas’ev, 1950), and a gabbro-plagiogranite formation of Upper Devonian age is distinguished (A. A. Kadenskii, 1956). For Rudny Altai there are a number of works (N. A. Eliseev, 1938; B. Ya. Khoreva, 1954; P. F. Ivankin and others), in which a detailed petrographic and petrochemical characterization of the gabbro-plagiogranite complex is given. The occurrence of gabbro-plagiogranites in the southern part of the Western Sayan is discussed in the article by A. P. Lebedev and I. V. Ginzburg (1953).

In conclusion, we note that in all cases the rocks of the complex mentioned are closely spatially associated with rocks of the spilite-keratophyre series. As V. N. Kotlyar (1955) notes, for such magmatic complexes confined to deep faults, it is “very characteristic that, when magma rises from great depths, their formation occurs, with rare exceptions, at comparatively shallow depths; therefore, along with intrusive complexes, subvolcanic ones are also formed.” Thus, the gabbro-plagiogranites of the Maina intrusion apparently represent an intrusive facies of the effusives of the spilite-keratophyre series spatially and genetically connected with them, which we unite into the Maina magmatic complex of the Middle Cambrian.

REFERENCES

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Corrections and Noticed Misprints

In S. A. Stroitelev’s article “Study of the Crystallization of Epsomite and Mirabilite,” for technical reasons curly brackets have been replaced by square brackets, for example: [100] instead of {100}, etc.