ON THE PETROGRAPHY OF THE MT. MALY KARATAG MASSIF

(GORNAYA SHORIA)

V. S. Chuvakin

(Presented by the Department of Petrography)

The article describes the complex, compositionally differentiated massif of Mt. Maly Karatag, which belongs to the Patynsko-Kul-Taiga intrusive complex, known in Gornaya Shoria and the Kuznetsk Alatau. The intrusive massif of Mt. Maly Karatag is situated within Gornaya Shoria, 11 km northeast of the settlement of Ust-Kabyrza (middle course of the Mras-Su River), and is expressed in the relief by a dominant summit with an elevation of 1217.2 m.

The Karatag massif is known in the literature as syenitic [9], and only recently, on the basis of work carried out by A. A. Panin, M. G. Dvoeglazova, N. V. Russkikh, and others (WSGU), augite diorites, granodiorites, and alkaline syenites have been distinguished within the massif. Yu. G. Shcherbakov [9] considers the age of the pluton to be close to that of the Patyn massif, located 22 km northeast of Mt. Maly Karatag [4]. The massif of Mt. Bolshoi Karatag, situated west of Maly Karatag, is composed essentially of alkaline syenites and volcanogenic rocks and is not considered by us in the present article.

The material forming the basis of this article was collected in the summer of 1961 during special petrological work organized by the Department of Petrography of the Tomsk Polytechnic Institute to study the petrography of Mt. Maly Karatag. The work was conducted under the supervision of S. S. Ilyenko.

Morphology of the Massif, Its Geological Position, and Mechanism of Formation

In plan view the massif has regular oval outlines; in size ($3.5 \times 7$ km) it belongs to plutons of the third order. With respect to the tectonic structure of the region, the massif is situated within the Mras anticlinorium and is confined to an anticlinal fold of northeastern strike, formed here by intricately dislocated limestones of the Yenisei Suite of Sinian age. The latter make up the core of the Mras anticlinorium [9].

The suite is predominantly carbonate in composition, with thin interbeds of quartzites, shales, and metaeffusives.

Contacts of the intrusion with the enclosing rocks were not observed directly. Taking into account the steep dip of the trachytoid structure in the gabbro-diorites inward into the massif, one may assume a steep dip of the southwestern contact likewise inward into it. Observation of marginal fractures near the southeastern contact, dipping inward into the massif at angles of \(30\)—\(50^\circ\), makes it possible to assume a steep dip of the southeastern contact of the intrusion [2]. Thus, judging from the position of the contacts of the pluton, it may be assigned conditionally to vertical stocks.

Along its long axis the massif is elongated in the direction of the folding of the enclosing rocks. Its northwestern contact, almost rectilinear (slightly concave inward into the massif), coincides with the strike of the bedding of the sedimentary sequence. The southwestern contact of the intrusion cuts the structures of the enclosing rocks almost at a right angle. The elements of the prototectonics of the intrusive body are also discordant with the elements of the external structures of the sedimentary formations.

With respect to the history of tectonic movements in the region, the massif belongs to the post-tectonic bodies and was formed under subplatform conditions. The magma substance filled a chamber, probably of tectonic origin, under conditions of a rigid frame.

The diversity of the rocks of the massif is connected with its formation in several intrusive phases and with processes of differentiation and assimilation. These same causes account for its coarsely zonal structure. The fact that rocks of different phases produced a single intrusive body indicates the emplacement of portions of magma through one feeder channel. In distinguishing the phases and determining the role of differentiation processes in the formation of the rocks of the massif, well-known difficulties were encountered, connected with the lack of data on the nature of the contacts between petrographic varieties of rocks. However, the formation of the complex of rocks of the massif is represented in the following sequence. The first phase is associated with the formation of gabbro and gabbro-diorites. Crystallization of the basic magma began already during its movement (before the complete filling of the magmatic chamber), as indicated by the presence in the gabbro and gabbro-diorites of primary-banded flow textures. Rocks of the first phase make up the extreme southwestern part of the massif and constitute no more than \(15\%\) of its area.

In the second phase of the intrusion the magma had a more acidic—monzonitic—composition; the monzonites that formed make up the greater part of the massif. Syenite-diorites are of subordinate importance. No direct contact between rocks of the first and second phases was observed, but their sharp transition was traced within an interval of one meter. Near the contact, both these and other rocks bear traces of weak crushing. The increased fracturing of the gabbro, gabbro-diorites, and monzonites and their abrupt transition within a narrow interval at the site of their presumed contact indicate its heterochronous character. In addition, the monzonites contain numerous xenoliths of altered rocks of basic and intermediate composition.

The formation of the monzonites was accompanied by assimilation of the enclosing rocks, as a result of which a narrow band of hybrid rocks—albite diorites—formed along the southern contact of the massif.

The third phase is associated with the formation of granites, which make up the northern part of the massif. The following facts indicate the intrusion of magma of granitoid composition: a) the monzonites in individual areas near the granites are strongly albitized; b) dikes of syenite-porphyries and diabases are absent in the granites, which can be explained by the later formation of the granites.

At the same time, one cannot fail to note the similarity in the structure and mineralogical composition of the monzonites and granites. As monzo-

nites and granites macroscopically the rocks are completely structureless, massive, have a porphyritic texture, often with the same grain size; in terms of the content of dark-colored minerals they belong to leucocratic rocks; in the dark-colored part they contain diopside, and they also show similarity in the composition of the feldspars. Their difference consists only in the content of quartz, which in the granites reaches 25%. The noted similarity is not accidental; it indicates the closeness in composition of the differentiates of the intrusion, enriched in the second case in SiO₂, or, finally, the formation of these rocks as a result of differentiation in place after the filling of the magmatic chamber by a monzonite–granite melt¹).

The fourth phase, in which pegmatitic alkaline syenites were formed, completes the formation of the intrusive rocks of the massif. Two small bodies of alkaline syenites have been noted, one of which is enclosed in the field of monzonites, the other—in the footwall side of the body of gabbro and gabbro-diorites at their contact with the enclosing rocks.

All dike and vein formations associated with the massif may be distinguished as a separate fifth phase.

In terms of the depth of formation, the massif belongs to near-surface hypabyssal bodies, as indicated by the wide distribution of porphyritic structures in the rocks of the massif.

The zonal structure of the massif, caused by a change of rocks from the basic to the acidic series from southwest to northeast, confirms the conclusion of M. A. Churilin and L. V. Pshenichnyi (8) on the influence of the rotation of the Earth on the formation of intrusive massifs and on the localization of mineralization associated with them. This conclusion is as follows: “Accumulations of differentiates of basic composition and of magmatic ores proper are generally observed in the southwestern parts of intrusions. Postmagmatic, pegmatitic, and pneumatolytic-hydrothermal formations, including ore-bearing ones, occur for the most part in the southeastern and more rarely in the northeastern or southwestern endo- and exocontacts of intrusive massifs” (8). The Patyn massif possesses a similar regularity, in which “… the gabbro most enriched in titanomagnetite is located in the southwestern endocontacts...” (8).

Petrography of the Rocks of the Massif

In the composition of the Karatag massif the following groups of rocks have been distinguished: 1) gabbro, gabbro-diorites; 2) monzonites, syenite-diorites, albitic diorites; 3) pyroxene granites; 4) pegmatitic alkaline syenites; 5) dike and vein formations.

1. Gabbro and gabbro-diorites have a gray, dark-gray color and a uniformly medium-grained texture. In the near-contact parts their trachytoid character is clearly manifested. The amount of dark-colored minerals is variable and fluctuates within the limits of 35–30%, dropping in the leucocratic varieties to 20%. Under the microscope the rocks have a gabbroic structure or a prismatic-granular one. Plagioclase (65–70%) is andesine-labradorite (No. 47–58), sometimes zonal in the gabbro-diorites. The composition of the marginal zones corresponds to No. 25–35. The angle of the optic axes is large, more often positive. Twins according to the albite and Carlsbad laws.

Pyroxene (13–20%) is monoclinic in optical properties ($2V$ 52–54°; $Ng = 1.715$–$1.718$; $Np = 1.689$–$1.695$; $c : Ng = 40$–42°), salite-augite (the refractive index of the minerals in all cases is determined in the principal sections of the optical indicatrix). In some pyroxenes the angle of the optic axes increases to 59°, the refractive index—

¹) The author’s materials of 1962 confirm this conclusion.

…varies rarely toward a decrease in it. The composition of such pyroxenes is evidently closer to diopsides. They occur as idiomorphic crystals or are bounded by the faces of plagioclases.

Magmatic hornblende is not characteristic of the rocks described; it was noted only in two grains, colored an intense brown, forming rims around pyroxene in the leucocratic variety. More commonly uralitic hornblende occurs, forming partial or complete pseudomorphs after pyroxene.

Biotite (6–8%)—meroxene-lepidomelane \((2V = 10—12^\circ;\ Ng = 1.645;\ Np = 1.599)\)—is clearly secondary. Its large tabular grains penetrate plagioclase, overgrow pyroxene with partial replacement of it, and, not infrequently, replace secondary amphibole. It forms reaction rims around magnetite grains at their boundary with plagioclase. Chlorite is only slightly developed and appears later than the other secondary minerals. In plagioclases, chlorite fills microcracks, and in the form of small spots is developed in the saussurite aggregate; it overgrows and partly replaces pyroxene, amphibole, and biotite. Along microcracks and in plagioclase, prehnite is often developed \((2V = +67^\circ;\ Ng = 1.637;\ Np = 1.612;\ a : Np \approx 0^\circ)\).

Epidote occurs in single grains \((-2V = 82^\circ,\ Np = 1.725)\). Ore minerals (up to 4% and less) are represented by magnetite and titanomagnetite; they are usually present as xenomorphic grains and sometimes form lattices that intersect like the cleavage of calcite. Inside, such a lattice is usually filled with a mixture of carbonate and chlorite; on the outside it is, as a rule, overgrown by a narrow rim of chlorite.

Among accessory minerals, apatite and sphene are noted.

Alteration of the gabbros and gabbro-diorites is connected with the processes of albitization, biotitization, and chloritization. In the altered varieties, the central parts of plagioclase grains are replaced by saussurite and sericite; externally the plagioclase crystals are albitized. Albitization is manifested both in the acidification of plagioclase—i.e., a decrease in the anorthite component in its composition—and in the formation of albite rims. In addition, albite develops along cracks. The process is accompanied by a change in the structure of the rock: idiomorphic plagioclase grains lose their crystallographic outlines, their edges become uneven and sinuous. In parallel with the destruction of plagioclases, pyroxene is uralitized, and biotite and chlorite develop. Ultimately, the primary magmatic minerals are established only as relics or are completely destroyed.

A comparison of the chemical composition of the rocks described with typical representatives of the gabbro group shows a somewhat elevated content in them of ferrous iron and the sum of alkalies, and a reduced content of magnesia and calcium oxides. The increase in alkalies and decrease in calcium are connected with de-anorthitization of the plagioclases and the development of albite and biotite.

2. Syenite-diorites and monzonites. Syenite-diorites have been mapped as a small body in the northwestern corner of the massif. They differ from monzonites in texture and mineralogical composition. In appearance these are gray and light-gray, uniformly medium-grained rocks. Under the microscope, a prismatic-granular texture is established, obscured by secondary processes of mineral alteration.

Plagioclase (84–70%) is an intensely saussuritized and sericitized oligoclase (No. 20–30; \(+2V = 82—84^\circ\)). Potassium feldspar (0–17%)—sodic orthoclase \((Ng = 1.5325,\ Nm = 1.527,\ Np = 1.526)\)—is sharply distinguished from plagioclase by the brown products of decomposition, and is also inferior to it in the degree of idiomorphism of the crystals.

Pyroxene–diopside ($c:Ng = 43^\circ$; $2V = +61^\circ$; $Np = 1.698$), with a content of 55% of the hedenbergite molecule, forms isometric grains; its content in the rock does not exceed 5%. Magmatic hornblende ($c:Ng = 13^\circ$; $c:Np = -62^\circ$; $Ng = 1.707$; $Np = 1.681$; $Ng$—dark brown, $Nm$—brownish-brown, $Ng$—light yellow; $Ng > Nm > Np$) forms rims around pyroxene grains. Green fibrous hornblende of the uralitic type and brown biotite are noted.

The ore mineral magnetite (2–4%) usually develops near hornblende and pyroxene. Apatite is present, and among secondary minerals there is low-iron muscovite ($2V = -45^\circ$).

Monzonites predominate among the rocks of the Karatag massif; they are pinkish-gray in color, massive in texture, and, as a rule, porphyritic in structure. Under the microscope they are characterized by a monzonitic texture with sharply expressed idiomorphism of plagioclase. In quantitative mineralogical composition (plagioclase—44%, K-feldspar—30, quartz—11, pyroxene—11, biotite and ore mineral—4%) they differ from typical Tyrolean monzonites (after Brøgger) by the predominance of plagioclase over K-feldspar, a higher content of quartz, and a lower content of mafic minerals. In compositional features they approach augite syenites of the “Gröba” type.

Plagioclase in the monzonites is andesine (No. 37–45, more often 39–43; $2V = +80^\circ$ for No. 42), sometimes zoned; its tabular crystals are often overgrown by rims of secondary albite. The potassium-sodium orthoclase ($Nm = 1.5265$; $Np = 1.522$; 40% $Ab$) occurs in less idiomorphic boundaries than plagioclase and is usually more weakly pelitized. Incidentally, we note that perthites are not characteristic of the rocks of the subalkaline and acidic series of the massif. Both in monzonites and in granites and alkaline syenites, the K-feldspars are represented chiefly by sodic orthoclases, in which the albite component occurs as an isomorphous admixture. Perthites and microperthites are rare.

Quartz is usually present in xenomorphic grains and sometimes forms micrographic intergrowths with feldspar.

Pyroxene, according to its optical properties ($2V = 53–54^\circ$; $Ng = 1.713$, $Np = 1.691$), is salite-augite; it forms isometric grains and is replaced by uralitic hornblende. During this replacement free magnetite is formed. Among the other minerals, flakes of brown biotite are noted, less ferruginous ($Ng = 1.635$, $Np = 1.589$) than in the gabbros and gabbro-diorites; magnetite in xenomorphic grains, apatite, and zircon are also present.

Monzonites form gradual transitions to augite–hornblende syenites and syenite-diorites. In the first case, among the feldspars K-feldspar predominates over plagioclase (K-feldspar—57%, plagioclase—19, quartz—6, pyroxene—5, amphibole—12, biotite and ore mineral—4%), while in the mafic part of the rock ordinary hornblende ($c:Ng = 19–20^\circ$, $Ng = 1.661$, $Np = 1.646$, $Ng$—dense yellowish-green, $Nm$—yellowish-brownish-green, $Np$—light yellow; $Ng \approx Nm > Np$) predominates over pyroxene. In the second case, conversely, plagioclase predominates over K-feldspar (plagioclase—56%, K-feldspar—18, quartz—8, pyroxene—12, amphibole—2, biotite and ore mineral—4%); in the mafic part there is salite-augite ($Ng = 1.713$, $Np = 1.691$; $2V = 48^\circ$, rarely increasing to 54°). The K-feldspar of the syenite-diorites is sodic orthoclase ($2V = -61^\circ$, $Ng = 1.5265$) with a content of 25% albite [6].

The albitic diorites are, in external appearance, gray fine-grained rocks. Their texture is pseudoprismatic, with traces of recrystallization. Quantitative mineralogical composition: plagioclase—74%, pyroxene—7, amphibole—14, biotite and magnetite—5%.

Plagioclase-albite No. 8–10 ($Ng = 1.540$, $Nm = 1.535$, $Np = 1.5315$, $2V = +84^\circ$) occurs in irregular grains with wavy extinction. A general tabular form of the crystals, disturbed by blastesis, is often observed.

Pyroxene in rounded grains, according to optical properties ($+2V = 57$–$59^\circ$, $Ng = 1.722$, $Np = 1.696$), is diopside-salite with a hedenbergite content of 40%; as a rule it is rimmed by borders of lepidomelane ($Ng = 1.652$, $Np = 1.598$) or hornblende overloaded with a dusting of magnetite. The amphibole is close to kersutite ($Np = 1.688$, $c > r$). This dark-colored hornblende, with sharp pleochroism from black-brown along $Ng$ to yellow along $Np$, forms irregularly shaped grains; upon further oxidation it is replaced by an ore mineral. The ore mineral is represented by magnetite; apatite is noted in thin needles, and carbonate in cracks.

1. Pyroxene granites are light grayish-pink or white rocks with a fine- to medium-grained structure, almost always distinctly porphyritic. The microstructure is granitic, with sharp xenomorphism of quartz.

Plagioclase (38%), of oligoclase composition (No. 18–30, $2V$ large negative), not infrequently more acid—to No. 10—forms tabular, well-bounded grains, often zoned. K-feldspar (23%)—orthoclase—$2V = -73^\circ$; $Nm = 1.524$; $Np = 1.520$, with a content of 20% albite, rarely orthoclase-perthite, occurs in prismatic crystals and sometimes forms simple twins. The feldspars are usually decomposed. Quartz (25%) occurs in xenomorphic grains of various shapes.

Pyroxene (4%)—diopside ($2V = 56$–$58^\circ$; $Ng = 1.705$, $Np = 1.675$)—forms irregular or rounded grains with numerous cracks. Among the secondary minerals, pale-green actinolite, biotite, magnetite, and zircon are noted.

1. Alkaline syenites are light-pink leucocratic rocks, coarse-grained, pegmatitic. The size of K-feldspar crystals reaches 2 cm. The mineralogical composition is fairly uniform and consists almost entirely of sodium orthoclase ($Ng = 1.532$, $Nm = 1.529$) with a content of 50% albite molecule, and orthoclase-perthite. Plagioclase in the perthites is finely twinned albite (No. 3–5).

In isolated grains quartz, albite, hedenbergite ($+2V = 62$–$64^\circ$; $Np = 1.718$), calcite, actinolite, and biotite with a fine dusting of magnetite are observed. Hematite fills the cracks.

1. Dike and vein rocks, in the order of their formation, are represented by deeply albitized diabases, syenite-porphyries, microsyenites, diabases, microdiabases, and diabase porphyrites. They are confined to cracks of submeridional strike, have steep dips and thicknesses not exceeding the first 3–4 m. The main mass of the dikes is located in the field of monzonites.

Deeply albitized diabases have a greenish-gray color and a massive texture. Rarely, porphyritic phenocrysts of plagioclase stand out from the groundmass. Under the microscope, relics of diabase structure and of primary minerals, more often pyroxene, are identified. Plagioclase is completely albitized. Epidote is noted. As a result of these rocks, metasomatic albitites are ultimately formed. By the degree of alteration, these dikes belong to the earliest.

Syenite-porphyries are colored pink and, as a rule, have a porphyritic structure; in the porphyritic segregations there is weakly zoned albite-oligoclase. The groundmass has a structure similar to granoblastic and consists of semi-decomposed feldspars. Sometimes pyroxene of the diopside-hedenbergite series is noted. By the presence

xenomorphic or rounded quartz grains, quartz-bearing and quartz-free varieties of syenite-porphyries are distinguished.

Microsyenites form branching veinlets, the thickness of which does not exceed the first centimeters. In their composition and structure they correspond to the main mass of the syenite-porphyries.

Among the dikes of basic composition there are fresh varieties of diabase porphyrites and weakly diagenetized diabases, microdiabases, and diabase porphyrites. Fresh diabase porphyrites in porphyritic phenocrysts contain plagioclase (No. 50—60) and augite ($2V = +47^\circ$; $c : Ng = 43^\circ$; $Ng = 1.720$, $Np = 1.697$). The groundmass (60% of the rock volume) has an ophitic structure and consists of plagioclase No. 45, augite, and a small amount of chlorite. Fresh diabases have a doleritic structure and the following composition. Plagioclase (No. 75—90, more often No. 85, $2V = +89^\circ$, zonal—the composition in the marginal parts of the grains decreases to No. 40—45) makes up 55%, monoclinic and rhombic pyroxene—30% (rhombic slightly predominates). Monoclinic pyroxene has the following properties—$2V = 58^\circ$; $c : Ng = 42—43^\circ$; $Ng = 1.713$; $Np = 1.691$; it forms rounded grains and decomposes with formation of a fine dusting of ore mineral. Up to 15% is brown biotite.

Diagenetized dikes of basic composition—diabase, microdiabases, and diabase porphyrites—have, as a rule, a hyalopilitic groundmass, which consists of a mixture of albitized plagioclase, monoclinic pyroxene, chlorite, and ore minerals. In the porphyritic phenocrysts of the porphyrites—plagioclase (No. 40—50) and augite.

In concluding the petrographic description, let us briefly note the composition of the products of contact metamorphism of the intrusion. At the contact with the massif, the black limestones of the Yenisei Formation become lighter, are transformed into marble, or are strongly silicified. The thickness of the zone of contact alteration does not exceed 200 m. On the northern contact of the intrusion with the enclosing rocks, a thin band of skarns has been noted. The skarns contain grossular (augite and magnetite predominate).

Some features of the petrochemistry of the rocks of the massif

Table 1 gives the chemical compositions and the results of their recalculation according to A. N. Zavaritsky for the rocks composing the massif of Mt. Mal. Karatag. The analyses were carried out in the chemical laboratory of the Krasnoyarsk Geological Administration. The following samples were analyzed: 1—gabbro, 2—gabbro-diorite, 3—gabbrodiorite, 4—diabase porphyrite, 5—diabase, 6—diabase, 7—monzonite, 8—monzonite, 9—monzonite, 10—porphyritic syenite, 11—monzonite, 12—monzonite, 13—alkaline syenite, 14—15—pyroxene granite, 16—17—metasomatic albitite. Note: analyses 11 and 12 are borrowed from the report of Yu. G. Shcherbakov.

In terms of the features of its geological position, morphology, and composition, the Karatag massif is close to the Patynsko-Kul-Taigin intrusive complex, known in the south of the Kuznetsk Alatau. The same conclusion follows from analysis of the petrochemical diagram according to the regular swarm of vectors, after A. N. Zavaritsky. The rocks of the massif as a whole, in their chemistry, are close to the average compositions of the rocks of P. Daly.

All rocks of the massif belong to rocks of the normal series undersaturated in Al$_2$O$_3$ (the vectors of the $asb$ plane are oriented to the right). The comparatively gentle orientation of the vector in the $csb$ plane indicates a noticeable content of potassium in the composition of the alkalies, and their relatively close position to the $sb$ line indicates the general alkaline specialization of the rocks.

Table 1

Continuation of Table 1

of the massif. This is also indicated by the displacement of the figurative points to the right on the $asb$ plane with increasing silica content in the rocks. The displacement of the upper figurative points of the diagram in the $asb$ plane toward the vertex $S$ testifies to a clearly expressed pantelleritic tendency, which is characteristic of volcanic associations of rocks of the alkaline series. Other compositional features of the rocks of the massif are also clearly reflected in the diagram and in the numerical characteristics given in the table.

REFERENCES

1. A. N. Vinchell. Optical Mineralogy. Moscow, IL, 1949.

2. N. A. Eliseev. Structural Petrology. Leningrad, Leningrad University Press, 1953.

3. A. N. Zavaritskii. Igneous Rocks. Academy of Sciences of the USSR Press, 1961.

4. S. S. Il’enok. Principal features of the petrology of the Latyn massif. Geology and Geophysics, No. 4, 1960.

5. G. V. Polyakov, A. E. Teleshev. Magmatic complexes of the area of the Teiskaya group of iron-ore deposits (Kuznetsk Alatau). In: The Principal Ideas of M. A. Usov in Geology. Academy of Sciences of the Kazakh SSR Press, 1960.

6. V. E. Treger. Tables for the Optical Determination of Rock-Forming Minerals. Gosgeoltekhizdat, 1958.

7. G. S. Fedoseev. On the origin of the syenite-diorites of the Mount Mal. Kul’-Taiga massif. Geology and Geophysics, No. 12, 1961.

8. M. A. Churilin, L. V. Pshenichnyi. Regularities in the spatial distribution of endogenous mineralization relative to intrusions. Exploration and Protection of Mineral Resources, No. 10, 1961.

9. Yu. G. Shcherbakov. New data on the geological structure of Gornaya Shoria. Geology and Geophysics, No. 6, 1961.