ON THE PETROGRAPHY OF THE BUIBINSKY GRANITE MASSIF

(WESTERN SAYAN)

V. N. SMYSHLYAEV

(Presented by Prof. A. G. Sivov and A. M. Kuzmin)

In the territory of the Western Sayan, acidic intrusive magmatic rocks are widely distributed. However, up to the present time the granitoid magmatism of this part of the Altai–Sayan system has not received proper treatment in the literature. Over the course of two years the author studied the Buibinsky massif, which is a typical representative of a special type of granite first described by I. K. Bazhenov [1, 2, 3] at the mouth of the Dzhoi River under the name “Dzhoi granites.” Under the same name, the Buibinsky granites were first briefly characterized by A. G. Sivov [4], who carried out geological investigations in the basin of the Us River, a right tributary of the Yenisei. Within this massif he distinguished porphyritic and evenly grained granites, alkaline granites and adamellites, and, among rocks of the dike facies, microgranite-porphyries, albite-porphyries, aplites, and rarer pegmatites.

As a result of the investigations carried out, we were able to establish that the massif is more complex not only in composition but also in structure. Without touching in any detail upon questions of the formation of the intrusive body as a whole—which is intended to be done in a separate work—in the present article we attempt briefly to set forth only the material directly concerning the material composition of the massif.

The Buibinsky pluton is one of the largest granite massifs of the Western Sayan. The total area of the intrusive body, which in plan has the form of a somewhat irregular “African continent,” is about 800 sq. km. Its greatest transverse dimensions are 30–32 km in the latitudinal direction and 35 km in the meridional direction. The southern, considerably narrower (11–12 km), part of the massif has a latitudinal, crest-like branch of adamellites, the eastern massif of which, on the Sheshpir-Taiga Range, is separated from the main body by an effusive-sedimentary septum of Silurian–Devonian age. In geomorphological terms the Buibinsky pluton is situated in the high-mountain part of the Western Sayan, forming a significant part of the watershed of the Us River, a right tributary of the Yenisei, and of the rivers Shadata, Bolshaya and Malaya Taigishei, and Kebezh.

The rocks enclosing the intrusive body are represented in the east, west, and southwest by accumulations that have undergone regional metamorphism.

of the Syutkhol and Ishkin formations of the Upper Cambrian. In the southeast the massif borders on the Kyzylbulak predominantly effusive sequence \(D\) 1–2 and on the red-colored sandstones and conglomerates of the Shishtyk Formation of the Upper Silurian (Sheshpir-Taiga Range). The Cambrian rocks as a whole form a persistent structure of sublatitudinal trend, whereas the more southerly red-colored sediments and effusives of the Silurian and Devonian together form a structure of clear northeastern strike.

The eastern contact surface of the intrusive body is almost vertical and dips away from the intrusion to the east at angles of 82–88°. The western contact is also steep and plunges almost vertically beneath the granites. We have no observations on the attitude elements of the northern contact surface, but, according to O. M. Glazunov (1953), it dips steeply beneath the granites, in connection with which he concludes that the massif has an etmolith-like form. On the basis of general geological considerations, and taking into account the available observations, one may suggest that the Buiba pluton is hardly a deep-seated batholith. Most likely it is a large interformational massif emplaced along the surface of a structural unconformity.

All the enclosing rocks at the boundary with the massif underwent distinct contact metamorphism, forming zones of cordierite, quartz-biotite, and other hornfelses of varying thickness.

Composition of the intrusion. The massif is a complex, multi-stage pluton, whose formation took place in several intrusive phases accompanied by the formation of rocks of differing composition. At one time A. D. Smirnov (1953), and after him I. N. Kazakov (1957), believed that the formation of the pluton proceeded in two phases, the establishment of the second of which was accompanied by a series of various dike formations whose genetic relation to particular rocks of the massif had not been established. They assigned to the rocks of the first phase diorites, granodiorites, and numerous hybrid rocks of intermediate-acid composition, located in the extreme southwestern part of the intrusion. The basis for this was the circumstance that the dioritoids are closely spatially associated with the granites of the massif, by which they are cut and metamorphosed while in direct contact.

Although the relations of the dioritoids with the porphyritic granites are obvious and no one doubts the younger age of the latter, the question of distinguishing the dioritic rocks as the first phase is controversial. According to an oral communication from A. G. Sivov, he and I. K. Bazhenov observed, on the left watershed of the Krasnaya Rechka, basal conglomerates at the base of the Shishtyk Formation that contain a considerable quantity of pebbles of diorites and other similar rocks assigned to the first phase of the Buiba intrusion. Therefore there are as yet insufficient grounds for assigning the dioritoids to the Buiba pluton. Most likely they represent that type of magmatic formation which is distinguished as “small intrusions.”

To the second phase the above-mentioned investigators assigned the granitic rocks of the massif, connected with one another by mutual transitions and accompanied by a dike complex that included dikes of olivine and non-olivine diabases, aplitic granites, and granite-porphyries of various ages and compositions; it was noted that the latter (the granite-porphyries) are younger than the diabases.

Such an understanding of the order of formation of the Buiba intrusion is too simplified. According to our observations, several sharply distinct groups of rocks take part in the structure of the pluton,

including here also the dike rocks of the final stage. It proved impossible to clarify the relationships between some groups, and therefore some of the rocks are distinguished as independent formations only conditionally.

In general, the sequence of formation of the massif appears as follows: 1) coarse- and medium-grained porphyritic granites; 2) equigranular coarse- and medium-crystalline biotite granites without signs of porphyritic texture; 3) white two-mica granites; 4) silicic microgranite-porphyries; 5) red alkaline granites; 6) medium-grained pink adamellites. The dike complex, which is common to all rocks of the pluton, is divided by time of formation into two stages: 1) enstatite diabases, diabases, microdiabases, diabase and quartz porphyrites; 2) coarsely porphyritic granite-porphyries.

Porphyritic granites constitute the most widespread type of rock. Situated in the central, western, and northwestern parts of the massif, they occupy more than half of its area, sharply predominating over the other granitic rocks. Everywhere they have a uniform appearance, determined by the constancy of their textures and composition, and only in a narrow, 1.5–2-meter endocontact zone at the boundary with the enclosing rocks do they acquire a lighter coloration. The contacts are always sharp and crosscutting. Sometimes small apophyses (0.2–1.5 m thick), composed of leucocratic granites, penetrate along schistosity into the exocontact zone. No assimilation of the country rocks is observed, and the few xenoliths of schists, located in the marginal zones, preserve relict structural and textural features.

Anisotropy of the rocks in the form of plano-parallel textures is expressed very weakly, is observed rarely, and appears as an alternation of thin (1–2 cm) fine-grained bands enriched in biotite with broader (10–15 cm) bands composed of a coarser-grained aggregate of quartz, K-feldspar, and plagioclase with an insignificant mica content. As a rule, the banding is subordinate to gently dipping and sheetlike jointing. In addition to the latter, 2–3 systems of endokinetic steeply dipping fractures of meridional and northeastern strike are clearly developed.

Dike rocks in the porphyritic granites are represented by aplites and aplite-like microgranites. In addition, cutting of the porphyritic granites by diabases and by granite-porphyries of the final stage is often observed. Pegmatites are practically absent, occurring rarely only as nests and structural schlieren composed of a quartz–feldspar aggregate having a pegmatoid structure.

This type includes gray and light-gray rocks with a pinkish tint, in the coarse- or medium-grained groundmass of which are situated large (up to 3 cm), broad-tabular porphyritic phenocrysts of gray, cream-colored, rarely pink alkaline feldspar, constituting 20–25% of the rock volume. The groundmass is composed of white 2–6 mm plates of plagioclase, gray externally xenomorphic grains of quartz, and hexagonal 2–3 mm flakes of biotite. Under the microscope the granites reveal a granitic and granophyric structure of the groundmass. In places a myrmekitic texture is observed. The principal minerals are quartz, microperthite, plagioclase, biotite, and very rarely hornblende. Secondary minerals are represented by chlorite, sericite, and pelitic material. Among the accessory minerals present are sphene, orthite, xenotime, zircon, apatite, and magnetite.

The quantitative proportions of the principal minerals vary within narrow limits. The quartz content ranges from 30 to 38%, microperthite from 37 to 45%, and plagioclase from 15 to 18%. Dark-colored minerals make up 5–6%, accessories 1%, and magnetite 1%.

Plagioclases are represented by several generations. The largest (porphyritic) grains, in composition, belong to oligoclase-andesine No. 27–32, which forms isometric or slightly elongated, finely twinned tablets. As a rule, zoning is observed, with a decrease in basicity in narrow marginal zones to oligoclase No. 14–17. Rarely, the presence of small idiomorphic flakes of biotite is noted in it, although on the whole the degree of idiomorphism of both is approximately the same, and the plagioclase of the early generation imprints faces in the biotite. This circumstance may be regarded as indicating their almost simultaneous crystallization.

The second generation of plagioclase, in the form of very small (0.1–1 mm) xenomorphic grains, takes part in the structure of the groundmass. It is represented by fresh grains of albite and albite-oligoclase No. 6–13. The observed perthites of exsolution correspond in composition to albite No. 3–4.

Biotite forms idiomorphic or somewhat irregular flakes included in grains of plagioclase, potash feldspar, and quartz, but is also not infrequently encountered in the groundmass, occupying intergranular spaces. The flakes show sharp pleochroism in brown tones; some of them are twinned according to the mica law and contain inclusions of apatite, zircon, and magnetite.

Amphibole is represented by common hornblende, forming very rare and small (0.1–0.2 mm) xenomorphic grains confined to intergranular spaces and replaced by secondary biotite. Judging from the relationships of the amphibole with other minerals, its crystallization proceeded in the final stage together with the plagioclases of the second generation.

Alkali feldspars form both porphyritic segregations and smaller grains in the groundmass, filling interstices and containing frequent inclusions of oligoclase and biotite, and in some places also hypidiomorphic quartz, which imprints its faces on them. Most of the grains are weakly pelitized and have undergone perthitization to one degree or another, in connection with which homogeneous grains are rare. Measurements using conoscopic observation showed that the alkali feldspars are represented by soda-bearing and potassic anorthoclase and orthoclase, with residual orientation of their sodium varieties. The angles of the optical axes range from −50 to −88°, with a distinct maximum in the interval (−70°)—(−85°), corresponding to 85% of 63 measurements.

Quartz is represented by two generations. Grains of the first of them give regular quadrangular sections and imprint faces in albite and potash feldspar. Quartz of later origin, together with albite, fills intergranular spaces and partly corrodes potash feldspar, forming bay-like embayments.

Zircon occurs in the form of small (0.05–0.7 mm) prisms concentrated in biotite and around segregations of ore material. In cross sections it displays zoning with a decrease in birefringence in the central parts.

In the form of single grains, a prismatic orthite is constantly present in the rocks; in intergranular spaces it forms nearly isotropic aggregates. The grains are small (0.1–0.5, rarely 1.1 mm) and almost always zonal, with an isotropized core. Pleochroism is distinct: \(Ng\) is dark brownish-red, \(Np\) is orange. As is also the case with zir—

is surrounded by pleochroic halos. Xenotime occurs as colorless or weakly pleochroic, pink to colorless small grains that give square sections. It is found near ore segregations, and also in biotite, where it causes the appearance of pleochroic halos. Apatite is very fine and enclosed in biotite. Magnetite forms a sparse dissemination of small grains.

Fluorite. Fluorite-bearing quartz porphyritic granites were encountered in two places—near Lake Metkul on the left watershed of the Third Buiba River and on the right watershed of the Us River, 4 km below the mouth of the Third Buiba River. As a rule, the rocks are broken by microfractures, along whose walls smears of chlorite are observed. In the samples, fluorite is violet in color and forms a fine dissemination between grains of the principal minerals, here and there forming irregular accumulations or filling hairline cracks. Under the microscope, strong cataclasis is observed in the granites. Fluorite forms irregular, sometimes patchy, pale-pink grains located in the interstices, in boundary intergranular areas, or developing along cleavage cracks in anorthoclase. Judging from its relations with other minerals and the confinement of fluorite to weakened directions in the rocks, it is a postmagmatic mineral.

Dike suite of porphyritic granites. The porphyritic granites are accompanied by aplites and aplite-like microgranites, which form dikes and gently dipping sheetlike bodies absent from the areas of development of the other granitic rocks. The aplites fill joints, being confined to their vertical or steeply dipping systems, at whose intersections with sheet jointing “bedded” bodies are formed. The thickness of the bodies is variable and changes depending on the character of the primary fracture. The most common dikes are 2–10 cm thick, more rarely 1–1.5 m. The contacts are sharp; in a narrow endocontact zone the texture of the rocks is finer-grained, which indicates intrusion of the aplitic melt into already considerably cooled porphyritic granites with well-formed systems of steeply dipping and sheet endokinetic joints.

Macroscopically these are light-gray, light-yellow, pink, and white fine-crystalline rocks consisting of quartz, microcline, and albite-oligoclase, with an almost complete absence of dark-colored minerals. The texture is aplitic, allotriomorphic, sometimes microgranitic. Under the microscope, K-feldspar has a well-expressed microcline lattice; the grains are fresh, xenomorphic. Plagioclase No. 8–16 is present in amounts approximately equal to those of microcline. Quartz shows weak undulatory extinction. Small, slightly elongated crystals of almost non-pleochroic orange orthite, small flakes of biotite, and individual grains of sphene and zircon occur rarely.

Equigranular granites. The relations of the rocks of this group with the porphyritic granites have not been clarified with sufficient certainty, since the presumed contacts are concealed by talus. The earlier age of the rocks described, in comparison with other rocks of the massif (with the exception of the porphyritic granites), is established by the fact that the equigranular granites are cut by them and are in intrusive relations with them. The supposition of gradual transitions with the porphyritic granites seems to us improbable, since each of these rock groups possesses sufficiently distinct individual features, manifested both in external appearance, texture, and in the character of the accompanying dike offshoots, and also in the quite definite spatial position of the aforementioned rocks in the body of the intrusion. In area, the equigranular granites yield...

…they are understood only as porphyritic, occurring chiefly on the western slopes of the Baldyr-Taiga Range, on the watershed of the Iliktygem and Third Buiba rivers, and along the right side of the valley of the Third Buiba River.

Macroscopically these are leucocratic fine-, medium-, and much more rarely uniformly coarse-grained rocks of light-gray color, sometimes with a pinkish tint. In the northeastern endocontact with the enclosing Cambrian rocks, a marked decrease in grain size is observed. The contact is sharp; no assimilation phenomena are observed.

The mineral composition is fairly uniform. The principal minerals are dark-gray quartz (37.5%), cream-white or white microperthite (54%), plagioclase, poorly distinguishable from it macroscopically (6–10%), and biotite (1.5%). Sometimes the granites acquire an aplite-like appearance, contain almost no biotite, and in them the quartz content is considerably reduced (to 26%).

The microstructure of the rocks is typically granitic; in separate areas there are signs of an indistinct porphyritic texture, caused by the presence of somewhat larger and earlier idiomorphic crystals of sericitized andesine Nos. 36–38. The second generation of plagioclases is represented by very fresh and very small xenomorphic grains of albite Nos. 2–6, whose crystallization proceeded almost simultaneously with the microperthite.

Quartz is always relatively idiomorphic and is incised by faces into potassium feldspar and albite. Biotite is intensely chloritized along the cleavage, leaving as an inheritance to chlorite single crystals pleochroic halos around inclusions of zircon, orthite, and xenotime. In fresh flakes, biotite is pleochroic from dark brown to pale yellow, sometimes in orange-red tones. Potassium feldspar is represented by microperthite with optical orientation usually belonging to sodium orthoclase and anorthoclase. The grains are weakly pelitized and contain perthites of decomposition of the ribbon and fibrous type. Simple twins according to the Manebach and Baveno laws are often observed; sometimes a blurred microcline lattice appears.

Accessory minerals are represented by numerous grains of zonal orthite, zircon, sphene, xenotime, and magnetite; secondary minerals—by chlorite, sericite, and pelitic material.

The vein series of granites of the second group is represented by microgranites. Their dike bodies form two systems and are localized among the rocks described above, sometimes occurring in the endocontact zone of adjacent porphyritic granites. The first system of dikes has a nearly meridional northeastern or northwestern strike; the second has an approximately latitudinal direction. All dikes are steeply dipping and have considerable thickness, usually within 5–15 m, more rarely up to 25 m, which is especially characteristic of the approximately latitudinal system.

Externally these are fine-grained rocks of light-gray, grayish-white, or pink color, consisting of white grains of plagioclase 1–1.5 mm in size, gray quartz (up to 1 mm), and small, rare flakes of biotite. Pink potassium feldspar, occasionally forming grains up to 2–2.5 mm in size, accounts for the indistinct porphyritic texture of the rocks. Study under the microscope shows that, mineralogically, the microgranites differ in no way from the comagmatic equigranular granites, almost exactly repeating the features of their principal minerals. The formation of the microgranites was probably connected with comparatively shallowly situated portions of the granitic magma, which were opened by primary steeply dipping fractures arising in the cooling equigranular granites.

Two-mica granites have been encountered in two areas located in different parts of the massif. Their largest body, with an area of about 4 sq. km, is situated along the northeastern margin of the pluton and has an irregular, though isometric, form. The southeastern contact with the even-grained granites, which they cut, is sharp and dips beneath the two-mica granites at angles of 64–68°. In turn, the two-mica granites are cut by dike-like apophyses of cream-colored microgranite-porphyries, the stock of which is located adjacent to them.

Another exposure of two-mica granites is noted in the axial part of the watershed of the Us and Verkhnyaya Buiba rivers. Here the body formed by them, with an area of 1.5 sq. km and having the form of a stock slightly elongated in a meridional direction, occurs among medium- and coarse-grained porphyritic granites.

Microscopically, the rocks described are white or grayish-white medium-crystalline rocks consisting of quartz (25–30%), white prisms of plagioclase (25–28%), yellow or cream-colored grains of K-feldspar (35–40%), and flakes of biotite and muscovite (3–5%). Their color, composition, and texture change very little with distance from the contacts. A granulitic texture is developed everywhere, together with which a poikilitic texture is also observed. Plagioclases were among the first to crystallize, their formation continuing right up to complete crystallization of the melt. The earliest idiomorphic segregations belong to albite No. 7–8, which forms square or slightly elongated prismatic sections, along whose margins a narrow rim of structureless albite No. 0–1 develops. The later generation of plagioclase grains is also the largest in size—1–1.8 mm. It includes most of the grains of albite No. 2–6.

Quartz forms grains of two generations. The early crystals are the largest (up to 1.5 mm), contain inclusions of albite, and are cut into K-feldspar by their faces. The later xenomorphic quartz, in the form of small grains, together with K-feldspar fills the intergranular spaces.

Alkali feldspars are always xenomorphic; they form comparatively large (up to 4 mm) fresh grains belonging to potassium-sodium orthoclase and anorthoclase, with maxima of the angles of the optical axes of 65–70° (36%) and 80–85° (49%). In most grains a reticulate or fibrous perthitic texture is clearly manifested.

Biotite forms small flakes uniformly distributed in the mass of the rock. Muscovite occurs both as independent xenomorphic grains 1.1–1.5 mm in size and as pseudomorphs after biotite. — \(2v = 36, 36, 39, 40, 42^\circ\). Birefringence is 0.043–0.045. Accessory minerals are rare.

Cream-colored microgranite-porphyries. This type of granite has, among the other rocks of the pluton, a comparatively limited distribution, occurring chiefly on the watershed areas between the Us and 2nd Buiba rivers, and between the 3rd Buiba and Iliktygem. The microgranite-porphyries form a series of small stocks with diameters measured in the first hundreds of meters and more rarely reaching 1–1.5 km. Sometimes they compose dike-like bodies of considerable thickness (200–250 m), cutting porphyritic, even-grained, and two-mica granites, which determines their younger age in comparison with the indicated rocks.

Macroscopically, this type of microgranite-porphyry is easily distinguished from other rocks by the cream-yellow color characteristic of its fine-grained groundmass, in which are located idiomorphic 1.5–2.5 mm grains of dark-gray or almost black quartz, rare but

fairly large (2–3 mm) flakes of biotite and cream-colored phenocrysts of K-feldspar (3–6 mm). This uniformity of external appearance is consistently preserved in the rocks described throughout the entire territory of the Buibinsky massif.

Under the microscope, the structure of the groundmass is granophyric, with transitions to microgranitic. The phenocrysts are often grouped in clusters, creating a glomeroporphyritic structure. Quartz gives polygonal idiomorphic sections into which, as bays, the resorbing groundmass penetrates. Some of its grains, like those of K-feldspar, are surrounded by narrow granophyric rims. The K-feldspar phenocrysts are represented by untwinned, weakly perthitized microcline with \(-2v = 82^\circ\). The porphyritic biotite is slightly chloritized. Smaller flakes of the same biotite are enclosed in the groundmass. Plagioclase No. 8–14 forms broad fresh tablets, only rarely containing single grains of epidote and sphene. The latter is always xenomorphic and occurs either in feldspars or is associated with ore matter. Very rarely, single grains of orthite are encountered.

Red alkaline granites were encountered in the easternmost part of the massif, on the right and left banks of the Us River, where they compose a body irregular in plan, slightly elongated in a northeastern direction, with an area of about 30–35 sq. km. In addition, analogous rocks are observed in the upper course of the Chap River, where they form a stock with transverse dimensions of \(1 \times 1.5\) km. Riebeckite varieties were encountered in the near-mouth part of the Chap River. It proved impossible to determine the relationships of the rocks described with the other rocks of the pluton directly in the outcrops. The replacement of even-grained granites by alkaline granites in the eluvium occurs abruptly, literally over a distance of 3–5 meters. In our opinion, the alkaline granites constitute an entirely independent group, sharply distinct both in outward appearance and in their mineralogical and chemical composition and other features from the remaining rocks of the massif. Their origin is probably connected with a special phase of intrusive activity within the Buibinsky pluton. This is also indicated by the appearance of a completely isolated stock-like body of alkaline granites among the Silurian-Devonian rocks in the upper reaches of the Chap River.

Macroscopically, the alkaline granites are red or yellowish-red, sometimes with a cream tint, leucocratic, nonporphyritic, coarse-grained rocks composed of red tablets of potassic feldspar (60–63%), irregular grains of xenomorphic quartz (32–35%), and a very small quantity of fine grains of amphibole or biotite (1–2%). In many cases the dark-colored minerals, as well as the plagioclases, occur in negligible amounts or are absent altogether, and then the role of the dark-colored mineral passes to magnetite.

Under the microscope the following mineral composition is established: quartz, K-feldspar, albite, amphibole, biotite, chlorite, sphene, zircon, orthite, xenotime, epidote, magnetite. The structure of the predominant part of the granites studied is granulitic. More rarely, granitic and graphic structures occur. Plagioclase is represented by albite No. 7–9 with \(+2v = 82^\circ\). It forms very rare small (0.5–1 mm), not quite regular tablets, captured during crystallization by quartz and K-feldspar. In some samples independent grains of albite are absent, and it occurs only in the form of perthites. Quartz forms two different-age generations. The first of them is represented by hypidiomorphic crystals 2–4 mm in size, embedded in grains of K-feldspar. A later generation of quartz is a component part of granophyric intergrowths. Alkaline feldspars are intensely peliti-

zoned. Within large grains, inclusions of smaller grains of the first generation are often observed; apparently these were crystalline “seeds.” Many grains are twinned according to the Manebach law. Large crystals of K-feldspar, in contact with one another, form a fanciful sutured line caused by the mutual penetration of perthites from neighboring grains. It has been established that the alkali feldspars are represented by sodium-rich orthoclase and anorthoclase with moderate (60–70°) and large (80–89°) angles of the optic axes. Amphibole is represented by xenomorphic, very small grains confined to intergranular spaces. Judging from its relations with other minerals, its formation took place during the final stage of crystallization. In optical properties the amphibole belongs to representatives of the riebeckite series. Biotite is rare: in places it is observed as xenomorphic flakes together with amphibole and is almost always chloritized. In it, as in the amphibole, intergrowths of radioactive zircon occur; during chloritization, grains of magnetite and epidote are released. Orthite is present as isolated idiomorphic crystals confined to intergranular spaces. Sphene is rare and gives quadrangular sections. Sphene is situated along grain boundaries, forming irregular and wedge-shaped grains. Magnetite is sometimes present in considerable amounts (4–6%) and plays the role of a vicarious mineral.

Adamellites and granodiorites. Pink adamellites compose the southeastern margin of the Buiba intrusive, occurring in an endocontact zone 1.5–2 km wide on the right bank of the Chap River, not far from its mouth, and then passing onto the left bank of the Us River, where they are exposed on the watershed ridges of its left tributaries as far as the axial part of the Sheshpir-Taiga Range. Another body, isolated from the main massif and analogous to the first, cutting through the deposits of the red-colored Shishtyk Formation, is known in the upper reaches of the upper left tributaries of the Chap River, where it is also located in the axial part of the Sheshpir-Taiga Range. In plan the body has an elliptical form with transverse dimensions of \(4 \times 9\) km, and its long axis is oriented in a northeastern direction, conforming to the general structure of the enclosing strata. The northwestern contact with Upper Silurian sandstones is active, sharp, and dips to the northwest at angles of 45–50°. Judging from measurements of jointing elements, the southwestern part of the adamellite body is a periclinal slope, the axis of which dips gently beneath the red-colored sandstones. The adamellites have a direct continuation to the southwest, where they join their main massif. The connecting saddle has not yet been completely exposed by denudation processes, but its existence is indicated by the presence of numerous small (from 20 m to 1 km across) cone-shaped protrusions of adamellites, which cut through and metamorphose the overhanging sedimentary roof in the area between the two above-mentioned bodies.

The time of formation of the adamellites is not yet entirely clear. It has been established that in the western part of the Sheshpir-Taiga Range they cut through hybrid rocks of the diorite intrusion, as well as even-grained granites of the second phase, and probably represent the latest intrusive phase.

Macroscopically the adamellites are very uniform. The grayish-red color, caused by the presence of red xenomorphic K-feldspar, tablets of white plagioclase, and grains of light-gray xenomorphic quartz in combination with a small amount of biotite; the ubiquitously observed even-grained textures; and the insignificant manifestation of the dike facies, represented by brick-red fine-grained...

microgranites—all these are highly characteristic features by which adamellites differ from the other rocks of the massif. In the narrow endocontact zone, measured in meters, the grain size of the rocks decreases somewhat; hornblende appears together with biotite; the content of alkali feldspar decreases; and the adamellites gradually pass into granodiorites, which sometimes send out concordant apophyses into the sandstones of the Shishtyk Formation.

Microscopically, the rocks are characterized by a hypidiomorphic-granular structure approaching granitic. The predominant mineral is zoned, slightly sericitized plagioclase No. 38–11, individual tabular grains of which reach 5 mm, which under the microscope gives the rocks a certain porphyritic character. Between the larger grains of andesine-oligoclase there are granular areas composed of albite No. 7–8, xenomorphic quartz, and pelitized microperthite containing small grains of plagioclase and biotite flakes. The potassium feldspars belong to orthoclase, sometimes with an elevated sodium content, and more rarely to potassic and sodic anorthoclases with moderate, and sometimes somewhat reduced, optical axial angles. Xenomorphic quartz is present in amounts of 30–32%. Biotite flakes are either included in the potassium feldspars or are squeezed into intergranular spaces, where, as a rule, they are chloritized, with development along cleavage of xenomorphic grains of sphene, epidote, and ore substance. The biotite content is 5–7%.

In the endocontact parts of the massifs, xenomorphic sieve-like twinned grains of hornblende appear, with \(cNg = 16^\circ\) and \(-2v = 68\text{–}69^\circ\). In immediate contact with the sandstones, in the form of a fine scattering, there are isometric grains of pale-green diopside and numerous grains of xenomorphic sphene. Their origin is apparently connected with assimilation of some quantity of sandstones that contained calcite as cement.

The accessory minerals, besides sphene, are represented by numerous grains of zoned orthite and the rarer zoned zircon.

Dike facies of adamellites. Vein offshoots of adamellites are very rare and are represented by single dikes of felsite-like or fine-grained leucocratic brick-red microgranites. Usually they have a northeast strike and a steep, almost vertical (80–86°) dip to the northwest. In the near-contact parts, very narrow zones of a finer-grained fabric are observed. Under the microscope the structure is microporphyritic, caused by the presence of slightly resorbed but, in general, idiomorphic small (0.2–0.8 mm) grains of quartz and potassium feldspar, which are immersed in a brownish quartz-albite mass. The microphenocrysts are surrounded by spherulitic rims. Individual spherulites and their aggregates also occur independently, “cemented” by a microfelsitic groundmass, in which small grains of magnetite, sphene, and chlorite are included here and there.

Dike complex. To the dike complex we assign vein rocks that are common to the entire Buibinsky massif. They cut the rocks of all intrusive phases and even extend beyond the limits of the intrusion; therefore it is not possible to connect their appearance with any one of the groups of granitic rocks. Their specific composition, features of their position in the pluton, and youngest relative age lead one to suppose that, genetically, the rocks of this complex are associated with a deeper magmatic source than that whose residual magma crystallized to produce the vein offshoots of each of the preceding intrusive phases.

In composition and time of formation, the rocks of the dike complex can be divided into two groups, formed as independent stages. The first group includes dike rocks of basic composition; the second, acid rocks (granite porphyries).

Dike rocks of the first group. These include numerous dikes of diabase, diabase and quartz porphyrites, which are widely distributed over the entire area of the massif, sometimes extending beyond it into the exocontact zone. They form two systems, one of which has a meridional trend with slight deviations to the northwest and northeast (within 5–10°), while the other has a northeasterly trend (20–35°). Both systems are steeply dipping, with the contact surface dipping to the west-northwest at angles of 65–90°. Dike bodies of the aforementioned rocks occur equally often both in the marginal parts of the massif and in the center, cutting all rocks of the pluton, with the exception of dikes of coarse-porphyritic granite porphyries. The relationships among the individual representatives of this group are unclear, since their mutual intersections are nowhere observed.

The thickness of individual dikes varies considerably, from 0.2 to 50–70 meters. Thick dikes can be traced for considerable distances, sometimes up to 500–600 meters, although more often their extent does not exceed 150–200 meters. It is interesting that solitary dikes of basic rocks occur relatively rarely and, conversely, series of several subparallel bodies are common.

Dike rocks of the second group. Dike formations of this group are developed everywhere. They are especially often observed in the southern, southeastern, and northeastern endocontact zones of the intrusive body, in places extending beyond its limits. They are encountered somewhat less frequently in the central part of the massif among porphyritic biotite granites. Petrographically, dikes of the second group are represented by only one rock type—coarse-porphyritic granite porphyries, which were previously described by A. G. Sivov [4] as an independent intrusion of granite porphyries and quartz porphyries of Late Variscan or even Ancient Cimmerian (?) age.

The overwhelming majority of granite-porphyry dikes belong to a single system of northeastern strike, with steep northwestern dip angles (62–90°). Morphologically, the dike bodies are characterized by good persistence along strike, reaching 500–600, and sometimes more, meters in length, with thicknesses from several meters to 15–30 m. An example is a 15-meter-thick granite-porphyry dike that cuts even-grained granites in the near-contact northeastern part of the massif; it can be clearly traced for more than 600 meters and is confined to a system of steeply dipping fractures that emphasize the position of the contact surface.

Their relationships with the widely distributed diabase dikes are clearly established in many places. It has been noted that the granite porphyries intruded along the same fracture paths as the diabases, frequently cutting the latter. Such relationships are observed 4 km west of Lake Metkul, in the mouth part of the Chap River, and in other places. A distinctive feature is that the granite porphyries contain more or less melted-and-dissolved xenoliths of diabases even where they are not adjacent to diabases or where the latter are entirely absent nearby. This evidently indicates that the diabase dikes do not always reach the present denudation surface, whereas the intrusive granite porphyries, using the same pathways, carry up their fragments.

From other granite-porphyry rocks, the dike rocks of this group differ in their extremely characteristic external appearance. This

coarse- and large-porphyritic rocks of red, pink, and yellowish-red coloration. The phenocrysts are represented by flesh-colored, red, pink, and yellowish potassium feldspar, the grains of which reach 1–3 cm; by rarer grayish-white platy plagioclase (1–1.5 cm); and by gray bipyramidal quartz crystals. Mafic minerals are rare and are represented by biotite and, more rarely, idiomorphic amphibole.

The contacts of the dikes are rectilinear and sharp; in their marginal parts chilled zones are well expressed, which in the most massive dikes reach 40–50 cm. Immediately at the endocontact the rocks have a felsitic structure and either contain no phenocrysts at all, or these occur as single grains measuring 0.5–1 mm. The transitions from the chilled zone to the inner parts, although sharp, are gradual. The main mass of the inner parts is comparatively well crystallized. The noted structural features of the granite-porphyries indicate that their intrusion always took place into cold rocks. The absence of phenocrysts in the endocontacts and their appearance in the more internal parts of the dikes, with a simultaneous increase in their size and quantity, testifies to crystallization of the phenocrysts directly at the site where the dikes were formed. This, in fact, explains the absence of primary anisotropic structures, which would undoubtedly have occurred during the intrusion of a melt containing tabular feldspar crystals.

In addition to the structures noted, the rocks described display a considerable variety of structures in the groundmass, in connection with which varieties with spherulitic-granophyric, microgranophyric, felsitic, and microgranitic structures may be distinguished.

Porphyritic segregations of potassium feldspar are represented by broad-tabular crystals of micro- and cryptoperthite, sometimes coated with pelite. On the Fedorov stage it is established that microperthite belongs to potassium orthoclase and anorthoclase. Plagioclase phenocrysts are slightly sericitized and are represented by andesine No. 34–38. Porphyritic segregations of quartz in the form of idiomorphic, slightly resorbed grains up to 1 cm in size are very common. Idiomorphic crystals of hornblende are rare, do not exceed 2 mm, and are completely replaced by an aggregate of chlorite, epidote, and sphene. Biotite is chloritized. In the principal quartz–feldspar mass, epidote crystallizes in microporous areas. Occasional single small grains of zonal orthite, zircon, sphene, and magnetite are encountered.

Such, in broad outline, are the features of the petrographic composition of the Buibinsky massif. In conclusion it may be noted that the plutons of the “Joi” granites are more complex formations than has hitherto been believed. In particular, the Buibinsky intrusive is a multiple pluton, the formation of which took place as a result of several not quite simultaneous intrusions of high-temperature granitic magmas, arriving from a single magmatic source or from deeper levels of the intrusive chamber. A feature of this type of granite is its inability to assimilate, despite the comparatively high temperature of the granitic magma, which led to the formation of broad zones of cordierite and quartz–feldspar hornfelses. At the final stage of intrusive magmatism, rocks of the dike complex arise, genetically associated with a deeper magmatic focus.

Age of the intrusive. The age position of the Buibinsky intrusive is determined by the fact that adamellites cut and metamorphose the effusive rocks of the Kzyłbulak Formation \(D_{1-2}\). At the same time, pebbles of the Buibinsky granitoids, according to observations by G. M. Vladimirsky, occur in the basal conglomerates of the Upper Devonian in the upper reaches of the B. Ury River. Thus

Thus, the Variscan age of the granites described is confirmed by an ever larger number of geological observations.

REFERENCES

1. I. K. Bazhenov. Report on geological investigations in the area of the Maina copper deposit. Izv. Sib. Otd. Geolkoma, vol. 4, no. 1, 1924.

2. I. K. Bazhenov. Preliminary report on geological investigations in 1924 in the southwestern Sayans. Izv. Sib. Otd. Geolkoma, vol. 4, no. 5, 1925.

3. I. K. Bazhenov. Western Sayan. Essays on the Geology of Siberia, Academy of Sciences of the USSR, 1934.

4. A. G. Sivov. Geology and age of the intrusions of the southeastern slope of the Western Sayan. Materials on the Geology of the West Siberian Territory, no. 33, 1936.