NEW DATA ON THE AGE OF PORPHYRITIC GRANITES IN THE SOUTHEASTERN ALTAI

N. S. Korzhnev

Introduction

The question of the presence of an Upper Paleozoic granite intrusion in the southeastern part of the Gorny Altai still remains far from resolved. The age of the intrusive massifs of porphyritic granites here has always been determined only by analogy with similar granites of the Rudny Altai and Kalba, as Upper Paleozoic (Variscan) \[4; 5; 7\].

In the summer of 1947, the author of the article carried out geological surveying work in the area of the Chikhachev Range. As a result of the work performed, new facts were obtained that make it possible to speak with greater confidence about the Variscan age of the intrusions of porphyritic biotite granites distributed in the southeastern part of the Gorny Altai.

Brief features of the stratigraphy of the area

Within the area studied, formations of the Lower, Middle, and Upper Paleozoic, as well as Quaternary deposits, have been recorded. The stratigraphic sequence of the formations may be presented as follows (from bottom to top):

a) Kurai Formation—green and, rarely, violet shales and sandstones, limestones, injection gneisses, and metamorphic shales;

b) Yustyd Formation—\(D_2^2\)—black argillaceous shales with flora, sandy-shaly rocks, sandstones, and dark-gray limestones with fauna;

c) Bogutin Formation—\(C_1\)—dark-gray and gray carbonaceous and argillaceous shales, sandy-shaly rocks, and fine-grained sandstones;

d) Quaternary deposits—\(Q\)—alluvial and glacial deposits.

Kurai Formation

The rock complex described is, in appearance and lithology, analogous to the Kurai Formation distinguished by V. A. Kuznetsov in the Kurai Range \[1\], to which he assigned a Lower Silurian age. Some analogy may be drawn with the metamorphic sequence of rocks of the South Chuya Alps.

The formation is the direct eastward continuation of the metamorphosed sandy-shaly and tuffaceous rocks of the Lower Silurian described by N. N. Smirnova (1932) under the name of the green-violet formation. For the deposits described, we retain the name Kurai Formation.

The Kurai Formation occupies the northern part of the area and is represented by green, more rarely violet, argillaceous shales and sandstones, gray-

…with conglomeratic sandstones, marbled light-gray and gray limestones, tuffaceous rocks, chlorite, chlorite-sericite and epidote schists, and injection gneisses.

The main area of distribution of the formation is occupied by injection gneisses and metamorphic schists, while the other rocks make up a narrow belt located along its southern boundary.

In the area where green schists and sandstones are distributed, it has a northeastern, nearly latitudinal strike and a monoclinal, steep dip to the northwest. On the south this monocline is cut off by a tectonic disturbance of the fault type and brought into contact with Devonian deposits. In the area where injection gneisses are distributed, it is impossible to establish the original structure of the formation.

The formation contains no fossil organic remains; therefore, only by analogy with neighboring regions can it be regarded as Lower Silurian.

Yustyd Formation

The Devonian deposits of the Southeastern Altai have repeatedly been studied during geological investigations in previous years and, in particular, by P. S. Krasnopeeva. On the basis of an analysis of the abundantly represented fauna, P. S. Krasnopeeva was able to distinguish several series in the Devonian, in particular the Buguzun Series, distributed at the mouth of the Buguzun River (Kyzyl-Shin gorge), and the Yustyd Series, encountered in the area of the Yustyd River. Each of them was divided into two members. The fauna of the lower members of the Buguzun and Yustyd series is characteristic of the Givetian stage of the Middle Devonian, and that of the upper members—for the Frasnian stage of the Upper Devonian [3]. We are inclined to assign to each member of P. S. Krasnopeeva’s Yustyd Series the significance of independent formations. In the Yustyd Formation we include only deposits of the Givetian stage, and for the deposits of the Frasnian stage of the Middle Devonian we recommend assigning another name.

The formation occupies a large part of the area studied. It is composed mainly of greenish-gray and black argillaceous schists with flora, dark-gray bedded sandy-schistose rocks, more rarely gray fine-grained sandstones, and rare, thin interbeds of dark-gray limestones and calcareous schists, which often contain abundant fauna.

These deposits are broken by disjunctive disturbances of the step-fault type, of nearly latitudinal strike, into three oriented blocks differing in their internal plicative structure. The first two blocks are narrow belts 2 to 4 km wide, elongated in a latitudinal direction and situated in the northern part of the Devonian outcrop area. They are characterized by intense folding of northeastern and northwestern strike, with dip angles of 50–70°. The third block occupies all the remaining area of distribution of the formation, south of the first two. Although within this block there are also a number of tectonic zones of disturbance, they do not delimit independent blocks but merely complicate the structural form of the third block. Here the rocks form a complex folded structural form of almost meridional strike, consisting of a large number of brachyanticlines and synclines.

From a number of localities we collected a flora and fauna assemblage. The fauna was identified by staff members of the paleontological office of WSGU, L. N. Kraevskaya, M. P. Lomovitskaya, and A. P. Zhmaev. They established the following forms: Spirifer mucronatus Conr., Spirifer cf. mucronatus Conr., Spirifer martianoffi Stuck., Spirifer cf. martianoffi Stuck., Spirifer

(Cyrtospirifer) ex gr. verneuili Murch., Spirifer (Delthyris) sp., Atrypa reticularis Linn., Atrypidae, Athyris concentrica Buch., Leptaena rhomboidalis Wilck., Strophomenidae indet., Schiophzoria sp., Schuchertella indet., Brachiopoda indet., Dalmanites sp., Kodonophyllum sp., Favosites ex gr. sibirica Peetz., Crinoidea.

On the basis of these determinations they establish the age of the deposits as the very uppermost horizons of the Zhivetian stage of the Middle Devonian, and correlate them with the lower division of the Yustyd suite of P. S. Krasnopeeva [3]. At the southernmost point of the area investigated, an Upper Devonian form, Spirifer ex gr. verneuili Murch. ($D_3^1$), was found. Thus there are grounds for supposing that in the southern part of the district, and farther to the south, Frasnian deposits appear—the upper division of the Yustyd suite of P. S. Krasnopeeva. The outcrop with the above-mentioned Upper Devonian spirifer is the only one on our planchet, and therefore its description is not given. In our opinion, these deposits, as was mentioned above, should be separated into an independent formation under another name.

Plant remains—Asteroxylon sp., Asteroxylon elberfeldense Kräus et Weyl., Psilophyton princeps Daws., Rhynia sp.—characterize the age range $D_1^2—D_2^2$. Thus, the evidence of the flora and fauna does not contradict one another, and we may accept an age of $D_2^2$ for the Yustyd formation.

Boguty Formation

The Boguty formation is distinguished and described for the first time by the author. The reason for separating the sequence described below into an independent formation was the presence of a significant break between the Yustyd and Boguty formations, and the distinctive lithological composition and character of the plicative structures of the sequence described.

The Boguty formation occupies a small area in the upper reaches of the Yustyd and Boguty rivers. Lithologically it is represented by gray, fine-grained, dense sandstones; bedded and massive siliceous dense dark-gray shales, which ring when struck with a hammer; clayey and carbonaceous shales; and sandy-shaly rocks.

Microscopic study showed that the sandstones have a fine-grained psammitic structure and consist mainly of angular, unrounded grains of quartz, feldspar, and mica flakes ranging in size from 0.1 to 0.25 mm. Their cement is carbonate-clayey and amounts to about 30% of the total composition of the rock.

The sandy-clayey rocks and clayey shales are microscopically a pelitomorphic clayey mass, sometimes with carbonate present. In the shales, in amounts up to 30–35%, and in the sandy-shaly rocks in greater amounts, angular grains of quartz and feldspar, 0.01–0.05 mm in size, are noted. In addition, fine and coarser grains of carbonaceous matter are recorded (up to 3%).

The carbonaceous shales consist almost 50% of carbonaceous matter and 50% of carbonate and clayey material, the carbonaceous matter being arranged in parallel bands.

This complex of sediments forms a complex synclinal folded structure, complicated by second-order folds. The dip of the fold limbs varies from 15 to 60°. In general, however, the Boguty formation lies more calmly than the underlying Yustyd formation. In the outcrops of one of the ravines (the Kok ravine—the right tributary of the Boguty River) it is clearly seen that it lies with angular unconformity on the steeply dipping shales of the Yustyd formation (Fig. 1).

In places the formation is crumpled at the contact with the Devonian, which can evidently be explained by local movements along the contact of heterogeneous and different-aged complexes of sediments.

From two points within the area of distribution of the formation described, we collected flora represented by impressions of large stem portions of ferns of the Cardiopteris type.

In our opinion, the age of the Boguty Formation is undoubtedly Lower Carboniferous, for the following reasons: 1) the formation lies with angular unconformity on the Yustyd Formation, whose age falls within the limits of \(D_2^2\); moreover, the deposition of the Boguty Formation was undoubtedly preceded by a long hiatus; 2) the formation shows considerable similarity to the Lower Carboniferous Bukhtarma Formation of Rudny Altai [5]; 3) a definite confirmation of our opinion may be the fact that, west of the study area, at the mouth of the Buguzun River, red-colored Upper Devonian is widespread [3]. These formations rest conformably on the Yustyd Formation and are older than the Boguty Formation.

Fig. 1. Schematic section across the valley of the Koz stream

1—Boguty Formation—\(C_1\); 2—Yustyd Formation—\(D_2^2\).

The measured thickness of the deposits of the Boguty Formation is about 1500 m.

Quaternary deposits

The Quaternary deposits are represented by alluvial and glacial deposits. The former are confined to river valleys and make up channels and terraces, while the latter are recorded both in river valleys and on watersheds. In river valleys they form ridges of former lateral and terminal moraines, and on watersheds they sometimes occupy considerable areas, forming a distinctive moraine landscape that fringes the foot of the axial part of the Chikhachev Range.

Igneous rocks of the region and their age

Porphyritic granites are predominantly developed in the region, forming large bodies; gabbro-diorites and dike rocks are weakly developed.

Gabbro-diorites

Gabbro-diorites were encountered by us only in a few outcrops. These are dark-gray rocks of fine-grained texture. Microscopic studies showed that they consist of 60–70% plagioclase and 30–35% mafic minerals. In some cases the plagioclase is andesine, in others labradorite. The mafic minerals are represented by brown and dirty-green uralitic hornblende, and also

among the latter, unaltered grains of monoclinic pyroxene are recorded. Epidote and zoisite are deposited along cracks. The structure of the rock is hypidiomorphic-granular or close to ophitic.

Gabbro-diorites in direct contact with dikes that cut through their granites undergo alteration, expressed in the acquisition of granoblastic textures and in a change in mineral composition, caused to a considerable degree by introduction. In the gabbro-diorites near the contact there appear quartz, biotite, a large quantity of ore minerals, and, along the contact itself, chlorite in the form of veinlets.

In the outcrops it is clearly visible that the gabbro-diorites are cut by a large number of vein-like bodies of varying thickness, represented by medium-grained leucocratic granites, which are offshoots of the Builugem granite massif. In addition, closer to the contact, xenoliths of gabbro-diorites are recorded in the granites. The gabbro-diorites make up small bodies no more than 3 km² in area, of irregular shape, arranged along one line of northeastern strike.

Their age is more ancient than the age of the porphyritic granites, since they are intruded and metamorphosed by the latter. The lower age limit of the gabbro-diorites is determined by the fact that they intrude the Kurai Formation. Therefore, by analogy with the diorites of the Kurai Range [1], which also intrude the Kurai Formation, we regard the age of the gabbro-diorites as Caledonian.

Porphyritic Granites

Porphyritic granites play an essential role in the geological structure of the region, occupying considerable areas. They form four independent, spatially separated massifs, which we call the Yustyd, Karagai-Burgazy, Builugem, and Ak-Kul massifs. The granite massifs have not been studied and contoured by us sufficiently completely, and therefore their form and dimensions have not been finally established.

Most of the massifs are situated in the axial part of the Chikhachev Range and are excellently exposed.

The granites of all the massifs possess considerable external similarity, and also, as optical studies have shown, the same mineralogical composition and structure. In fresh fracture they have a light-gray and sometimes yellowish color and a coarse-grained porphyritic fabric. The microscope showed that they have a porphyritic texture and consist of idiomorphic tabular grains of albite-oligoclase, large grains of microcline and microcline-perthite, and xenomorphic grains of quartz. Among the dark-colored minerals biotite is present, sometimes partly replaced by chlorite. Accessory minerals are represented by sphene, apatite, and magnetite. The percentage contents of the principal rock-forming minerals for all the granites are approximately expressed by the following figures: plagioclase—from 5 to 35%, microcline and microcline-perthite—35–65%, quartz—20–40%, biotite—1–10%. Most often the granites contain 5–10% plagioclase. Consequently, all the massifs are composed of comparatively homogeneous microcline granites.

In some granite massifs, in the peripheral parts, leucocratic aplite-like granites of fine-grained texture and granite-porphyries are noted.

Within tectonic zones and near them, the granites have undergone cataclasis of varying degree, up to the formation of mylonites. Microscopically the mylonites are a tectonic meshing, where larger fragments float in crushed fine material; moreover, fragments of quartz under the conditions of the tectonic zone coalesce with one another, forming parquet-like

structures and, as it were, cement the fragments of feldspars. Extremely characteristic of mylonites is the stretching—rolling-out—of the fragments composing them, which creates the impression of a flow structure. In the cataclased granites the primary structure can still be discerned, since the degree of crushing is less and rolling-out is absent. Along cracks in the cataclased granites, and in the mylonites within the main mass of crushing, there are epidote and sericite, which are products of hydrothermal processes.

Since the form and dimensions of the granite bodies have not been fully clarified, we can give only a general idea of them. Most of the massifs are discordant, but in some cases (the Karagai-Burgazin massif) they are elongated in accordance with the general direction of the plicative structure of the main field of Devonian rocks. Their form may be represented as large stocks of irregular outline in plan. The contacts of the granite bodies recorded in outcrops have a sinuous, steep dip toward the intrusion or toward the enclosing rocks, with vein-like offshoots of gentle or steep dip.

Porphyritic granites intrude and metamorphose all the strata of the district from the Lower Silurian up to the Lower Carboniferous; moreover, the character of alteration of the Lower Silurian formation, in our opinion, differs considerably from the character of alteration of the Devonian sedimentary complexes.

The direct alteration of the Lower Silurian rocks near the contacts with intrusive bodies of granite is the formation of amphibolites and quartz-feldspar-biotite hornfelses. The usual alteration of the enclosing rocks is the formation of injection gneisses, which arose as a result of intense injection from the intrusion of porphyritic granites.

The alteration of the Devonian and Lower Carboniferous strata is expressed in the formation of a belt of hornfelses and hornfelsed rocks 1.5–2 km wide. The Devonian strata are intruded and metamorphosed by the Karagai-Burgazin granite massif, and the Lower Carboniferous by the Yustyd massif.

The latter fact deserves special attention and was observed by us at the headwaters of the Yustyd rivers. Here porphyritic granites intrude and metamorphose the sandy-shaly deposits of Lower Carboniferous age—the Boguta Formation—collected into brachyfolds. The relationship of the Boguta Formation to the porphyritic granites can be well illustrated by a schematic section along the left headwater of the Yustyd River (Fig. 2).

Fig. 2. Schematic section along the left headwater of the Yustyd River.

1—porphyritic granites; 2—Boguta Formation; 3—Yustyd Formation; 4—hornfelses

Thus, the supposition of a Variscan age for the intrusion of porphyritic microcline granites in the Southeastern Gorny Altai is confirmed by new, more convincing facts.

Dike rocks

Dike rocks are developed mainly in the endo- and exocontact of intrusive granite bodies.

In the endo- and exocontact of the granite intrusions there are aplite-like granite-porphyries, microgranites, aplites and pegmatites, aphanitic diorite porphyrites. Away from them, in particular, in the area of distribution of the de-

...the zone near the shatter zone, small dikes of quartz porphyries were noted, in places colored by copper green.

Without going into the petrographic characterization of these rocks, we note that aplites and pegmatites are widely distributed, and are associated with the area enclosed between the Ak-Kul and Builyugem massifs; moreover, the latter have distinct vein-like forms of occurrence. Microgranites and aplite-like granite-porphyries are less widespread, while aphanitic diorite porphyrites are poorly developed.

LITERATURE

1. Kuznetsov V. A.—Mercury deposits of the Gorny Altai, Deposits of Rare and Minor Metals of the USSR, Publishing House of the Academy of Sciences of the USSR, 1939.

2. Kuznetsov V. A.—New data on the geology and metallogeny of the Altai, Bulletin of ZSGRT No. 1–2, 1941.

3. Krasnopeeva P. S.—Middle and Upper Devonian bryozoans of the Altai, Materials on the Geology of ZSK No. 20, 1935.

4. Nekhoroshev V. P.—Principal features of the geology of the Gorny Altai, “Oirotia,” Gorny Altai, Proceedings of the Session of the Council for the Study of Productive Forces on the Study of Productive Forces of the Oirot Autonomous Oblast, Publishing House of the Academy of Sciences of the USSR, 1937.

5. Nekhoroshev V. P.—Explanatory note to the geological map of the basin of the upper reach of the Irtysh (Bolshoi Altai) at a scale of 1:500,000, ONTI, NKTP USSR, TsNIGRI, 1936.

6. Obruchev V. A.—Altai Studies, vol. 2, On the tectonics of the Russian Altai, Zemlevedenie, vol. 3, 1915.

7. Semenov A. I.—New data on rare-metal mineralization in the Zyryanovsk district in the Altai, Soviet Geology No. 6, 1945.