GEOLOGICAL STRUCTURE OF ONE OF THE PALEOVOLCANOES IN THE SOUTHEASTERN PART OF THE SOLGON RIDGE

(EASTERN SAYAN)

V. A. GAVRILENKO

(Presented by Prof. A. M. Kuzmin)

In recent years, interest has increased considerably in areas where volcanogenic formations are developed, with which numerous deposits of nonferrous, rare, and radioactive metals are often associated [1, 2, 3, 4, 5, 6, 7, 10, 11]. A characteristic feature of such deposits is their close spatial connection with volcanic edifices, which, being situated along regional faults, constitute the geological structures most favorable for localization of ore mineralization [2, 11]. The identification of ancient volcanic edifices and the careful study of their morphology, internal structure, tectonic features, and the patterns of distribution of volcanites in space are necessary in order to carry out purposeful searches for mineral deposits in areas of analogous geological structure [9, 10].

Over a number of years, the author, as a member of a group of geologists of the Department of Mineral Deposits of the Tomsk Polytechnic Institute, took part in detailed geological mapping work carried out in the southeastern part of the Solgon Ridge, within the belt of development of Lower–Middle Devonian volcanogenic-sedimentary formations. This territory is characterized by a number of features caused by its location in the area of junction of the marginal part of the folded zone of the East Sayan anticlinorium and the North Minusinsk depression; in particular, by the presence of two clearly expressed structural stages, one of which—the lower—is composed of Proterozoic deposits, and the upper of Devonian volcanogenic-sedimentary rocks [8].

The Devonian formations make up the northeastern limb of a gentle synclinal structure within the eastern end of the North Minusinsk depression. On the west-southwestern flank of the limb of the syncline, a regional fault of northwestern strike has been established; in the southeastern part of the territory described it changes its direction to sublatitudinal, almost northeastern. The fault is characterized by an exceptionally complex structure, caused by a system of branching dislocations of various orders and directions. The fault zone has a considerable extent—more than 70 km, with a width of up to 1 km—and is manifested in intense fragmentation and brecciation of the rocks. Along its entire length, the fault controls a chain of small massifs of syenite-porphyries, as well as dikes of various composition.

Among the tectonic structures of the second order, a series of large transverse disturbances stands out in particular; these are confined to the fault or cut across its splays, probably established in the lower structural stage and expressed in the overlying volcanogenic–sedimentary formations as zones of intense fracturing of the rocks. Along the fault, at places where it is coupled with or intersected by feathering transverse disturbances, there is a series of paleovolcanoes, of which the largest are the Kukuer, Sobolii, Zhulet, and Maltat volcanoes.

Below is a characterization of a volcanic center situated in the southeastern part of the studied territory, in the headwaters of the Solonechnaya River, in the zone where the above-mentioned regional fault is articulated with a large feathering disturbance of submeridional trend.

The area described is a volcanic structure about \(3.5 \times 4.0\) km in size, somewhat elongated in a submeridional direction. Its distinguishing feature is the gentle (\(5—10^\circ\)) attitude of effusive bodies along the periphery of the edifice, with a sharp steepening (\(40—60^\circ\)) toward the center. In its northeastern and southeastern parts, the effusive bodies dip respectively to the southwest and northwest, while in the southern and southwestern parts reverse dips appear. In the south the structure is bounded by a fault; its axial part is cut by a zone of the feathering disturbance.

The geological structure of the volcanic edifice includes a complex of volcanogenic rocks of the Byskar Series of the Lower–Middle Devonian, represented by a thick effusive–pyroclastic sequence. The latter is cut and injected by numerous intersecting dikes and interstratal bodies of complex morphology, similar in composition and close in time of formation to the erupted rocks. The vertical section of the sequence is characterized by heterogeneity in lithological composition. Basic rocks predominate in its lower parts, and subalkaline and alkaline rocks in the upper parts.

Formations of basic composition are fairly widespread, but are recorded at the surface only along the periphery of the area described. In the central and northeastern parts of the structure they are overlain, in the section, by subalkaline and alkaline rocks. The basic effusive rocks are represented by alternating flows of olivine basalts, plagiobasalts, and trachybasalts, with extensive development of amygdaloidal slag-like varieties separating individual flows from one another. In the upper parts of the basic-rock horizon, observed in the southwest of the structure, pyroclastic formations are developed. In external appearance the basalt varieties are similar. They are massive, fine-grained rocks, often with indistinct traces of flow emphasized by the orientation of phenocrysts of olivine, plagioclase, and kalishpat. The slag formations differ from basalts by an aphanitic groundmass and by a porous, often amygdaloidal texture. The orientation of the amygdules is, as a rule, subparallel. Most of them are filled with calcite and chlorite. The pyroclastics are represented by fine- to medium-clastic and agglomeratic tuffs or lava breccias composing thin (\(3—5\) m) interbeds. They consist chiefly of fragments of plagiobasalts. The cement of the rocks is either lava or fine-clastic, chlorite–pelite–carbonate–ash in composition.

Subalkaline and alkaline formations predominate in the area described. In petrographic composition and conditions of occurrence they are divided into two groups: a) subalkaline rocks—trachyliparites, quartz orthophyres, vesicular lavas (slag formations—

…felsite-porphyries, various tuffs and lavas, and b) substantially alkaline varieties—alkaline trachytes and phonolites.

The predominant forms of occurrence of the subalkaline and alkaline rocks are also sheets and flows. The character of their occurrence and their internal structure vary in different parts of the paleovolcano. Lava varieties of volcanics predominate over pyroclastic ones; the latter, together with trachyliparites, are widely developed in the central and northeastern parts of the volcanic edifice. The trachyliparites are characterized by fluidality and an oligophyric structure, and contain sparse phenocrysts of plagioclase. Toward the upper parts of the sheets, the trachyliparites gradually pass into vesicular, slag-like lavas, intensely altered, with an aphanitic groundmass and an aphyric structure. Quartz orthophyres are spatially confined mainly to areas where intrusive bodies of syenite-porphyries occur, being, in all probability, their extrusive analogues. They are porphyritic rocks, massive or sometimes fluidal.

Effusive formations of acid composition—felsites and felsite-porphyries—occur as isolated exposures over small areas and are localized in complexly built tectonic nodes in the central and southeastern parts of the structure. These are dense, massive rocks, sometimes fluidal, glassy, rarely with sparse porphyritic segregations of feldspar. Subalkaline lava breccias, tuffs, tuffaceous sandstones, and tuff breccias compose lenticular bodies of gentle dip (10–15°), discontinuous along strike, alternating with one another or facies-replacing one another, overlain by lava sheets.

Of considerable interest is the presence, in the central parts of the structure, in individual beds of tuffs and especially tuffaceous varieties, of a distinct sorting of material (rhythmic bedding), from agglomerates at the base of the section to aleuritic tuffs at its top. In external appearance these are fine-clastic (ash or lithocrystalloclastic tuffs) or coarse-clastic (agglomeratic xenotuffs) rocks with gradual mutual transitions, consisting of 50–60% fragments of various size and shape, represented by orthophyres, trachyliparites, crystals of quartz and plagioclase, glass, and ash particles. The cementing part of the rocks is represented by ash or lava material.

Alkaline trachytes and phonolites are distributed mainly in the northeast of the structure. They make up individual blocks also in the central parts of the latter. These rocks are characterized by porphyroid, dense, massive textures. Porphyritic segregations of rounded or elongated form are represented by potassium feldspar or by rare hexagonal plates of nepheline (alkaline trachytes); moreover, individual varieties of them (phonolites) contain up to 15–20% nepheline. The groundmass of the rocks is semicrystalline, in places felsitic, with a distinct microfluidal texture. Somewhat west of the block composed of phonolites, horizons of their tuff breccias appear.

In the central and northeastern parts of the paleovolcano, subvolcanic bodies of trachyliparites have been found, having sharp contacts with the volcanics enclosing them. The rocks are characterized by more distinct recrystallization than their sheet facies, but it is very uneven, as a result of which they acquire a pseudofluidal, banded appearance. Diverse dike formations in composition are especially widespread. In the central and northwestern parts of the structure they are represented by syenite-porphyries,

quartz orthophyres and microdiorites; in the central and northeastern parts—trachyliparite-porphyries and phonolites; in the south—anamesites and diabase porphyrites. All dikes, with the exception of varieties of basic composition, are characterized by small thicknesses (0.5–4.0 m), steep dips (75–80°), and also by a semicircular arrangement in plan, especially in the northern part of the structure. Along the periphery of the volcanic structure, the effusive-pyroclastic formations are cut by a series of small stock-like bodies of quartz syenite-porphyries. These rocks have an even-grained porphyritic texture and contain up to 20–30% phenocrysts of K-feldspar, more rarely plagioclase and mafic minerals. Their groundmass is fine-grained, with a trachytoid or granitic structure.

Explosive breccias and secondary quartzites occupy a quite separate position. The explosive breccias (agglomerate xenoclastolavas) form rupturing bodies of very complex morphology, often branching, with swellings and pinches, ranging in thickness from several centimeters to tens of meters. Outwardly they are characterized by an agglomeratic lithoclastic structure, a breccia texture, and contain up to 60–70% fragments. The fragments are represented by trachytes, quartzites, carbonaceous-siliceous basement shales, basalts, trachyliparites, sandstones, etc. The cement of the rocks is fine-clastic, dark gray in color, and is distinguished by the presence of a considerable amount of carbonaceous-siliceous material along with abraded material of the enclosing rocks. Secondary quartzites, in the form of thin lens-shaped bodies, have been encountered in the eastern part of the territory described, in the exocontact of a stock of syenite-porphyries with felsites. They are typically white or yellowish-gray in color, have a saccharoidal appearance, a conchoidal fracture, a dense massive texture, and a quartz-sericite composition.

Thus, in the southeastern part of the Solgon Ridge, within the area of development of Lower–Middle Devonian volcanogenic-sedimentary formations (the Byskar series), a volcanic edifice is established, spatially tending toward the zone of intersection of a regional sublatitudinal fault and a major transverse disturbance of submeridional trend. Its internal structure is characterized by a complex facies composition of volcanogenic formations, caused by the presence of a sequence of effusive-pyroclastic rocks cut by subvolcanic bodies, dikes, and sills of complex morphology and varied composition. The volcanites filling the central parts of the structure are represented by the final magmatic differentiates—trachyliparites, orthophyres, and their fragmental varieties—agglomeratic xenotuffs, as well as lava breccias and explosive breccias [2].

Of considerable interest is the spatial arrangement of subvolcanic and dike bodies, as well as lava breccias of subalkaline composition. Practically all small intrusive bodies of syenite-porphyries tend toward the peripheral parts of the structure, as if tracing its boundaries. Dike formations and bodies of lava breccias are often characterized by a semicircular arrangement, healing the few ring disturbances probably connected with subsidence of the vent.

The presence of rhythmically bedded tuffogenic-sedimentary formations in the central part of the edifice indicates a rather significant interruption of volcanic activity in the Lower–Middle Devonian. The development of secondary quartzites within the structure points to ancient solfataric-fumarolic alteration of the rocks.

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