ON PALEOVOLCANOES OF THE SOUTHEASTERN PART OF THE SOLGON RIDGE

V. G. KRYUKOV, Yu. A. FOMIN, V. K. CHEREPNIN

(Presented by Professor A. M. Kuzmin)

Questions of the reconstruction of volcanic edifices constantly attract the attention of researchers [1, 2, 3, 4, 5, 6], which is explained to a considerable extent by the frequent confinement of accumulations of ore mineralization to such structures. These questions acquire particular urgency in the study of ancient centers of volcanism, whose structure, as a rule, proves to be obscured by subsequent processes.

The present article, devoted to the characterization of paleovolcanoes, is based on material collected by the authors as a result of a three-year study of Devonian effusives in the southeastern part of the Solgon Ridge (the environs of the village of Stog). This territory is situated in the zone of junction of the Eastern Proterosayan with the North Minusinsk depression, which accounts for the broad manifestation within it of inherited faults of northwestern and sublatitudinal strike, controlling the position of centers of volcanism and the distribution of products of volcanic activity.

Below is given a description of a number of complex polygenic paleovolcanic edifices of the central type, the general layout of which is shown in Fig. 1.

Volcano I (Fig. 2a) is located at the intersection of two faults having northwestern and sublatitudinal strikes. It is characterized by an oval plan form, northwestern orientation, and considerable dimensions (5.0 × 7.5 sq. km). Geomorphologically, the central part of the edifice, composed of subvolcanites, is a weakly dissected depression of small size, bordered on the east by arcuate ridges. On the western side it is bounded by relief elements that emphasize superimposed sublatitudinal graben structures.

Within the proper vent facies of rocks, subvocalnic lavobreccias, quartz porphyries, and orthophyres are noted.

Subvent lavobreccias form a lens-shaped body measuring 800 × 3000 m, the position of which is controlled by a northwestern disturbance. The rocks are distinguished by a distinct clastic structure, with the composition of the clastic material and of the cementing lava being very similar. The size of the fragments reaches 10–20 cm, and their amount varies from 10–30% in the central parts of the body to 50–70% at its periphery; moreover, in the direction from the periphery toward the center, not only a smoothing of the clastic structure of the rocks is observed, but also a decrease in the size of the lithoclasts. As to the conditions of occurrence of the lavobreccias, it may be ju-

Fig. 1. Map of the geological structure of the area, with a diagram of the distribution of paleovolcanoes (compiled by V. G. Kryukov using materials of A. Smagin, V. M. Shutov, and Yu. A. Fomin).
1 — basement formations: marbles, quartzites, schists of the Proterozoic and granitoids of the Lower Paleozoic; 2 — basic effusives, \(D_1\); 3 — pyroclasts of acid and subalkaline volcanics; 4 — acid and subalkaline effusives of the Eifelian; 5 — basalt porphyrites of the Eifelian; 6 — sedimentary cover of the Middle and Upper Devonian and Carboniferous; 7 — post–Middle Devonian granitoids; 8 — faults; 9 — geological boundaries: a) separating Devonian effusives, b) of the sequence of volcanogenic formations; 10 — location of paleovolcanoes.

...judged by the character and behavior of the banding. In the marginal parts of the body the latter is expressed very clearly and is gently oriented (10–30°) toward the center, with such dimensions being recorded only on ridge summits. In the central parts of the body, as well as along its periphery in valleys, the banding is steeply oriented, sometimes vertical, and at the same time is characterized by very intricate bends.

Quartz porphyries cutting through the lava breccias occur as two necks, round in plan, about 600–800 m across, and as several dikes. The necks are confined to areas where sublatitudinal and northwestern disturbances intersect. The orientation of the southern contacts of the bodies indicates their steep dip (70–90°). The same dip is also characteristic of the dikes that trace the zone of the central sublatitudinal disturbance and, more rarely, emphasize radial structures. Petrographically, the quartz porphyries are characterized by an abundance of phenocrysts of quartz, feldspar, and biotite, and by a microcrystalline quartz–feldspar groundmass. With depth, there is found...

Fig. 2. Maps of the geological structure of paleovolcanoes: a—compiled by V. G. Kryukov, Yu. A. Fomin, and V. A. Gavrilenko; b—compiled by V. G. Kryukov using materials of V. A. Gavrilenko; c—compiled by V. G. Kryukov.

Volcanites: 1—basalt porphyries and their lava conglomerates; 2—trachyte porphyries and their tuffs; 3—quartz-bearing porphyries and their tuffs; 4—rhyolite porphyries, their tuffs, and plagio-porphyrites; 5—orthophyres, tuffs of subalkaline effusives; 6—felsites, ash tuffs, agglutinates; 7—andesite porphyries, basic effusives, and their tuffs. Subvolcanites: 8—basalt porphyries; 9—eruptive tuff breccias, agglomerate tuffs, and lava breccias; 10—orthophyres; 11—quartz porphyries; 12—rhyolite porphyries, crystalloporphyries, granophyre porphyries; 13—subalkaline lava breccias; 14—trachytoid phonolites. Intrusive formations: 15—dolerites, diorites, and gabbro porphyrites and microdiorites; 16—syenite porphyries, porphyritic syenites, granosyenites, granodiorites. New formations: 17—feldspathized rocks; 18—secondary quartzites; 19, a—disjunctive boundaries, b—geological boundaries, c—boundaries of metasomatic bodies.

there is an increase in the degree of recrystallization of the groundmass and an increase in the number of biotite phenocrysts.

The orthophyres, which are the youngest vent formations, make up small necks, as well as dikes cutting through lava breccias and quartz porphyries. Orthophyres are characterized by an abundance of albite and orthoclase phenocrysts and by a groundmass, essentially feldspathic with quartz, that has a trachytic or orthophyric structure.

Along with vent facies of rocks, the development of subvolcanic dikes, more rarely sills, is established within the paleovolcano. In particular, in the northeastern and southern parts of the edifice, numerous dikes of rhyolite porphyries and crystalloporphyries are observed, framing the body of lava breccias; moreover, these dikes often have arcuate forms in plan. The extent of the dike bodies of this composition is from 300–600 m to 1–3 km, with thicknesses from 10–20 m to 100–200 m; their dip is steep and directed toward the center of the paleovolcano. In the southern peripheral part of the apparatus, sills of granophyres are recorded, emphasizing mantle structures.

Very characteristic of this edifice are veins and dikes of agglomerate tuffs and eruptive breccias, confined to the contacts of bodies of quartz porphyries or to conical fractures on the periphery of the apparatus. Agglomerate tuffs are distinguished by an abundance of rounded or subangular fragments of acid and subalkaline effusives ranging in size from 1–5 cm to 0.5–1.0 m. Their cement is lava-tuffaceous, basal, or contact-type. The rocks are characterized by considerable strength; this explains the preservation of their remnants, rising in the form of “needles,” “walls,” “teeth,” etc. Eruptive breccias are distinguished by the angular form and comparatively small (up to 5–10 cm) size of the fragments, as well as by the ash composition of the cement. Along the contacts of veins of eruptive breccias that emphasize conical structures, slickensides are observed, indicating the normal-fault character of the displacements. The extent of the bodies of agglomerate tuffs and eruptive breccias amounts to the first hundreds of meters, with thicknesses of 5–15 m.

The effusive products of the paleovolcano are ash tuffs, rhyolite porphyries, quartz-bearing porphyries, and felsites. The tuffs encircle the ancient eruptive center in a ring, forming a hornitos. The erupted volcanites are recorded only in the southwest and south of the area, preserved in the downthrown tectonic sectors of the edifice.

It is of interest to note the development, although rather limited, of products of the activity of ancient solfataras and fumaroles. The latter tend toward the central part of the paleovolcano and are represented by secondary quartzites—white or light-gray dense formations having a quartz-sericite composition with dickite and pyrite. The vent facies of rocks—quartz porphyries, lava breccias, and orthophyres—as well as ash tuffs, were mainly subjected to replacement.

The volcanic formations in this area are cut by a small intrusive body of syenite porphyries of irregular shape. The position of this body is determined by a ring fault.

Volcano II (Fig. 2б) is located 9–10 km east of the first. Its form is oval, measuring 3.5 × 4.0 km, and has a submeridional orientation along its elongated axis. Geomorphologically the volcano is very poorly expressed, which is explained by the development of dike belts and tectonic zones of northwestern trend, which are expressed in the relief respectively by ridges and ravines. Only in the northeastern part of the area is the presence of arcuate ridges noted, emphasizing the ring faults of the area. The paleovolcano is divided by a northwest-trending disjunctive fault into two blocks: northeastern and southwestern.

The northeastern block is composed of basic effusives, which are characterized by the presence of slag crusts and banded textures. From the banding, their southwesterly dip at angles of 10–30° is established. The volcanites form sheets of northwestern orientation, which can be traced far beyond the area of the volcanic edifice. The basic effusives are intruded by granodiorites, manifested as a small stock-like body in the central part, and in the northeast as ring dikes.

Within the southwestern block lies the greater part of the paleovolcano, its vent and near-vent formations. The vent proper, oval in plan and about 800–1200 m in size, is composed of trachytoid phonolites. The contacts of the body, judging from isolated observations in its southern part, have an almost vertical dip.

The trachytoid phonolites are distinguished by a uniform brownish-gray color and a silky luster on cleavage planes. Their mineralogical composition is simple: K-feldspars, to a lesser extent alkaline mafic minerals, and also products of nepheline replacement. Similar to them in composition are lavabreccias that border the small dike-like body of trachytoid phonolites located in the northeastern part of the apparatus.

As a rim around the vent, feldspathized newly formed rocks are observed. They are characterized by banded or spotted textures, a brownish-reddish color, small quartz–feldspar veinlets of recrystallization, and also relics of the original effusives. The new formations develop over tuffs of acid and subalkaline effusives, and the fragmental structure of the latter is often inherited. The orientation of the banding in the newly formed rocks has a steep dip (70–90°) and strike, in accordance with the morphology of the vent.

Other manifestations of subvolcanites also gravitate toward the central part of the volcano: agglomerate xenotuffs, as well as intrusive rocks represented by granodiorites, syenite-porphyries, and dolerite porphyrites.

Of interest is a small body of agglomerate xenotuffs, oval in plan, with vertically dipping contacts. The clastic material contains lithoclasts of Devonian effusives and of marbled limestones, quartzites, and basement schists. The size of the fragments reaches 0.5–1.5 m; their shape is rounded or semi-angular. The contact cement is ash-like in composition. The body composed of xenotuffs is apparently an explosion pipe confined to the mobile zone of the paleovolcano.

Within tectonic zones of radial and mantle type, dikes of crystal porphyries and granophyre porphyries develop. The intrusive formations are confined to rupture structures of northwestern and sublatitudinal directions, as well as to disturbances of ring character.

The effusives composing the southwestern block are represented by tuffs of acid volcanites, felsites, and also trachyte porphyries. The latter are observed within the southwestern sector, bounded by radial disjunctives. The dip of the effusives is southwesterly at an angle of 20–30°.

Volcano III (Fig. 2c) is located 10–11 km southwest of volcano II. Its spatial position is controlled by a fault of northwestern strike. In contrast to paleovolcanoes I and II, the characterized structure is distinguished by a rounded form in plan and by the broad development of conical and radial disjunctives. Its size reaches 4.0 km across. The volcano is comparatively clearly expressed in the relief; the vent itself, 1.2–1.5 km in diameter, represents a [[unclear: text continues on next page]]

a saddle, surrounded on almost all sides by arc-shaped ridges. By a northeast-trending fault the paleovolcano is divided into two blocks: northwestern and southeastern.

The central part of the apparatus of the southeastern block is composed almost entirely of granophyric porphyries. The contacts of their body everywhere prove to be torn, steeply dipping. The rocks are characterized by uniform recrystallization in any part of the body. They are cut by agglomerate tuffs, eruptive tuff breccias, and intrusions of porphyritic syenites, microdiorites, and diorite porphyrites.

Of note are the eruptive tuff breccias and agglomerate tuffs confined to ring zones of collapse, where they have a vein-like mode of occurrence. The thickness of the “veins” reaches 5–15 m. Their boundaries with the enclosing rocks are sharp on the footwall side, while on the hanging wall they are often indistinct. The collapse of individual segments along such structures explains the “preservation” in the central part of the gently dipping pyroclasts of quartz porphyries. Three ring zones of collapse are recognized, with transverse dimensions of 800, 1200, and 2000 m.

In addition, within the southeastern block there is a body of porphyritic syenites, fairly isometric in plan, along the contacts of which dikes of microdiorites and diorite porphyrites develop with the agglomerate tuffs.

Along the periphery of the paleovolcano, in its southern part, metasomatites of feldspathic composition are observed. The position and shape of their body are determined by a ring-type fault; moreover, they make up the hanging wall of the disjunctive structure.

In the northwestern block, the continuation of the ring zones of collapse can be traced, but here the latter are expressed by brecciation or crushing of the rocks. The central part is made up of feldspathic neogenic formations, while along the periphery, in downthrown areas, vulcanites develop. Characteristic of the northwestern block are steeply dipping dikes of olivine-bearing basalt porphyrites, emphasizing radial faults. The thickness of the dikes reaches 1–10 m (rarely 15–20 m), and their length is measured in tens or hundreds of meters.

Within the northwestern block there is an oval, in plan, body of agglomerate tuffs. In this area the occurrence of such formations is most typical. The size of the body is about 600 m across. A steep dip (60–80°) toward the center of the northern and northwestern contacts is established. The shape of the body is reflected in the orientation of the clastic material, with dip angles varying from 30–60° near the contacts of the body (especially the southern ones) to 70–90° in its central part. It is necessary to note the considerable size of the fragments (up to 50–80 cm) and the purely tuffaceous character of the cement. At the same time, the latter is so firmly welded to the lithoclasts that, when the tuffs are destroyed, clasts with rims of cement are formed.

A substantial part of the area of the ancient apparatus is composed of effusives and volcanogenic-sedimentary rocks, which form a rather strongly dislocated (dip angles of the beds sometimes reach 70–90°) anticlinal fold. The paleovolcano is confined to its axial part.

Volcano IV (Fig. 2, г) is located 5–6 km southeast of volcano III. Its spatial position is controlled by a northwestern fault. The paleovolcano is characterized by the development of intrusive bodies, as well as by a very broad occurrence of secondary quartzites, which distinguishes it from the preceding ones. The central part of the apparatus is a bowl-shaped depression surrounded by ridges. The ring of the latter is about 3.5 km in diameter. Noteworthy in this respect is the radi-

...a radial system of the river network. All streams originate mainly at the foot of the ring ridge and flow in different directions. The entire structure is rounded in plan, with a transverse dimension of about 4.5–5.0 km.

The vent, about 500 m in diameter and circular in shape, is composed of porphyritic syenites. Its body has ruptured contacts dipping toward the center at an angle of 70–90°. In morphology it is close to a neck. On its southern and western sides albitites are recorded in the form of an ellipsoidal body measuring about 250 × 2500 m and having a northwestern orientation.

A fairly large body of secondary quartzites is confined to the central part of the apparatus. In addition, secondary quartzites, often together with feldspar neogenic formations, make up the overhanging sides of conical disjunctives within the southwestern part of the paleovolcano.

Subvolcanic rocks are represented by granophyre porphyries, which form concordant bodies with apophyses developed chiefly in the roof of the bodies. The sill type of occurrence of the granophyre porphyries determined their broad areal distribution.

The effusives that compose the area of the paleovolcano in the southwest are predominantly tuffaceous in nature. In addition to tuffs, horizons of porphyrites and sedimentary rocks are established. All these formations make up a rather complexly dislocated limb of an anticline, to the axial part of which the paleovolcano is confined. The northeastern limb of the fold is composed of rhyolite porphyries and their tuffs. At the same time, their calm and gentle (5–15°) plunge to the northeast is observed.

In addition to the paleovolcanoes characterized above, several more apparatuses are distinguished in the area. In particular, 4–6 km east of volcano II (Fig. 1), a remnant of monogenic nature is observed, the vent facies of which is represented by blocky lava breccias of andesitic porphyrites. The size of the blocks reaches 1–5 m across. The cement is lava. The rocks of the vent, together with the adjoining tuffs, are feldspathized and ocherized. The spatial position of the apparatus is determined by the same disjunctive that controls paleovolcanoes I and II.

At the extreme southeast of the area (Fig. 1) there is a complex structure which, in its morphological features, approaches volcano IV.

Thus, in the territory described, complex paleovolcanoes of the central type are widely developed. Their activity also accounts for the manifestation, chiefly, of acidic and subalkaline effusives of various facies (sheet, dike, sill, and vent). The spatial position of the paleovolcanoes is determined by sublatitudinal and northwestern faults.

Isometric, rounded apparatus forms in plan are a fairly widespread phenomenon. At the same time, features of the internal structure of ancient volcanoes are often clearly expressed geomorphologically (weakly dissected depressions, a radial river network, arcuate and ring ridges, etc.).

The presence of bodies of lava breccias of subalkaline composition, trachytoid phonolites, quartz porphyries, orthophyres, granophyre porphyries, agglomeratic lava breccias and tuffs, eruptive tuff breccias, secondary quartzites, as well as stock-like bodies of granodiorite intrusions, porphyritic syenites, and albitites, characterized by rupturing contacts with respect to the enclosing effusives, emphasizes the central, vent part of the apparatus.

The peripheral parts of the paleovolcanoes are “shaded” by ring structures, with which collapse breccias, feldspathic metasomatites, and also dikes of effusive and intrusive rocks are associated.

References

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