SOME FEATURES OF THE GEOLOGY OF DEVONIAN VOLCANOGENIC FORMATIONS IN ONE AREA OF THE SOUTHEASTERN SPURS OF THE SOLGON RIDGE

Yu. A. Fomin, V. G. Kryukov, V. L. Gavrilenko, V. K. Cherepnin

(Presented by Professor A. M. Kuzmin)

In 1966, in the indicated area, we studied volcanogenic formations known as the Byskar (Kop'ev, Chilan) series of the Lower–Middle Devonian \[4, 1, 10\]. Within these formations, along with basic rocks, acidic and subalkaline varieties are widely developed, which is highly characteristic of the mountain uplifts framing the Minusinsk trough, especially from the east \[6, 9\], and which, at the same time, has been rather poorly covered in the literature. Quite insufficient attention has also been devoted to the study of paleovolcanic structures with which volcanic activity is associated. A description of the compositional and structural features of the volcanogenic formations within a comparatively narrowly restricted area (Fig. 1) constitutes the main purpose of the present article.

The investigations carried out enabled us to subdivide the volcanogenic formations described into a number of members that differ in lithologic-petrographic features and in the sequence of deposition. In general, the studied section has the following form (from bottom to top):

1. Basic effusives, occupying considerable areas in the northern part of the region and represented by sheet-like bodies of olivine basaltic porphyrites, basalts, plagioclase-pyroxene and plagioclase porphyrites, interbedded in the lower parts of the member with thin horizons of tuff-conglomerates that likewise have a basic composition. Individual flows are characterized by heterogeneity of structure, expressed in the gradual upward replacement of massive varieties of the rocks by porous and then amygdaloidal ones, which fully agrees with the data of E. A. Schneider and B. I. Zubkus \[10\] on the basic effusives of neighboring areas of the Minusinsk intermontane trough. The thickness of the described rocks is on the order of 250–300 m.

Fig. 1. Schematic geological map of one of the areas of the southwestern spurs of the Solgon Ridge (compiled by V. G. Kryukov, Yu. A. Fomin, V. A. Gavrilenko): 1—tuffs, tuff breccias, eruptive breccias of trachytic porphyries; 2—quartz porphyries; 3—quartz-bearing porphyries and agglomerate tuffs; 4—alkaline rhyolitic porphyries and their tuffs, tuff breccias, tuff lavas; 5—tuffs and tuff conglomerates of trachybasalts; 6—ash tuffs and felsophyres; 7—basalt and andesite porphyrites; 8—dolerite and diorite porphyrites; 9—syenite porphyries, crystal porphyries: a) dikes, b) intrusions; 10—alkaline rhyolitic porphyries of subvolcanic appearance; 11—orthophyres; 12—tectonic disturbances: a) faults, b) large ring faults, c) other; 13—geological boundaries; 14—bedding elements: a) disturbances, b) fluidality and layering.

1. Coarse-clastic tuffs of basic composition. The rocks are of limited distribution, tending toward the western boundary of the area. Being represented by litho-crystal-clastic varieties, they contain, in the composition of the clastic material, along with andesite-basalts, basalts and, characteristically, trachybasalts, a significant amount of plagioclase. Bedding is absent in the indicated tuffs. Their thickness does not exceed 100 m.

2. Ash tuffs of acid composition, occurring with some unconformity upon the formations described above. Being very widely developed in the southern and especially in the central parts of the area, they are represented in the lower parts of the unit by pure ash tuffs with a characteristic finely knobby jointing, agglutinates, and pisolitic tuffs, alternating with cherry-gray oligophyric felsophyres. In the upper parts of the unit, crystal-clastic varieties of ash tuffs and agglutinates predominate, distinguished by an admixture of fragments of crystals of alkaline feldspars. These formations are often characterized by microbedding. The thickness of this unit reaches 300–350 m.

3. With a certain break, unconformably upon the ash formations lie greenish heterogeneous psephitic tuffs, tuffites, and tuffaceous sandstones with a total thickness of 10–15 m, not shown on the schematic geological map (Fig. 1). It should be noted that the indicated rocks are widely distributed in the areas adjacent to the east, where their thickness reaches 300 m.

4. Effusives of basic-intermediate composition, observed in the central and western parts of the area, form several sheet-like bodies of oligophyric basalt porphyrites, often with spheroidal jointing, and porous andesitic porphyrites with a total thickness of 120–150 m.

5. Acid effusives with an alkaline tendency are exposed as a broad belt, encompassing the central part of the area from the south and composed essentially of ash formations. They are represented by sheet-like bodies of compositionally persistent lilac-gray and cherry-red fine- to medium-porphyritic alkaline rhyolitic porphyries, in the lower parts of the unit interbedded with horizons of coarse-clastic bedded-taxitic tuffs, tuff breccias, and tuff lavas of the same composition. The thickness of this unit is 300–350 m.

6. Acid effusives, tending toward the southern boundary of the area and separated from the underlying alkaline rhyolitic porphyries by a thickness-persistent horizon of coarse-clastic (agglomeratic) heterogeneous tuffs, include reddish-brown, cherry-colored or cherry-gray quartz-bearing porphyries, felsites and felsite-porphyries, often having a “ribbon” structure. The thickness of the acid effusives together with the underlying agglomeratic tuffs is 300–350 m.

7. Tuffs of subalkaline composition, crowning the section of volcanogenic formations of the area, crop out at the surface in its central part and are represented by lithocrystal-clastic tuffs, tuff breccias, and eruptive breccias of trachytic porphyries. Their thickness reaches 150–200 m.

The effusives and pyroclastics described above, with a total thickness of 1500–1700 m, are cut by dikes and subvolcanic intrusions of varying composition and time of formation, some of which are comagmatic with the erupted varieties, while others represent considerably later formations.

The former include (from older to younger): diabases, alkaline rhyolitic porphyries, quartz porphyries, and orthophyres. Diaba-

...and diabase porphyrites compose dikes, varying in thickness, of northwestern strike, which cut through the basic effusives of the first member and correlate well with them both in petrographic composition and in time of formation. Hypabyssal alkaline rhyolite porphyries and granophyre porphyries, which are comagmates of the sheetlike alkaline rhyolite porphyries, form dikes that have an annular shape and bend around, from the south and north, the central part of the area, within which are concentrated isometric, in plan, bodies of quartz porphyries, representing the root parts of sheets of acid composition, in particular quartz-bearing porphyries.

The latter is reliably proved by the closeness of the petrographic composition of the one and the other. Their differences are reduced mainly to the abundance of phenocrysts and the comparatively high degree of recrystallization of the groundmass of the quartz porphyries; moreover, at lower hypsometric levels their crystallinity increases. The orthophyres composing the annular dikes are everywhere spatially associated with trachyte tuffs, as well as with the quartz porphyries that they cut.

Thus, within the volcanogenic formations of the Byskar Series of this area, in accordance with the views of M. A. Usov [7], it is expedient and entirely possible to distinguish extrusive, dike, and vent facies of rocks, assigning to the latter the quartz porphyries and eruptive breccias that compose the vent of an ancient volcano.

The post-Byskar dikes of crystallite porphyries, syenite porphyries, labradorite porphyrites, and diorite porphyrites established in the area cut the rocks of the series. In the spatial distribution of the young dikes a certain regularity is observed, consisting in their tracing of sublatitudinal tectonic disturbances.

Microscopic study of the volcanogenic formations indicates that the degree and character of alteration of all varieties of the extrusive, dike, and vent facies are approximately the same and correspond to the diagenetic phase of their state [7]. In addition, alterations caused by the solfataric-fumarolic activity of the volcano are manifested here quite widely.

The age of the described volcanogenic rocks is determined on the basis of comparison of the section presented with similar sections of Devonian formations in neighboring areas [4, 9, 10]. There is no need for us to dwell in detail on this question, which has been sufficiently covered in the geological literature [2, 8, 9, 10]. We shall only note, with reference to I. V. Luchitskii [4, 9], that analysis of the data available in the literature makes it possible to establish with sufficient confidence the boundaries of these formations within the Lower Devonian—Eifelian Stage of the Middle Devonian. Comparing this section with the section given by E. A. Schneider and B. I. Zubkus [10] for the North Minusinsk and Sydo-Erba depressions, it seems possible to us, by age, to correlate members 1, 2, and 3 with the Imir Formation ($D_1 im$), member 4 with the Shunet Formation ($D_1 sch$), and members 5, 6, 7, and 8 with the Ton Formation ($D_2 tn$). However, the content placed by the cited authors in these formations does not accord with our ideas.

It is most expedient to unite the volcanogenic formations of this area, in accordance with the views of Yu. A. Kuznetsov [3], into trachybasalt and trachyliparite formations, corresponding to the basaltic and porphyry terrestrial-volcanogenic formations in the conception of I. V. Luchitskii [4] and A. A. Mossakovskii [6]. The first includes the rocks of the two lower members, and the second all the remaining formations.

Very interesting material for clarifying the type of volcanism and the cha-

The character of its change over time is provided by the study of the structural features of the area. On the basis of orientation and time of formation, all the faults here have been divided by us into several groups.

The oldest of them is a major northwestern fault, having the character of a deep-seated fracture that was initiated as early as pre-Devonian time. It is precisely to this fracture that both bodies of quartz porphyries of the vent facies are confined. It is appropriate to note that the sheets of basic effusives lying at the base of the section, and the diabase dikes associated with them, are oriented in the same northwestern direction.

Faults of the ring-fault type, very clearly expressed geomorphologically, were initiated considerably later, already in Devonian time. Their formation is evidently associated with the origin, development, and then destruction of a volcanic edifice of the central type. These faults, as a rule, control ring dikes of alkaline rhyolite porphyries, granophyre porphyries, and orthophyres. This indicates that by the time of the appearance of mass outpourings of alkaline rhyolite porphyries the volcanic edifice already existed. The ash formations lying stratigraphically below the rhyolite porphyries are apparently already the result of its activity.

Sub-latitudinal tectonic zones of deep origin, manifested by intense crushing, foliation, and fracturing of the rocks, formed still later, probably in Late Lower–Middle Devonian time, as is indicated by the tracing of these zones by younger dikes.

Submeridional and northeastern faults are the youngest in this area. As a rule, no manifestations of magmatism are observed in association with them.

The material presented above permits the following conclusions:

1. Diagenetically altered volcanogenic formations of the effusive, dike, and vent facies of the Byskar series, dated to the Lower Devonian–Eifelian stage of the Middle Devonian, are combined into a lower—trachybasalt—and an upper—trachyliparite—formation, characteristic of orogens, chiefly geosynclinal mobile zones.

2. The formation of the rocks of the trachybasalt formation is associated with a fissure type of volcanism, as evidenced by the considerable thicknesses and characteristic composition of the outpourings, the elongated shapes of the bodies and their persistence over considerable areas, as well as the subordinate amount of pyroclastic material in the products of volcanism. The development of fissures in this case is most probably associated with the initiation of the oldest deep-seated faults, which in this area have a northwestern strike.

3. The formation of the rocks of the trachyliparite formation should be linked with the activity of a volcanic apparatus of the central type, spatially confined to the northwestern fault. That a remnant of such an apparatus is indeed present is indicated by the occurrence of bodies of quartz porphyries that fill the vent and are characterized by intrusive contacts with respect to the enclosing rocks; by the presence here, around the supposed vent, of ring structures controlling dikes; and also by the occurrence of agglomerate tuffs, tuff lavas, and eruptive formations. In addition, this is confirmed by the very widespread manifestation of solfataric-fumarolic activity, which affected rocks of both the effusive and subvolcanic facies.

References

1. A. I. Anatol’eva. On the stratigraphic position of Devonian volcanic rocks of the Sayan–Altai folded region. In: Problems of Paleovolcanology, Paleovolcanology Laboratory Transactions, issue 1, Alma-Ata, 1963.

2. G. A. Ivankin. On the stratigraphy of the Devonian of the western margin of the Minusinsk depression. Proceedings of Tomsk State University, vol. 146, geological series, 1960.

3. Yu. A. Kuznetsov. Principal Types of Magmatic Formations. Nedra, 1964.

4. I. V. Luchitskii. Volcanism and tectonics of the Devonian basins of the Minusinsk intermontane trough. Publishing House of the Academy of Sciences of the USSR, 1960.

5. E. F. Maleev. Volcaniclastic Rocks. Gosgeoltekhizdat, 1963.

6. A. A. Mossakovskii. Tectonic Development of the Minusinsk Basins and Their Mountain Framing in the Precambrian and Paleozoic. Gosgeoltekhizdat, 1963.

7. The Basic Ideas of M. A. Usov in Geology (collection of articles). Academy of Sciences of the Kazakh SSR, 1960.

8. A. G. Sivov. Elements of the stratigraphy and tectonics of the Devonian deposits of the Minusinsk depression. Proceedings of Tomsk State University, vol. 132, geological series, 1954.

9. Comparative Paleovolcanology of Southern Siberia and Eastern Kazakhstan. Nedra, Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1966.

10. E. A. Shneider, B. I. Zubkus. Stratigraphy of the Lower and Middle Devonian deposits of the North Minusinsk and Sydo-Erbinsk basins. Materials on the Geology of Krasnoyarsk Krai, issue 3, Krasnoyarsk, 1962.