HISTORY OF THE FORMATION OF THE TALANOVSKY GRABEN

(NORTHERN SPURS OF THE KUZNETSK ALATAU)

V. G. KRYUKOV, V. Z. MUSTAFIN, N. S. LYKHINA

(Presented by Professor A. M. Kuzmin)

“From the nature and stratigraphy of volcanic products it is possible to reconstruct the type and qualitative course of eruptions.”

(A. Ritman, 1964)

The Talanovsky graben is located in the axial part of the northern spurs of the Kuznetsk Alatau [12], within the Pervomaisky Sinian–Cambrian intermediate volcanic zone [8]. The principal features of the geological structure of this feature were considered by us earlier [9]. To address the question posed, it is necessary briefly to recall them.

The Talanovsky graben is a tectonic wedge 1.5–4 km wide, extending in a meridional direction for 25 km and bounded on the west and east by branches of the regional Tersin zone of folding [4]. It is framed by Sinian and Lower Cambrian sedimentary-effusive, intensely dislocated and metamorphosed sequences. In individual areas (outside the graben) these sequences are cut by intrusions of plagiogranites and granodiorites of Cambrian–Ordovician [12] age, belonging to the Martayga complex.

The history of the geological development of the region in the pre-Devonian period is not considered by us, since it has been covered quite fully in the works of A. N. Churakov [13], V. A. Kuznetsov [5], A. A. Mossakovsky [8], T. N. Ivanova [3], Yu. D. Skobelev [11], and others.

Within the area of the graben there are widespread effusive formations of the Telbes series, subdivided by us into Lower and Middle Devonian sequences. The Lower Devonian effusives are represented by rocks whose composition changes from basic (lower parts) to alkaline (upper parts of the sequence). The Middle Devonian formations have the composition of basaltic and andesitic porphyrites. These sequences are separated by an interruption in the process of volcanism, recorded by the formation of red-colored and green-colored terrigenous rocks.

Dike and intrusive formations are abundant in the graben; moreover, the Lower Devonian sequence is cut by intrusions of quartz-free syenites and by dikes (from older to younger) of labradorite porphyrites, diabases, and diabase porphyrites. The Middle Devonian sequence is intruded—

are cut by bodies of quartz syenites (granosyenites) and by dikes of syenite-porphyries, microsyenites, microdiorites, as well as spessartites and diorite porphyrites. At the same time, the granosyenite intrusions are cut by dikes of the latter two types.

The age of the effusives in the graben has not been proven and is determined on the basis of comparison of the Talanovsky section with similar sections of Devonian formations in neighboring areas, where the age of the strata is substantiated by fauna or flora. Study of the geological features of the effusive section has allowed us to distinguish within the area of the graben a trachyandesite formation (Lower Devonian) and an andesite-basalt subformation (Middle Devonian) [6]. The history of the development of the volcanism that led to their formation is considered below (scheme of the geological development of the Talanovsky graben).

The features of the geological development of the area described were wholly determined by the confinement of the Talanovsky graben to the regional Tersin tectonic zone. The latter was apparently established as early as the Cambrian (Salair stage), when the Kuznetsk Alatau represented a geoanticlinal structure, on whose flanks and area, at its junction with geosynclines, regional deep faults arose [5]. The Caledonian cycle in the history of the Tersin zone, as in the Sayan–Altai region as a whole, was of low activity and consisted in denudation and dislocation of Sinian–Cambrian formations. The intensifying tectonic life of this submeridional zone by the beginning of the Devonian led to subsidence of the territory and to an outbreak of volcanic activity caused by the development of deep tectonic faults.

In Early Devonian time the structure of the graben was initiated, and lavas of basic composition were poured out onto the eroded surface of the Sinian–Cambrian formations, forming a series of flows. The first stage of volcanism led to the formation of labradorite and labrador-pyroxene porphyrites, as well as their clastolavas, lapillistones, pelitic and gravelly tuffs. The composition of all the listed rock varieties is more or less the same and corresponds to a basic one. The thickness of the effusive bodies of basic composition is rather great (100–140 m) and their extent considerable (18–20 km). It apparently formed over a long period of time, when the magma in the chamber was differentiating, acquiring an intermediate composition. A fissure character of eruptions, especially characteristic of basic magmas [10], is probable. It is not excluded that the paths for the outpouring of the basic lavas were tectonic dislocations (especially the western one) bounding the graben.

The eruption of a significant volume of magma, corresponding to the volume of the basic effusive sequence, caused a decrease in the internal energy of the magmatic chamber, which led to a certain quiescence of the volcanic apparatus. During the period of quiescence, apparently only weak explosive activity was manifested. Its result was the accumulation of a rather thick horizon of tuffaceous material, confined to the eastern flanks of the sequence of basic effusives in the central part of the graben along almost its entire length.

The first stage of volcanism ended with the emplacement of dikes of labradorite porphyrites along submeridional fractures, and then of diabases and diabase porphyrites along northwestern dislocations.

The subsequent internal life of the volcanic chamber promoted a further rise of the magma remaining in it. However, since the energy of the chamber had been substantially exhausted, the magma remained insufficiently…

directly in the fissure vent. Only in individual sectors did the magma break through the basic effusives, sometimes entraining their xenoliths. The solidification of the magma that remained in the vent, confined to the western tectonic boundary of the graben, contributed to the formation of rocks of intermediate composition—plagioclase–hornblende porphyrites. Thus, the latter represent a vent facies of the erupted basic effusives.

In general, the history of the area’s development during this period is characterized by relative quiescence, except for weak explosive activity, as evidenced by thin horizons of clastolavas and a comparatively thick horizon of agglomerate lavas.

After the formation of the sequence of basic effusives and their vent facies, represented by rocks of intermediate composition (plagioclase–hornblende porphyrites), there began a period of relative quiescence of the volcanic center. During this period, the magma remaining in it apparently underwent further differentiation, as a result of which, becoming enriched in alkalies, it acquired an alkaline composition. At the same time, the previously formed sequence of basic effusives probably underwent denudation, as indicated by the almost complete absence of tuffs and lava breccias of basic composition over a considerable area of the graben.

Tectonic activity in the area continued even during a certain quiescence of the volcanic center. With the formation of plagioclase–amphibole porphyrites, the pre-existing fissures proved to be sealed. As A. Rittmann notes [10], in most cases a fissure serves for the ascent and outpouring of magma only once. A new eruption occurs along a new fissure. Therefore subsequently, along with the renewal of large submeridional fissures, new tectonic dislocations of submeridional and northeastern trends formed, which served for the renewed ascent of magma.

The second stage of volcanism in the graben area is characterized by the fact that magma of alkaline composition penetrated along the newly formed fissures, its extrusions occurring differently in different sectors: in some places products of alkaline magmas are entirely absent, while in others they appear more or less widely. Thus, in the axial part of the graben, in its northern half, alkaline lava forms a dike-like body of trachytic porphyries of submeridional strike, whose structural-textural features allow it to be assigned to subvolcanic facies. Alkaline lavas also form extrusions near the eastern boundary in the central and partly southern parts of the graben. In these sectors, comparatively thin lenses of clastolavas of alkaline composition are recorded, as well as their agglomerate, psammitic, and pelitic tuffs, especially characteristic of the southern half of the area described. The formation of the latter represents a manifestation of a comparatively calm explosive process.

Near the eastern boundary of the central part of the graben, as well as at its northern end, both extrusive (trachytic porphyries and their clastolavas) and subvolcanic (quartz-free syenites) facies of alkaline rocks are developed. Part of the alkaline magma probably did not reach the surface at all and, forming stock-like bodies, cooled at shallow depth. Subsequent erosion exposed them, and they are now recorded as small stocks of coarse-grained (pegmatoid) and medium-grained quartz-free (alkaline) syenites.

These same rocks evidently also represent the vent facies of the alkaline effusives.

Table 1

Scheme of the geological development of the Talanovsky graben

The final products of the development of alkaline magma are apparently sill and dike bodies of trachytoid phonolites, recorded both in the central and in the northern parts of the graben, where spatially they tend toward the eastern and western boundaries, respectively.

If, for the sequence of effusives of basic composition, the formation of the vent facies is preceded by a comparatively short break, during which the magma underwent differentiation, then in the formation of the vent facies of alkaline rocks no such break is established. This is confirmed by the following facts: 1) the rather gradual transition from clastolavas and lavas to trachyte porphyries, and then to alkaline syenites; 2) the similarity in composition of the noted series of rocks, which differ only in structural-textural features.

All the rocks described above are united into the trachyandesite formation, whose formation is completed by the emplacement of dikes of trachytoid phonolites.

Probably, the volume of alkaline magma that was poured out, ejected, and released in the form of solfataras and fumaroles is somewhat less than the volume of expended magma of basic and intermediate composition. As a result of these two major eruptions, the magmatic chamber was emptied to such an extent that its roof part, experiencing a colossal load, collapsed. As a result, in the north of the graben, apparently, a caldera formed in the place of a small “bocca.” The process of its formation, caused by subsidence (collapse) of the roof part of the chamber, probably began after the discharge of lavas of intermediate composition from the chamber. At the same time, not only near-meridional but also relatively large tectonic disturbances of northwestern strike arose. Thus, a division of the graben into a number of blocks is outlined, the main ones being the northern, central, and southern blocks.

Each block is distinguished by a certain peculiarity of geological history. The northern block is characterized by the most complex development and, correspondingly, structure, where both stages of volcanism are fully manifested.

Apparently, by the time the formation of the trachyandesite formation was completed, corresponding to the end of the Lower Devonian, the tectonic life of the territory became more active, which led to the emergence of near-meridional tectonic disturbances that contributed to the further shaping of the described area as a negative (graben) structure. However, because of the depletion of the magmatic chamber, active tectonic activity did not lead to new eruptions. In the history of the development of volcanism in the graben, as in similar structures of other areas of the Kuznetsk Alatau, [1] a prolonged break occurred at the boundary of the Lower and Middle Devonian. To this time, apparently, belongs the manifestation of relatively weak plicative tectonics, which, together with the intensive processes of denudation that developed during the break, determined the asymmetry of the profile of the Devonian effusives. The latter acquire an eastern dip, the angles of which gradually increase from gentle to steep in the direction from west to east.

The duration of the break is indicated by the accumulation of a tuffaceous-sedimentary sequence, especially thick and widely developed in the north of the graben within the caldera. In the central and southern parts of the graben, the thickness and areas of distribution of terrigenous material are insignificant. In time, the break apparently corresponds to the beginning of the Middle Devonian.

At the base of the tuffogenic-sedimentary sequence lie conglomerates containing pebbles of earlier-formed effusive rocks of basic, intermediate, and alkaline compositions. Toward its roof, the size of the fragments decreases to psammitic and psephitic, and throughout the entire section of the sequence an admixture of tuffogenic material is noted. The presence of the latter indicates a weak manifestation of explosive activity.

The coloration of the rocks of the sequence is mainly red-colored; however, gray and grayish-green interbeds of psammitic sandstones are sometimes observed in it. They, as well as poorly preserved floral remains found in the north of the graben, indicate formation of the sequence under lagoonal conditions. Apparently, at the beginning of the Middle Devonian, the sea that existed at the beginning of Eifelian time in the western part of the Minusinsk intermontane trough [7] penetrated this far to the north of the Sayan-Altai region.

The formation of the tuffogenic-sedimentary sequence within the Talanovsky graben also indicates that at the end of the Lower and the beginning of the Middle Devonian the graben, as a negative structure, had already been formed. As a result, the material undergoing denudation was not carried beyond the area but accumulated in the newly formed negative structure.

A new (third) stage of volcanic activity is manifested in the Middle Devonian (Eifelian?). It leads to the formation of rocks of the andesite-basalt subformation. At this time the magmatic source supplies new portions of basaltic magma, but of a more basic character than at the beginning of volcanism in the graben. This is evidenced by the fact that, in the composition of the rocks formed at the beginning of the Middle Devonian (basalt-pyroxene-olivine porphyrites and aphyrites), olivine, pyroxene, and biotite make up as much as 20--40%.

The forms of occurrence of the basaltic magma of the third stage are varied: in the northern block of the graben it is represented by sills; in the central block it forms fissure vents and flows; in the southern block, fissure vents, sills, and flows. Numerous apophyses, expressed as low-thickness, not very extensive, pinching-out dikes, often branch from the basaltic sills.

A somewhat later phase in the manifestation of basaltic magma is the formation of dike and sheet bodies of andesite porphyrites. Not only temporal but also spatial separation of these from the basaltic porphyrites is indicated. True, the differentiation of the magma in this case was not so complete, and the differences in the composition of these rocks are insignificant. Andesite porphyrites formed in almost all parts of the graben.

The concluding period in the development of the territory described is characterized by the renewal of submeridional and the formation of northwestern tectonic disturbances. The final phase of magmatism, and possibly an independent cycle, was the emplacement of a post-Eifelian (?) intrusive complex of acid composition (granosyenites). Its intrusions are located in earlier-formed regional northwestern fractures.

The dike complex, formed simultaneously with the formation of the granosyenites, is represented (from older to younger) by microdiorites (submeridional), microsyenites (the same), and syenite-porphyries (northwestern). The noted dikes and the intrusions of granosyenites are cut by northwestern tectonic disturbances, along which the youngest dikes in the graben, of spessartites and diorite porphyrites, were emplaced. Probably, with their formation the history of the tectonomagmatic development of the described territory comes to an end, as—

since, in crossing the tectonic boundary of the graben, these dikes undergo neither displacement nor crushing.

The absence of younger formations in the territory described does not allow one to judge the history of its further development.

Conclusions

1. Volcanic activity within the graben area, manifested in three stages, developed under continental conditions and is closely connected with the deep faults bounding the structure.

2. Eruptions within the Talanovsky graben in Early Devonian time indicate a continuous evolution of magma from basic to acidic and ultra-alkaline compositions (trachyandesite formation).

3. Middle Devonian volcanism is characterized by a repetition of the Early Devonian evolution of magma, leading, however, to the formation of products of a different composition (andesite-basalt subformation).

4. The forms of occurrence of the effusives of the described formations and their comparatively gentle bedding indicate the predominance of a fissure type of eruption, with less developed eruptions of central-type volcanoes.

In conclusion, we express our deep gratitude to Associate Professor N. N. Solov’ev of the Moscow Geological Prospecting Institute for the consultation he provided during the collection of material.

References

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3. T. N. Ivanova. History of the geological development and magmatism of the Kuznetsk Alatau in the Paleozoic. In: Problems of Magmatism, Metamorphism, and Ore Formation. Gosgeoltekhizdat, 1963.

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10. A. Rittmann. Volcanoes and Their Activity. Mir, 1964.

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13. A. N. Churakov. Kuznetsk Alatau. The History of Its Geological Development and Its Geochemical Epochs. Publishing House of the Academy of Sciences of the USSR, 1932.