GEOLOGY AND GEOCHEMISTRY OF DEVONIAN VOLCANOGENIC DEPOSITS IN THE UPPER REACHES OF THE MALY TASHTYP AND BALYKSU RIVERS

V. I. GERYA

(Presented by the seminar of the Department of Petrography)

Middle Paleozoic volcanogenic deposits of the southwestern marginal part of the Minusinsk intermontane trough form, in the basin of the Tashtyp and Balyksu rivers, a distinctly isolated structure of meridional strike, separated from the strongly dislocated terrigenous-carbonate sequences of the Upper Proterozoic—Lower Cambrian by regional faults of long development (7). This block, known as the Balyksu graben-syncline (7), the Balyksu depression (4), and the Balyksu intermediate zone of the South Minusinsk intrageosynclinal trough (6), is one of the fairly independent elements framing the Minusinsk depression.

The study of the structure of the effusive sequence in the upper reaches of the Balyksu and Tashtyp rivers, carried out by the author in recent years, has made it possible to clarify the internal structure of the Balyksu block, which is an anticlinal structure of meridional strike, connected through intermediate folds with the more gently lying Devonian deposits of the Minusinsk depression.

The absence of organic remains, the very diverse material composition, the complex tectonic structure, and thicknesses reaching in places 2500–3000 m make the stratification of these formations difficult. Nevertheless, as a result of investigations in recent years concerning both the Minusinsk depression proper and its western framing, published in [1, 4, 5] and known from the works of Yu. V. Makarov, I. K. Kokodzeev, and others, it has become possible to correlate more confidently the volcanogenic deposits of this area with the complex of synchronous sediments of the depression. In particular, immediately south of the area, in the middle part of the Devonian effusive sequence, Yu. V. Makarov distinguishes the Imek Formation, composed of variegated, greenish-gray sandstones, siltstones, gravelites, black limestones, and marls. The deposits combined into the mentioned formation may be preliminarily correlated with the upper part of the Kamenka Formation, distinguished within the area described (see below), in which, along with effusives, a substantial role is played by psammitic-psephitic sediments that occasionally contain thin beds of clayey limestones.

Correlation of individual sections, petrographic and geochemical features, and changes in the material composition of the formations by

The thicknesses of the effusive sequence make it possible to distinguish two major stages (megacycles) in the history of volcanic activity. Of these, the older encompasses the period of accumulation of the deposits assigned to the Oya and Kamenka formations, which apparently belong to the Koblenzian stage. During the second stage, probably corresponding to the Eifelian Age (or its first half), a thick sequence of volcanogenic rocks of the Tarlan Formation accumulated. The initial stage of each stage is, in general, characterized by outpourings of products of relatively weakly differentiated basaltic magma. In the middle and final stages of the megacycles of volcanism, owing to deeper differentiation of the magma, effusive sheets fairly varied in composition were deposited, with a noticeable admixture of layers of pyroclastic material.

In the second half of the early and late stages of effusive activity, individual cycles of eruptions are indicated; most of them begin with outpourings of products of basic composition, later replaced by accumulation of effusives of somewhat lower basicity, and not infrequently end with the formation of acidic differentiates of the basic magma. The volcanogenic sequence formed under continental conditions; the deposition of coarse-clastic sediments with interbeds of siltstones, tuffs, and, rarely, limestones took place during the period of maximum subsidence of the land. The most favorable in this respect were the periods at the boundaries of individual cycles or pauses in volcanic activity.

The cyclicity of the structure of the volcanogenic sequence is confirmed by the nature of changes in the concentration of individual elements, the content of which is closely related to the composition of the rock.

In the adopted subdivision scheme, the stratigraphic boundaries of the distinguished (from bottom to top) Oya, Kamenka, and Tarlan formations coincide with the boundaries of the largest cycles of effusive magmatism.

The limited number of chemical analyses of effusive rocks from the southern part of the area, known from the works of G. V. Polyakov and A. E. Teleshev, Yu. V. Makarov, and the author, and the absence of their precise stratigraphic control, allow conclusions to be drawn only about the petrochemistry of the complex as a whole. Judging from the available analyses, it belongs to a series intermediate between alkaline and alkaline-earth magmatic associations, with a noticeable pantelleritic tendency in the variation of the parameter \(a\). This complex approaches the trachyandesite group of formations of Yu. A. Kuznetsov [3], to which the Devonian volcanogenic series of the Minusinsk intermontane trough [5] belongs, especially its association close to the trachyandesite one. In contrast to the latter, the most basic rocks of the complex described have a noticeably lower alkalinity, increasing with an increase in \(s\). Conversely, the effusives of intermediate composition, represented by the trachyandesite group, are distinguished by a high value of \(a/c\), which in the acidic differentiates again decreases somewhat in comparison with analogous formations of the series of the Minusinsk intermontane trough.

Oya Formation

The base of the volcanogenic sequence is exposed in the south of the area, on the right slope of the valley of the Oya River, where, on the eroded surface of sandy-shaly deposits of the Lower Cambrian, red-colored sandstones lie with sharp angular and azimuthal unconformity; these are overlain by a series of effusive formations of basic composition.

The lower subformation is represented by a member of the aforementioned red-colored sandstones, which compose the flattened core of the Oya anticline-

... The base of the member, 25–30 m thick, is composed of gravelly sandstones with a thin layer of basal conglomerates containing flat, well-rounded pebbles of the underlying shales, limestones, quartzites, and, more rarely, variegated sandstones and siltstones. Above, conformably, lies a monotonous member of brown-lilac bedded polymictic sandstones with interbeds of siltstones and gravelites. In its upper part this member contains an interbed of fine-pebble conglomerates with pebbles of plagiogranites, acid effusives, and variegated sandstones.

The thickness of the subformation reaches 180–200 m. The subformation may be correlated with the lower terrigenous member of the Chilan volcanogenic series of A. I. Anatol’eva [1] and with the lower parts of the Chilan Formation of N. A. Belyakova and V. S. Meleshchenko [5].

The upper subformation, which combines the volcanogenic deposits of the initial period of the early megacycle of magmatism, occupies mainly the axial parts of the Balyksu and Oya anticlines and is represented by a monotonous sequence of basic effusives, among which, in the area of the Oya River, sheets of brown olivine basalts, porphyrites, and their lava breccias play the principal role. An almost complete absence of amygdales in the rocks is typical. In the basin of the Tashtyp and Balyksu rivers the subformation is composed predominantly of sheets of diabase porphyrites and, more rarely, olivine basalts. In the upper part of the subformation, sheets of lilac andesine-labradorite and labradorite porphyrites are sometimes observed. In the roof of the sequence, local sheets of orthophyres and plagioporphyres are occasionally present. Clastic rocks are not typical of the subformation. The thickness of the subformation west of Khayleol apparently exceeds 700–900 m; in the lower reaches of the Oya River its full thickness decreases to 350 m.

Kamenskaya Formation

The deposits of the formation, which make up the limb parts of the Balyksu and Oya anticlines, formed during the middle and final stage of the early phase of effusive activity. In addition to magmatic products, terrigenous, chiefly continental, sediments play an important role in the composition of the formation; these tend toward the middle and upper parts of the section of the sequence.

A distinctive feature of the material composition of the formation is the appreciable development within it of lilac-brown labradorite porphyrites and their lava breccias, as well as acid effusives. The latter, in the area of the mouth of the Dresvyanka River, constitute somewhat more than 20% of the thickness of the section. No less typical is the presence of sheets of dark-gray dense olivine diabases (dolerites) with a characteristic microdoleritic structure. Diabases, diabase porphyrites, and varieties of effusives intermediate to average composition are somewhat less widespread. Among the acid differentiates of basaltic magma, felsites, orthophyres, plagioporphyres, and trachyandesites are observed; they form sheets in the roof of individual volcanogenic members, at whose base sheets of labradorite porphyrites are usually found. Beds of conglomerates occurring in the middle part of the formation contain numerous pebbles of granitoids, of various effusive rocks of acid composition similar to the effusives of the Teya region [8], of basic effusives, sandstones, and siltstones. North of the settlement of Khayleol, in pebbles of shales from conglomerates, Yu. V. Indukaev found remains of brachiopods of the genus Cyrtospirifer; here also, in the upper parts of the subformation, there are thin interbeds of gray limestones. According to Yu. V. Indukaev, on the basis of preliminary data, the age of the fauna is estimated as ranging from the lower part of the Upper Devonian to the lower part of the Middle Devonian.

The thickness of the suite varies from 1300 m in the south of the area to 300–400 m north of Khaileol.

The upper part of the suite can be correlated with the Imek suite, distinguished by Yu. V. Makarov in the middle part of the Devonian volcanogenic sequence immediately south of the area. In this connection it is necessary to take into account the conclusions of A. I. Anatol’eva [1] concerning the probable Early–Middle Devonian age of the Imek suite.

Determination of the absolute age of the felsites in the top of the suite yielded 292 million years (bulk sample; determination by V. M. Kisenko, Central Laboratory of ZSGU), which, apparently, indicates a significant loss of argon by potassium feldspar of the rock.

Tarlan Suite

The volcanogenic deposits of the late megacycle of magmatism, assigned to the Tarlan suite, are situated on the limbs of the Balyksu and Oya anticlines and in the core of the Tatyk synclinal structure. It is necessary to emphasize the sharply predominant lilac and lilac-brown coloration both of the effusive rocks and of the terrigenous-carbonate rocks of the suite, as well as the extremely widespread development of amygdaloidal varieties of effusives.

Lower sub-suite unites deposits of the first half of the late megacycle of volcanism, represented by a thick sequence of relatively weakly differentiated effusives of basic composition, with individual beds and lenses of terrigenous sediments. The effusive rocks are represented by numerous flows of amygdaloidal lilac-brown and dark-lilac, more often aphanitic or very fine-crystalline olivine basalts, porphyrites, labrador and andesine-labrador porphyrites and their lava breccias; more rarely, by confluent diabases, plagioclase porphyrites close to intermediate composition, and also plagioporphyres confined to the top of the sub-suite. Lenses of conglomerates at the base of the sub-suite, overlain by persistent beds of sandstones, tuffaceous sandstones, and siltstones, contain moderately and poorly rounded pebbles of the underlying rocks, chiefly felsites of the Kamensk suite. The thickness of the sub-suite, reaching 825 m at the mouth of the Dresvyanka River, decreases in the south of the area to 300 m.

Upper sub-suite. Within the area, the section of the volcanogenic deposits is completed by a series of effusive flows interbedded with rather thick packets of terrigenous rocks. Among the effusives, lilac-brown lavas and lava breccias of basic composition predominate, with subordinate development of olivine diabases, andesine-labrador and andesine porphyrites. Diabases, labrador porphyrites, and also acidic varieties of effusives are not very characteristic of the sub-suite. In the north of the area, where sedimentary rocks make up about 40% of the thickness of the sub-suite, its exposed part is composed of three rhythmic packets, at the base of which lie red-colored polymictic psammitic-siltstone rocks. The middle part of the packets is represented by flows of basic-composition effusives of varying thickness, and the top by thin flows of lilac-gray andesine porphyrites or plagioporphyres. The thickness of the sub-suite in the basin of the Oya River reaches 800–1000 m and decreases in the north of the area to 450–550 m.

The deposits of the suite may be regarded—taking into account the correlation of part of the Kamensk suite with the Imek suite of Yu. V. Makarov—as an effusive facies of the Tolochkov suite of N. A. Belyakov and V. S. Meleshchenko [5].

Geochemical Features of Devonian Effusive Rocks

Spectral analyses carried out by I. G. Koryakova and N. I. Erofeev (TsGFE KGU) established Ca, Mg, Cu, Zn, Co, Ni, Cr, Mn, Ti, and V in all samples of volcanogenic rocks (90 samples); in most samples, in addition, Zr, Pb, B, and Ga are present, and Sr and Mo occur considerably more rarely.

Analysis of variations in the contents of individual components makes it possible to establish the relationship of their concentration to the material composition of the sample; in particular, the elements of the first group show a distinct positive correlation with the basicity of the rock. The nature of the vertical distribution of a number of elements indicates cyclicity in the fluctuations of their contents through the section, which, in turn, confirms a definite regularity in the change in composition of the products of effusive activity over time.

On the basis of the range of concentration variations and the degree of association of the component with the composition of the rock, several groups of elements may be outlined; among them the pair Ca, Mg stands out by having the maximum ranges—excluding Zn—of variation in contents (0.1–10.0% for Ca and 0.02–5.0 for Mg, respectively, in acidic and basic varieties of effusive rocks). Thus, the quantity of these rock-forming elements, closely connected with the composition of the rock, is significantly differentiated and quite convincingly reflects the regularities in the change of eruption products over time. Close to this group is zinc, which is noticeably correlated with calcium and is distinguished by the broadest range of concentration fluctuations (on average from 0.02 to 0.2%), which also definitely depends on the composition of the sample.

The behavior of the second group of elements—Ni, Cr, Co—is to a considerable degree governed by the composition of the enclosing rock; however, the change in their contents is limited by narrower ranges: less than 0.001–0.02% for Ni, 0.001–0.05% for Co, and traces–0.02% for Cr. The close mutual relationship of these components indicates the well-known commonality of their geochemical history.

Ti, Mn, and V are characterized by a comparatively stable distribution; only in felsites, orthophyres, and plagioporphyrites does their content decrease noticeably, while some increase in Ti is noted only in individual samples of labradorite porphyrites.

Copper, despite a known slight increase in its amount with increasing basicity of the rock (2), does not show a definite relationship to the composition of the effusive rocks. Individual local maxima of Cu, as well as Zn, are possibly connected with postmagmatic and polymetallic mineralization, which is extremely typical of these formations.

The geochemical column presented, unfortunately, covers only the large “middle” part of the section of the volcanogenic sequence; therefore, the most general geochemical features of the initial and final stages of the Middle Paleozoic effusive magmatism of the region are described from data from a limited number of samples taken from other sections and correlated with the data. The distribution of the elements mentioned in the effusive rocks of the Oya Formation in the basin of the Oya River in the southeastern part of the region somewhat resembles that in rocks of the lower subformation of the Tarlan Formation, which is consistent with a certain closeness in the general composition of the complex of these deposits, represented by products of the initial periods of the early and late megacycles of magmatism. Only a slight overall decrease in the contents of Mg, Ni, Co, and Cr in the rocks of the first half of the late megacycle may be noted in comparison with the corresponding complex of the early stage.

The relatively weak differentiation of magma in the early periods of each stage of effusive activity is confirmed by the small ranges of variation in the contents of elements in the rocks of the lower subformation of the Tarlan Formation, especially Ti, V, and partly Cr, Ni, Co, and Mg. A sharply differentiated distribution of components, with numerous extreme values of their concentrations in the complex of deposits of the late periods of the metacycles, is well illustrated by the character of the geochemical column of the Kamenka Formation and the upper subformation of the Tarlan Formation, which include both acidic and the most basic varieties of effusives.

In the section of the Kamenka Formation, intervals are distinguished that are composed of effusive products of three independent eruption cycles. The lower part of these intervals is usually represented by covers of labradorite porphyrites, characterized by maximum contents of Mg, Cr, Ni, Co, and partly Zn. Higher in the section, owing to the decrease in the basicity of the effusives, the concentration of these components declines to minimum values in orthophyres, plagiophyres, and felsites, which complete the volcanic cycle. Maxima of Ca, as a rule, are confined to the middle part of the intervals. A certain analogy may be noted in the character of component distribution and in the structure of the upper subformation of the Tarlan Formation and the base of the Kamenka Formation.

The complex of products of the first half of the second metacycle (the lower subformation of the Tarlan Formation) differs from the underlying deposits by its comparatively homogeneous composition, with less clearly expressed cyclicity. The lesser contrast in the composition of the effusives corresponds to narrower average ranges of variation in the contents of V, Ti, Ca, and Mg. Anomalously high Zn concentrations in the clastic deposits of the base and upper part of the subformation are apparently associated with hydrothermal mineralization.

Among the numerous geochemical rhythms that reflect a certain periodicity in the history of volcanism, individual intervals stand out with distributional features of certain components that are peculiar only to them. In this respect, the upper part of the Kamenka Formation is the most individualized, with a stable, clearly expressed opposite tendency in the vertical distribution of Ti and Ca in the middle part of the interval and their positive correlation in the remaining intervals. Also noteworthy, as a diagnostic feature, is the composition—considerably homogeneous with respect to Ti and V—of the deposits of the lower subformation of the Tarlan Formation, and a certain decrease in Ca content toward the top of this part of the section.

REFERENCES

1. A. I. Anatolyeva. Stratigraphy and certain problems of the paleogeography of the Devonian of the Minusinsk intermontane trough. Proceedings of the Institute of Geology and Geophysics, issue 2, 1960.

2. A. P. Vinogradov. Average content of chemical elements in the principal types of igneous rocks of the Earth’s crust. Geochemistry, No. 7, 1962.

3. Yu. A. Kuznetsov. Principal types of magmatic formations. Nedra Publishing House, 1964.

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

5. V. S. Meleshenko. On certain questions of the stratigraphy of the Devonian deposits of the Minusinsk basin. Proceedings of VSEGEI, collection Paleontology and Stratigraphy. Gosgeolizdat, 1953.

6. A. A. Mossakovskii. Tectonics of the Minusinsk depressions and their basement. Tectonics of Siberia. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, 1963.

7. D. I. Musatov. Certain features of deep faults, exemplified by the folded regions of the south of Krasnoyarsk Krai. Tectonics of Siberia. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, 1963.

8. G. V. Polyakov, A. E. Teleshev. Magmatic complexes of the region of the Teya group of iron-ore deposits (Kuznetsk Alatau). Basic Ideas of M. A. Usov in Geology. Publishing House of the Academy of Sciences of the Kazakh SSR, 1960.