Petrochemical Features of Late Precambrian Effusive Rocks of the Eastern Slope of the Kuznetsk Alatau

V. E. Nomokonov, L. V. Peshekhonov

(Presented by the scientific seminar of the departments of general geology, historical geology, and fossil fuels)

As a result of many years of investigations carried out by a large group of geologists [1, 2, 3, 4, 5, 8, 9] on the eastern slope of the Kuznetsk Alatau, several stages of volcanism have been distinguished, the products of which have a definite stratigraphic confinement. In one of the recent works by V. E. Nomokonov et al. [7], no fewer than seven volcanogenic complexes were distinguished; for each of them the features of material composition and the specific conditions of formation were characterized [6, 7]. However, until recently little attention has been paid to questions of the petrochemistry of the effusive rocks of these complexes. The petrochemical analysis of Precambrian effusive rocks presented in this work to some extent fills this gap. On the basis of the chemical-analysis data for effusive rocks available to us, the weight quantities were recalculated into numerical characteristics, and a vector diagram was constructed showing the petrochemical features of the Precambrian volcanogenic rocks of the eastern slope of the Kuznetsk Alatau (Fig. 1).

The earliest volcanism within the eastern slope of the Kuznetsk Alatau manifested itself under conditions of a trough zone of a geosyncline that was laid down at the beginning of the Late Proterozoic (Lower Riphean). The volcanogenic products of this stage are derivatives mainly of fissure-type volcanism that took place in submarine facies. These include the effusive rocks of the Kommunarovka Series. In its composition there predominate, to varying degrees, albitized, amphibolized, and chloritized diabases, diabase, pyroxene-plagioclase and plagioclase-pyroxene porphyrites, as well as spilites with relict pillow and cushion jointing. Tuffs and lava breccias of basic composition, thin bodies of quartz porphyries, quartz albite porphyries, quartz keratophyres, bituminous limestones, and black lydite-like siliceous shales take part in the structure of the series in insignificant amounts. The latter are confined to the upper part of the Kommunarovka Series (the Ungur Formation), which indicates a relative waning of volcanism by the end of Kommunarovka time. Without dwelling on the petrography of these volcanites, since this information has already been covered in the literature [4, 5, 6], we note that, in terms of structural and textural features and features of material composition, the effusive rocks of this type of volcanism are united by many researchers into a spilite-diabase formation [2, 4, 5]. Com-

Fig. 1. Summary vector diagram of the chemical compositions of Late Precambrian effusive rocks of the eastern slope of the Kuznetsk Alatau. Vectors of chemical compositions: 1 — Kommunar complex, 2 — Poludennyi complex, 3 — Kulbyurstyuk complex. Variation lines of rock compositions: I — normal series of igneous rocks, after Daly; II — Etna; III — Kommunar complex; IV — Poludennyi complex; V — Kulbyurstyuk complex.

of the effusive rocks mainly corresponds to leuco- and mesocratic basalts (the value of \(v\) ranges from 21.7 to 27.1). Varieties corresponding to andesite-basalts and dacites are present in insignificant amounts. The rocks of this series are represented by single samples; therefore their petrochemical features are not considered. All initial points of the vectors of the chemical compositions of the rocks of the formation are located on the petrochemical diagram at a considerable distance from the variation curve of the rocks of the normal series. This is explained by the fact that the rocks of the formation are very poor in alkalis and lime, in connection with which the parameters \(a\), \(c\), and the alkalinity index \(a/c\) have small values. The parameter \(a\) varies from 2.9 to 7.7 and averages 5.1; the index \(a/c\) ranges from 0.8 to 3.8 and averages 1.4; the parameter \(c\) varies from 2.0 to 4.0 and averages 3.0. With an overall low alkali content, the basaltoid rocks of the Kommunar series are characterized by a noticeable predominance of sodium over potassium, which is clearly shown in the left part of the diagram by the steep slopes of the vectors. The basaltoids of the Kommunar series are normally aluminous, oversaturated with silica (the numerical characteristic \(Q\) is always positive and not less than 19.7), and low-titanium. Moreover, a directly proportional dependence is established between titanium content and the content of total iron and magnesium. The high silica content (up to 54.37%), the small value of the ratio of magnesium oxide to calcium oxide (0.52 on average), the low content of magnesium oxide (average—3.7), and the presence of normative quartz indicate that the basaltoids of the Kommunar series belong to the tholeiitic type of basalts. Deviations in the contents of alkalis and feldspathoid lime from the contents of these components in normal basaltoids are explained by the deep greenstone alteration of the effusive rocks of the spilite-diabase formation, often to the greenschist facies.

The next stage of volcanism in the region manifested itself in the middle of the Late Proterozoic (in the Middle Riphean) over considerably larger areas than the preceding one, which testifies to the expansion of the rift zone of the Kuznetsk-Alatau geosyncline. The volcanogenic products of this stage are derivatives of explosive-extrusive activity that occurred essentially under submarine conditions. We have studied in greatest detail the effusive rocks composing the Poludennaya series in the area of the middle course of the Bely Iyus River and united into the spilite-keratophyre formation [7]. The effusive rocks of the formation form two distinct groups of rocks, of dacitic and basaltic composition. Volcanites of basic composition are represented by albitized diabase, basalt, plagiobasalt porphyrites, spilites, and diabases. Rocks of dacitic composition are represented by quartz keratophyres, keratophyres, felsites, and felsite-porphyries. As a whole, the Poludennaya series is characterized by a roughly rhythmic structure (about 5 rhythms are distinguished). In each rhythm, effusive rocks of basic composition precede rocks of acid composition.

The basaltic series of rocks of the spilite-keratophyre formation has a large scatter of figurative points in the right-hand field from the variation curve of rocks of the alkaline-earth series. In contrast to the basaltoids of the Kommunar series, the figurative points of the effusive rocks described are displaced to the right not only from the line of the alkaline-earth series, but also from the variation curve of the intermediate type, which separates the lime-alkaline rocks from the alkaline ones, a feature characteristic of alkaline basaltoids. With \(v\) equal to 27.3–29.0, the relative number of atoms of alkali metals \(a\) is 11.5–13.4. Another distinctive feature of the basic effusive rocks of the Poludennaya series is an increased content

Table 1

Chemical analyses of the effusive rocks of the northeastern slope of the Kuznetsk Alatau: 1–7—communal series (according to A. F. Korobeinikov et al., 1965); 8–46—spilite; 6–9, 407, 17, 14—diabasic porphyrites; 16–638—diabase; 399 (6–21)—quartz porphyry; 8–16—spherulitic porphyry; 12–66—albitized porphyry; 56—albitized diabase, Rechnaya series; 60—albitized porphyrite, Rechnaya series; 523, 527a, 256—quartz keratophyres, area of the vicinity of the Rudnik Kholmichev settlement, 1968; 6514, 5403, 2889, 623—albitized tuffitic schists (according to V. L. Kholmichev, 1964); 17–31—Kultaibor suite (region of the middle course of the Belyi Iyus); 523/1, 523/14, 519/13, 516/16, 523/5, 519/4—basaltic porphyrites (according to A. F. Belousov); 14–66—almond-shaped diabase; 22–66—felzitic porphyry; 523/18, 523/16, 523/17, 523/12, 518/19, 523/11—porphyries (according to A. F. Belousov); 516/29—porphyrite (according to A. F. Belousov).

Table 2

Numerical characteristics according to A. N. Zavaritsky

magnesium, which is 1.5–2 times greater than in rocks of the same name of the Kommunarovskaya series. They are undersaturated with silica (the numerical characteristic \(Q\) in them is negative), normally titaniferous, only mesocratic. The titanium content in them is directly dependent on the total iron content. Like the basaltoids of the Kommunarovskaya series, the Poludennaya basaltoids are characterized by a considerable predominance of sodium over potassium (parameter \(n\) ranges from 85.2 to 96), and by normal alumina saturation.

The acid effusives of the Poludennaya series have their own petrochemical features. They belong to the alkaline-earth series, but are oversaturated with silica and alumina. The alkali content on average does not exceed the norm, and only individual samples show an elevated alkali content (\(a\)—15.3). The alkali ratio is unstable, varying from 63.8 to 91.0, but in most rocks sodium markedly predominates over potassium. According to the values of other parameters, the acid rocks of the Poludennaya series are poor in magnesium, feldspathic lime, ferrous and ferric iron, and titanium.

The concluding volcanism in the Late Precambrian (in the Upper Riphean) was that of Kulbyurstorg time [7], manifested over still larger areas of the Kuznetsk-Alatau geosyncline than the preceding volcanism, which testifies to its continuing expansion. The effusives of this time are united into the Kulbyurstorg Formation. Among them predominate spilites, diabase and basalt porphyrites of submarine facies, albitized to varying degrees, and diabases, which make up the lower part of the section of the formation. The upper half of the section of the formation is composed mainly of greenstone andesite-basalt and andesite porphyrites and their tuffs. The section of the formation is completed by a thin member (50–80 m) of quartz porphyries, quartz albitophyres, felsite-porphyries, and porphyry-latites. Despite the considerable scatter on the diagram of figurative points reflecting the chemical composition of the effusives of the Kulbyurstorg Formation, the incipient variation curve occupies an intermediate position between Daly’s variation curve for differentiation of basaltic magma of the normal alkaline series and the variation curve of the intermediate type separating lime-alkaline rocks from alkaline rocks. Therefore the Kulbyurstorg effusives differ little from a consistently differentiated normal basalt–rhyolite series, characterized by a wide variation in composition with respect to acidity. This is typical of the effusives described, in which the amount of silica by weight percent varies from 45.5 to 73.0%. Greater variations are observed in the contents of alumina, alkalies, feldspathic lime, and chemical components. The basic and intermediate rocks of the formation, in terms of alumina content, belong to normally aluminous rocks. In the group of acid effusives, rocks oversaturated with alumina predominate, but normally aluminous rocks also occur.

The broad variation in the content of femic components is indicated by the values of parameter \(в\), fluctuating within the limits from 4.0 to 27.2. The alkali ratio is relatively stable and, with rare exceptions, very high (a steep inclination of the vector on the lime plane), indicating the predominance of sodium over potassium. The alkalinity index \(a/c\) is unstable and increases from basic rocks to acid ones.

Noteworthy is the stable and low content of feldspathic lime, especially in rocks of acid composition, which is clearly shown by the position of the figurative points on the left side of the diagram.

In general, the effusives of basic composition of the Kulbyurstorg Formation are poor in alkalies, normally aluminous, corresponding to leuco- and mesocratic basalts. The alkalinity index \(a/c\) in them is expressed by figures

1.2–4.8, averaging 1.9. They are not saturated with silica, are normally ferruginous, normally titaniferous, and in this respect differ little from the basaltoids of the Poludennaya series. Effusive rocks of intermediate composition (andesites) are usually moderately rich in alkalies, less often rich in alkalies (sample 516/29), and are undersaturated with silica. Their alkalinity index \(a/c\) ranges from 4.7 to 9.1; the parameter \(Q\) does not exceed 5.9 and is often negative. Rocks of acid composition are usually oversaturated with alumina and are rich in, or oversaturated with, alkalies. Noteworthy is the low magnesium content: from 0 to 0.4%, and feldspar lime: from 0 to 0.5%.

As can be seen from the petrochemical-analysis data presented, the Late Precambrian effusive rocks of the eastern slope of the Kuznetsk Alatau are derivatives of tholeiitic basaltic magma, whose differentiation, regardless of the time of volcanism, proceeded toward rhyolites with a constant tendency toward the quartz–feldspar eutectic. At the same time, a change through time is observed in the chemical composition of the derivative magmas toward increased alkalinity and decreased lime content.

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