PETROCHEMICAL FEATURES OF THE CAMBRIAN 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 combustible minerals)

Within the eastern slope of the Kuznetsk Alatau, volcanism manifested itself repeatedly during the Cambrian. Geological studies of recent years [1, 2, 4, 6, 7] have quite convincingly demonstrated the presence of Lower and Middle Cambrian effusive rocks on the right bank of the Belyi Iyus River (Tyurim–Efremkino district), in the area of the village of Katoshkino, the Erbinskaya station, the village of Verkhnyaya Erba (Batenevsky Ridge), and on the Azyr-Tal Range. Moreover, both Lower and Middle Cambrian effusive rocks occur at several stratigraphic levels [2, 5, 6]. Although the stratification has been studied in sufficient detail, as have the features of the petrographic composition and the conditions of formation of the volcanogenic sequences, little attention has until recently been paid to questions of their petrochemistry. The factual material at our disposal makes it possible to identify the principal petrochemical features of compositionally diverse Cambrian volcanogenic products formed in four stages, of which two are Early Cambrian (Usinsk and Efremkino) and two Middle Cambrian (Karasuk and Berikul).

The volcanogenic products of the Usinsk stage of volcanism are the effusive rocks of the Kolodzhul Formation of the Belyi Iyus River basin, distributed in the area of Mount Koshkulak and the village of Efremkino. They compose the lower part of the formation section (up to 200 m) and are represented predominantly by greenstone, more rarely spilitized, diabases, pyroxene–plagioclase and plagioclase–pyroxene basalt porphyrites, and their tuffs, interbedded with dark-gray bituminous limestones containing abundant fauna of Early Cambrian trilobites. Near the village of Efremkino, in the core of the Iyus syncline, where a reduced section of the Kolodzhul Formation is observed (a 120 m member of red beds), in addition to pyroxene–plagioclase basalt porphyrites forming a flow 20 m thick, there occur numerous weakly rounded fragments of orthophyres and felsite-porphyries (in gravelites and conglomerates), possibly representing the most acid differentiates of the basaltic rocks described. Volcanism of this stage manifested itself under submarine and partly subaerial conditions within intra-geanticlinal uplifts of the Kuznetsk-Alatau geosyncline, which was established at the beginning of the Late Precambrian. Moreover, Usinsk volcanism was spatially confined to those geoanticlinal uplifts in whose place typical eugeosynclinal volcanism had appeared in the Late Precambrian.

According to the chemical-analysis data available to us and the data from recalculating their weight amounts into numerical characteristics by the method of A. N. Zavaritskii (Tables 1, 2), the vector diagram shows a large scatter of the figurative points representing the chemical composition of the effusives of the Kolodzhul Formation. The geometric axial line of the initial points of the vectors lies between the variation curve of Daly’s normal series and the variation curve of the intermediate type, separating calcareous-alkaline rocks from alkaline ones. Therefore, the effusives of the Kolodzhul Formation as a whole are similar to the basaltoids of the alkaline-earth series, with their characteristic variation in acidity from melanocratic olivine basalts, weakly undersaturated with silica, through leucocratic silica-saturated basalts to andesite-basalts and leucocratic, weakly silica-oversaturated andesites (silica content

Fig. 1. Summary vector diagram of the chemical compositions of Cambrian effusives of the eastern slope of the Kuznetsk Alatau. Vectors of chemical compositions: 1—of the Usinsk (Kolodzhul) complex, 2—of the Efremkino complex, 3—of the Berikul complex. Variation lines of rock compositions: I—of Daly’s normal series of igneous rocks, II—of Ethne, III—of the Usinsk (Kolodzhul) complex, IV—of the Efremkino complex, V—of the Berikul complex.

Chemical analyses of effusive rocks of the eastern slope of the Kuznetsk Alatau

Table 1

Note. 1—6 — Koldzhul suite: 1—6 — albitized pyroxene porphyrite; 9—66 — albitized diabase, village of Efremkino; 11—66 — albitized diabase, village of Efremkino and R. Torma; 513/11, 513/10; 513/8 — porphyrites, R. Torma (after A. F. Borsuk); 7—18 — Azirtal suite: 7—18 — porphyrites, 11—72 — pyroxene-plagioclase porphyrite; 538/4 — spilitic porphyrite; village of Efremkino; 668/9, 668/8 — augite-plagioclase porphyrites; R. Erbinskaya (analyses 9—12 by V. M. Yaroshevich, 1962); 3511, II—62, II—20, 4009, 1a — pyroxene-plagioclase porphyrites, D. Katochkin (after D. N. Tsarev, 1969); 19—28 — Berikul suite: 0765—A — amygdaloidal porphyrite, 9059, 0376, 10044, 10428, 10225, 0639—A, 0631, 10431, 11072 — pyroxene andesitic porphyrites, village of Koshkulak.

Table 2

Numerical characteristics according to A. N. Zavaritskii

varies from 42, 92 to 62%). Greater variations are observed in the alumina content, which is confirmed by the appearance, in the group of effusives of intermediate composition, of varieties oversaturated with alumina. In the area of andesite-basalts, the geometric axial line of the initial points of the vectors, in comparison with the normal series on the right-hand plane of the diagram, is noticeably shifted to the right, reflecting an increase in alkalinity in the more acidic effusives of the basaltoid composition of the Kolodzhul Formation. This is also indicated by the values of the alkalinity index $a/c$, which reach 5.25. The alkali ratio, expressed by the index $n$, varying from 81.0 to 90.0, shows a marked predominance of sodium over potassium. This is reflected on the left-hand part of the diagram by the almost vertical inclination of the vectors. On the limestone plane of the diagram, the initial points of the vectors in the area of the andesite-basalts are noticeably shifted to the right. This is connected with a sharply decreasing content of feldspar limestone in the more acidic differentiates of the basaltoids of the Kolodzhul Formation (the values of parameter $c$ decrease to 1.1). In terms of titanium and magnesium content, the effusives of the Kolodzhul Formation should be considered normal.

The volcanic rocks that formed in Efremkino time make up the base of the Efremkino and Azyrtal formations [2], interbedding with terrigenous red-colored and gray-colored rocks and reefogenic carbonate rocks containing a rich trilobite and archaeocyathid fauna of the Obruchev horizon of the Lower Cambrian [4, 7]. The effusives of this stage of volcanism form thin flows, lens-shaped packets of tuffs (tuff-conglomerate breccias), and products of their redeposition. Spatially they are confined to zones of rupture dislocations and have been described in the area of the village of Efremkino [4], near Erbinskaya station [7], in the area of Ka-

Tyoshkino [6], in the Azyr-Tal Range [2]. The effusives described are represented by diabases that have been diagenetized to varying degrees, more rarely weakly albitized; by pyroxene-plagioclase and plagioclase-pyroxene basalt porphyrites (sometimes with olivine); and by plagioclase porphyrites. Efremkino volcanism, as before, manifested itself within introgeanticlinal uplifts, which at that time were island ridges. Under these structural conditions, volcanism naturally appeared in both subaerial and submarine facies. The composition of the effusives of the Efremkino Formation corresponds mainly to leuco- and mesocratic basalts; andesite-basalts are developed to a lesser extent (the values of parameter \(b\) range from 16.6 to 30.2) [1]. On the variation diagram the chemical composition is expressed by a swarm of vectors located between the variation line of the average compositions of rocks of the normal series and the variation curve of the intermediate type, which separates lime-alkaline rocks from alkaline ones. At the same time, the geometric axial line of the initial points of the vectors is markedly close to the latter. In the field of andesite-basalts the vectors deviate appreciably to the right from the variation curve of the normal series, which indicates increased alkalinity, similar to that noted in characterizing the corresponding rocks of the Kolodzhul Formation. Like the Kolodzhul effusives, the Efremkino effusives are low in iron \((\mathrm{FeO}'' < 11\%)\). With the general similarity in the chemistry of the effusives of the Kolodzhul and Efremkino formations, it should be noted that the latter are consistently normal in alumina content. In the effusives of the Efremkino Formation the titanium content is reduced by almost a factor of 2 (\(t\) average equals 1.5) in comparison with its content in the effusives of the Kolodzhul Formation (\(t\) average equals 3.0). In the Efremkino effusives sodium predominates over potassium, but less sharply than in the Kolodzhul effusives. This is expressed by the value of the index \(n\), which varies within the limits 55.7–73.4, and only isolated values reach 80.3. Thus, an increased content of potassium oxide is very characteristic of the rocks described. The low titaniferousness and iron content of the basaltoids of the Efremkino Formation, as well as their increased alkali content, especially potassium, bring them closer to the basaltoids of the orogenic formation [3], for which differentiation in the trachytic direction is characteristic. The extremely acidic differentiates of the formation described received comparatively wide development in the area of the village of Katoshkino and were described in sufficient detail by D. I. Tsarev [6]. The chemical composition of the latter falls within the range from trachyandesites to trachytes and orthophyres. This tendency was already outlined in Kolodzhul time, but manifested itself most clearly in Efremkino time.

Middle Cambrian volcanism, as already noted above, also manifested itself in two stages: the early Middle Cambrian—Karasuk stage and the late Middle Cambrian—Berikul stage. The products of early Middle Cambrian volcanism are comparatively widespread and take part in the composition of thick terrigenous-volcanogenic sequences (the Bezymyannaya, Tolchinskaya, and Karasuk formations), which are characterized by a graywacke composition of the rocks and increased manganiferousness [5]. The volcanites of this stage (plagioclase and pyroxene-plagioclase basalt porphyrites, felsite porphyries, and their corresponding pyroclastics) are sharply subordinate to tuffaceous-clastic and normal sedimentary rocks and in most cases are accessory members of the above-mentioned sequences. We do not have data from chemical analyses of the effusives of the early Middle Cambrian and therefore can only say that the differentiation of the basaltoids of this time (judging by the petrographic composition of the rocks) reached the level of rhyolites.

Late Middle Cambrian (Berikul) volcanism led to the ob-

the formation of the thick (1.5–2 km) effusive–clastic Koshkulak Formation, distributed in the area of Mount Koshkulak [4]. The Koshkulak Formation is represented by cherry-gray, diagenetically altered pyroxene–plagioclase andesitic porphyrites, which make up the upper two thirds of the thickness of the section. Plagioclase and andesite–basalt porphyrites are developed to a lesser degree. In the lower part of the formation section basalt porphyrites occur; however, lava breccias and tuff conglomerates of andesitic porphyrites and the products of their reworking are especially widespread. The volcanics of the Koshkulak Formation are products of stratovolcanoes of the subaerial type. Their composition corresponds mainly to andesites \((b\) varies within 12.3–14.7), to a lesser degree to andesite–basalts \((b\) equals 17.6–17.8) and leucocratic basalts \((b\) equals 20.2–25.7). Despite the great scatter of plotted points on the ASB plane of the variation diagram, the basaltic series of the formation’s effusives is located near the variation curve of the normal series. In the series of andesite–basalts and especially andesites, there is a tendency for the plotted points to shift to the right toward the variation curve of the intermediate type, separating lime–alkaline rocks from alkaline ones, which indicates increased alkalinity of the more acidic differentiates of the basaltic rocks. In this connection, the values of parameter \(a\) for these rocks increase from 13.2 to 15.5 (one value reaches 16.1). The alkalinity index \(a/c\) has low values (from 1.6 to 4.1) and averages 2.6. The alkali ratio, expressed by parameter \(n\), whose value varies from 49.5 to 83, shows a predominance of sodium over potassium, but in an even smaller proportion than in the effusives of the Efremkino Formation. In terms of feldspathoid lime content, the effusives of the Koshkulak Formation differ little from basaltoids of the normal series. Therefore, on the lime plane the initial points of the vectors are located mainly near the variation curve for the composition of rocks of the normal series. In terms of silica content, the volcanics described belong to the class of silica-saturated rocks; weakly undersaturated and weakly oversaturated silica rocks are developed to a lesser degree. The value \(Q\) in them varies from \(-6.7\) to \(+10.9\). In terms of titanium dioxide content (0.79–1.43%), the effusives of the Koshkulak Formation belong to basaltoids poor in titanium; in terms of iron content (calculated as ferrous iron), to low-iron rocks (less than 11%). Along with this, reduced magnesian character is markedly expressed. Many of these petrochemical features are similar to those noted in the characterization of the Efremkino effusives. The petrochemical features of the effusives of the Koshkulak (Berikul) Formation indicate that they also belong to a typical andesitic formation of the orogenic stage of development of the region. Specific features of chemistry for this type of formation had already begun to appear in the effusives of Kolodzhul time; they became more distinctly manifested in Efremkino time and received their final, complete expression in Berikul time.

Thus, Cambrian volcanism on the eastern slope of the Kuznetsk Alatau is represented by several volcanogenic complexes that are derivatives of basaltic magma of the tholeiitic type, whose differentiation over time acquired increasingly clear features of the basalt–trachyte trend.

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