SOME PETROCHEMICAL FEATURES OF VOLCANIC ROCKS IN THE AREA OF ELEVATION 808.5 (BATENEV RIDGE)

D. I. TSAREV

(Presented by the Department of General Geology)

To the east of the village of Katyushkino, in the area of elevation 808.5, volcanic rocks are exposed in the closure of a Lower Paleozoic syncline. In the lower part they are represented by pyroxene-plagioclase and plagioclase porphyrites, their tuffs and tuffites with lenses of limestones containing a fauna of archaeocyaths and trilobites of the Obruchev complex, among which Middle Cambrian trilobites occur. The porphyrites are cut by dike-like and pipe-shaped bodies of explosion breccias (5), by bodies of quartz syenites, syenite-aplites, microsyenites, and by dikes of acidic, intermediate, and basic composition. Explosion breccias, in addition to cross-cutting bodies, occur above the porphyrites together with semi-fused tuffs of alkaline composition.

On the basis of petrographic features and the geological setting of rock formation, the volcanogenic sequence has been divided by the author into two volcanic complexes: 1) the Katyushkino basal­toid complex—\(Cm_{1-2}\), and 2) the acid-alkaline complex—\(Cm_2\) (?). To the first complex are assigned pyroxene-plagioclase and plagioclase porphyrites and their volcaniclastic formations; to the second—explosion breccias, tuffs of alkaline composition, extrusive orthophyres and felsites, syenites, and dikes of acidic and alkaline rocks. The rocks of the acid-alkaline complex were previously assigned by some earlier investigators and by the author to the Devonian (5). On the basis of petrochemical analysis of the rocks, the author is now inclined to consider the gap in age between these complexes to be considerably smaller.

Katyushkino volcanic complex \((Cm_{1-2})\)

The composition of the rocks of the Katyushkino complex on the diagram of A. N. Zavaritsky (Fig. 1) is expressed by a series of vectors which, in the \(asb\) plane at the greatest value of the coefficient “\(b\),” lie on the variation line of the average compositions of rocks of the normal series, according to Daly (2), corresponding to plateau basalts. The vectors then deviate to the right, which indicates the increased alkalinity of the rocks being characterized. The greater part of the rocks is normally saturated with alumina (rightward inclination of the vectors). Their composition varies from plateau basalts to andesite-basalts, according to Daly. The alkalinity interval ranges from rocks very poor in alkalis to alkaline rocks (2). In titanium dioxide content

(1–2%) porphyrites should be assigned to basaltic rocks poor in titanium, and, in terms of iron content (\(\mathrm{FeO}^{\prime\prime} < 11\%\)), to low-iron rocks (7). Low-titanium and low-iron basalts are commonly regarded as a characteristic feature of orogenic series (1). In addition, an increased content of alkalis, especially potassium, characterizes porphyrite formations in which basalt–trachyte series occur. Usually in these series there is a gap between the basalt and trachyte groups, but within each group differentiation is clearly expressed in the same basalt–trachyte direction (7). In our case, differentiation of magma of basaltic composition toward trachyte is clearly indicated: a decrease in the magnesium content of the rocks leads to an increase in the alkali content. This phenomenon is quite clearly revealed in the behavior of the vectors on the \(asb\) plane of A. N. Zavaritsky’s diagram (Fig. 1), and in the fact that porphyrites with a low content of pyroxene phenocrysts, and those containing none at all, predominate in the upper horizons of the volcanic complex.

Acid–alkaline volcanic complex (\(\mathrm{Cm}_2?\))

In A. N. Zavaritsky’s diagram (Fig. 1), the variation line of the compositions of the rocks of this complex is, as it were, a continuation of the variation line of the Katyushkinsky porphyrites. In all probability, the rocks of the acid–alkaline complex are differentiates of magma that

![Fig. 1 and Fig. 2 diagrams]

Fig. 1. Variation lines of rock compositions: 1—of the Katyushkinsky and acid–alkaline volcanic complexes; 2—average compositions according to Daly; 3—lavas of intraoceanic islands; 4—lavas of intracontinental volcanoes (after G. S. Gorshkov). Rock compositions: 5—of the Katyushkinsky complex; 6—of the acid–alkaline complex.

Fig. 2. Silica–“femic index” diagram according to Yu. M. Sheinmann. Fields of magma classes: 1—andesite–basalt; 2—olivine–basalt; 3—ultrabasic–alkaline. Rocks of volcanic complexes: 4—Katyushkinsky; 5—acid–alkaline.

gave rise to the basaltoids of the Katyushka complex. The composition of the rocks ranges from trachyandesitic (Nos. 1, 2) to trachytic and rhyolitic. A large part of the rocks is saturated with \(\mathrm{SiO_2}\) and rich in alkalies (2). The orthophyre–syenite group of rocks, which forms part of the fragments of the explosive breccias, falls entirely into the class saturated with \(\mathrm{SiO_2}\) and into the group rich in alkalies. The intrusive and dike rocks of the complex are for the most part oversaturated with \(\mathrm{SiO_2}\), and in alkali content vary from rich to poor. All rocks of the complex are leucocratic. On A. N. Zavaritskii’s diagram, the vectors of the syenite–orthophyre group of rocks of the explosive breccias and tuffs (Nos. 5, 6, 7, 8) lie almost at one point and have a leftward inclination. This circumstance indicates the comagmatic character of the rocks that solidified at depth and at the surface (alkaline syenites and orthophyres). In composition this group of rocks comes closest to nordmarkite and alkaline trachyte, according to Daly (2), but differs from them mainly in a higher \(\mathrm{K_2O}\) content, a lower \(\mathrm{Na_2O}\) content, and oversaturation with alumina.

As for the group of intrusive and dike rocks of the complex, here a certain scatter of the vectors along the variation line is observed in the direction of decreasing alkali content and increasing silica from the intrusive rocks to the dike rocks. The final product of differentiation—the felsite dikes—has a composition close to rhyolitic. The loss of alkalies in the latest magmatic products of the volcanic complex and the increase in their silica content may be explained by D. S. Korzhinskii’s hypothesis of infiltration–diffusion interaction between the melt and the enclosing medium contrasting with it in composition [3, 4].

The variation curve through the vectors of the Katyushka and acid–alkaline rock complexes (Fig. 1), in its curvature, corresponds to the variation curves for lavas of intraoceanic islands (1).

On the diagram of Yu. M. Sheinmann [6], constructed in the coordinates \(\mathrm{SiO_2}—\alpha/\beta\) \(\left(\alpha = \mathrm{K_2O} + \mathrm{Na_2O};\ \beta\right.\) is the sum of oxides, calculated as forming mafic minerals: \(\mathrm{MgO}\), \(\mathrm{FeO}\), \(\mathrm{Fe_2O_3}\), and the remainder \(\mathrm{Al_2O_3}\) or \(\mathrm{CaO}\) after formation of feldspars), from 800 analyses, the rocks of the complexes under consideration fall mainly in the field corresponding to rocks of the olivine-basalt class of magma (Fig. 2).

REFERENCES

1. G. S. Gorshkov. Phenomena of volcanism and the upper mantle. Proceedings of the Geochemical Conference “Chemistry of the Earth’s Crust,” vol. 2, dedicated to the centenary of the birth of V. I. Vernadsky. Nauka, 1964.

2. A. N. Zavaritskii. Recalculation of Chemical Analyses of Igneous Rocks. Gosgeoltekhizdat, 1960.

3. D. S. Korzhinskii. Acid–basic interaction of components in silicate melts and the direction of cotectic lines. Reports of the Academy of Sciences of the USSR, No. 2, 1959.

4. D. S. Korzhinskii. Theory of the Processes of Mineral Formation. Third Reading in Memory of V. I. Vernadsky. Publishing House of the Academy of Sciences of the USSR, 1962.

5. D. I. Tsarev. Paleovolcanism of the area of the Yuliya mine (Batenev Ridge). Geology, Geophysics, Geodesy (abstracts of reports of the Novosibirsk Conference of Young Scientists and Specialists), Siberian Branch, Academy of Sciences of the USSR, 1966.

6. Yu. M. Sheinmann. Possible connections of magmas with the structure of the Earth’s outer shells. Proceedings of the Geochemical Conference “Chemistry of the Earth’s Crust,” vol. 2, dedicated to the centenary of the birth of V. I. Vernadsky. Nauka, 1964.

7. D. S. Shteinberg. On the chemical classification of effusive igneous rocks. Proceedings of the Institute of Geology, Ural Branch, Academy of Sciences of the USSR, issue 72, 1964.

Table 1

Chemical composition of rocks of the Katyushkinsky volcanic complex and their numerical characteristics

Continuation of Table 1

Note: In the table, in addition to the numerical characterization according to A. N. Zavaritsky, certain characteristics according to D. S. Shteinberg are included (\(\alpha\)—the anorthite number and FeO″—total conventional ferrous oxide), as well as the alkalinity modulus \(\gamma\) according to I. V. Luchitsky.

Table 2

Chemical composition of volcanic rocks of the acid-intermediate complex and their numerical characteristics according to A. I. Zavaritskii

Continuation of Table 2

Note. \(a\) — anorthite number according to D. S. Shteinberg (7);
\(r\) — iron module according to I. V. Luchitskii;
FeO″ — total iron according to D. S. Shteinberg,
1, 2, 3, 4 — andesito-basalts of the exhalation breccias,
5, 6 — fragments of siltstones in the exhalation breccia,
7 — fiamme of the exhalation breccia (ignimbritic rocks),
8 — orthophyric cement of the exhalation breccia,
9 — inclusions of orthophyres in the exhalation breccia,
10 — fluidal orthophyres,
11, 12 — intrusions of quartz syenites,
13, 14 — dikes of orthophyres,
15 — dike of microsyenites,
16 — dike of banded felsites.