PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION
AND ORDER OF THE RED BANNER OF LABOR
S. M. KIROV POLYTECHNIC INSTITUTE

Vol. 297
1975

SOME PETROCHEMICAL FEATURES OF THE ALKALINE ROCKS OF TYRDANOV ULUS

(KUZNETSK ALATAU)

A. A. MITYAKIN

(Presented by the scientific seminar of the Department of Petrography)

The alkaline rocks of Tyrdanov Ulus, in geological terms, are confined to the endocontact zone of the Uibat granitoid pluton at its northwestern termination. The rocks enclosing the pluton in this area are Precambrian limestones and dolomites. Geographically, the area is situated approximately in the middle course of the Sukhaya Erba River, on its right bank.

After these rocks were first described in the 1930s by Yu. A. Kuznetsov, they were studied by quite a number of researchers; however, two points of view still exist regarding the origin of these rocks. The first of them—that the alkaline nepheline-bearing rocks are a product of contact interaction of an acidic magma with carbonate rocks—was first expressed by Yu. A. Kuznetsov. The second is that these rocks have no genetic connection with the granitoids, but are a constituent part of the Devonian gabbro-syenite formation.

In this article, some petrochemical features of the rocks will be considered that confirm the first point of view on their genesis.

The Tyrdanov Ulus area is characterized by a great diversity of rocks, represented by nepheline syenites, nepheline diorites, nepheline-free syenites and diorites, quartz syenites, melanocratic rocks with nepheline of the melanocratic diorite type, and others.

The features of the chemistry of these rocks, based on a diagram (Fig. 1) constructed by the method of Zavaritsky, are as follows. On the \(ASB\) plane, the vectors of the analyses, with minor exceptions, have a gentle inclination to the right, which indicates the great role of calcium in the composition of the dark-colored components, expressed in the value of \(C^1\). Thus, for example, in analysis No. 2 the value of \(C^1\) is 42.30 instead of 21.70 in a rock of similar chemical composition according to Daly; in analysis No. 6 it is 36.50 instead of 21.70, etc. Nepheline syenites are represented by points 1, 2, 3, 6, 7, 16, 22, 28, 29. On Zavaritsky’s diagram of average rock types, according to Daly, by bulk chemical composition the nepheline syenites mostly gravitate toward ordinary nepheline-free rocks—umptekite, trachyandesite, banakite—and only one analysis, No. 22, gravitates toward the typical nepheline syenite—lujavrite. With the exception of this last analysis, the vectors corresponding to nepheline syenites are usually located on the \(ASB\) plane to the left of the vectors of typical nepheline syenites. This feature, together with the increased value of the quantity

1, 2, 3, 6, 7, 16, 22, 28, 29 — nepheline syenites; 8, 19, 20 — nepheline diorites; 18 — albite; 30 — melanocratic rock with nepheline; 12, 13, 14 — quartz syenites metasomatic; 4, 5, 9, 10, 11, 17, 21, 23, 25, 31 — aegirine syenites, diorites, syenite-diorites, melanocratic diorites.

Table 1

Fig. 1. Petrochemical diagram by the method of A. N. Zavaritskii for the alkaline rocks of Tyrdanov Ulus. The points of the chemical compositions correspond to Table 1.

\(C^1\) in most nepheline syenites indicates the great role of calcium in the formation of these rocks. Another feature of the nepheline syenites is the elevated value of the quantity \(Q\), approximately twice as high as this value in average rock types according to Daly. Thus, in analysis No. 1 it is equal to 15.90 instead of 8.50 according to Daly, etc.

In the nepheline diorites—analyses Nos. 8, 19, 20—an increased value of \(C^1\) is also observed in comparison with closely related average rock types according to Daly. The same can be said of the melanocratic rocks with nepheline, which are atypical and, in bulk chemical composition, correspond to alkaline basalts.

The above-mentioned features do not apply to certain nepheline-free rocks—syenites and diorites. This is due to the fact that, during the formation of the Uybat pluton, at the final stage there occurred an intensive potassic metasomatism, accompanied by the development of metasomatic potash feldspar and, at the same time, by silicification of the rocks. Potassic metasomatism manifested itself in all rocks, including those containing nepheline. In this process, nepheline was replaced, and the mafic minerals were also decomposed and replaced, with development of metasomatic potash feldspar. As a result of this process, the rock became more leucocratic and its silica content increased. Thus, in the course of metasomatism, components such as iron, magnesium, and calcium were removed from the rock. Therefore, the vectors of metasomatically altered rocks on the plane usually have a normal inclination corresponding to average rock types. Examples are rock analyses Nos. 12, 13, 23.

The excess calcium content in the rock is also manifested in the value of \(C\). As an example one may cite analysis No. 1, with a value of \(C\) equal to 4.70 instead of 0.1 for a rock of similar composition according to Deli; in analysis No. 3, 5.85 instead of 0.1; in analysis No. 6, 6.00 instead of 2.80; in analysis No. 7, 4.30 instead of 0.2, etc.

On the \(CSB\) plane, the vectors of rocks that have undergone lime metasomatism are approximated to the \(SB\) axis. These include analyses Nos. 5, 9, 11, 12, 13, 14, 24.

Thus, the position and inclination of the vectors on the \(ASB\) and \(CSB\) planes indicate an excess calcium content in the rocks in comparison with compositionally similar average types according to Deli. The abundance of calcium, which has a greater affinity with silica than sodium, contributed to the crystallization of the feldspathoid mineral—nepheline.

This peculiarity of the chemical composition of the rocks of the Tyrdanov Ulus, together with the features we noted earlier—the gradual transition of nepheline-bearing rocks to nepheline-free ones, the presence of minerals with a high calcium content, garnet andradite with a CaO content above 30%, pyroxene with a CaO content up to 26%, an elevated content of this component even in potassic feldspar—above 9%, the presence directly at the contact with carbonate rocks and close to the contact of such varieties as nepheline pyroxenites, as well as atypical melanocratic diorites with nepheline—all this indicates that the nepheline-bearing rocks located in the endocontact zone of the granitoid pluton are the product of interaction between an acid magma and the enclosing carbonate rocks.