GENERAL GEOCHEMICAL FEATURES OF THE ALKALINE ROCKS OF THE BATANAYUL MASSIF

(Kuznetsk Alatau)

A. A. Mityakin

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

The Batanayul massif of alkaline rocks is located along the Batanayul and Kududet rivers, left tributaries of the middle course of the Uryup River. It consists of two principal bodies of complex petrographic composition, separated somewhat spatially. Both bodies are elongated, extending in a sublatitudinal direction, and have a total outcrop area of about 1 km². The rocks of the massif occur among effusive-sedimentary formations of the Lower Devonian. A great variety of rocks has been established within the massif; their formation took place in four successive phases, from earlier to later:

1st phase—theralite-porphyries, labrador bereshites, and andesinous bereshites;
2nd phase—leucocratic theralite-porphyries;
3rd phase—nepheline monzonites and nepheline syenites;
4th phase—calc-alkaline and alkaline syenites.

At the final stage in the development of the massif, dike rocks were formed—diabases, dolerites, alkaline syenites. A general feature of the rocks of the massif, which formed under near-surface conditions, is a distinctly porphyritic or fine-grained texture.

A general geochemical feature of the massif is that almost all elements found in its composition are through-going and, moreover, are present in approximately equal amounts in all rock varieties. The only exception in this respect is lithium, found in appreciable amounts only in one variety—bereshites. The through-going elements of the rocks of the massif are titanium, lead, zirconium, yttrium, strontium, barium, gallium, zinc, copper, beryllium, vanadium, and chromium. Two elements—nickel and cobalt—are contained in insignificant amounts, and their content is variable.

As for geochemical specialization, then, as is evident from the foregoing, almost all the listed elements are characteristic of basic and intermediate rocks. For alkaline rocks, only beryllium, yttrium, and gallium are characteristic, but they are present in small amounts, beginning with the early products of differentiation and ending with the late ones, and therefore do not determine the geochemical specialization of the rocks. Let us consider the behavior of the elements during the process of differentiation of the magmatic melt, taking into account the particular character of the set of elements and the prereq-

presumed connection of the rocks with the basic magma, the comparison by content will be made with the clarkes of basic rocks according to A. P. Vinogradov (1962).

Elements of the iron group. Of this group, titanium, vanadium, chromium, cobalt, nickel, and manganese are present in the rocks.

Titanium is present in all rock varieties in amounts below the clarke, and its content decreases somewhat in the direction from the early products of differentiation to the late ones—from 0.5 in bereshites to 0.25 in nepheline syenites. As spectral analyses show, titanium is contained in magnetite in an amount of 2% and in pyroxene—0.3–0.5%. Microscopic study of the pyroxenes established that in the rocks of the early phases it is more titaniferous than in the later ones, in particular in nepheline syenites, where hypersthene and aegirine are present. This, evidently, explains a certain decrease in its content in the later rock varieties.

Vanadium is present in all rock varieties in amounts considerably below the clarke values, and no regularity in the change of its content is observed. Spectral analyses show its greatest content in magnetite, reaching 0.01%. This element also enters into the composition of ferromagnesian pyroxenes. The considerably lower contents of vanadium in all rocks in comparison with the clarke testify to the poverty of the parental magma in this element. At the same time, the small differences in the content of ferromagnesian components and accessory magnetite in rocks of different phases did not promote accumulation of this element in any particular rock.

Analogously to vanadium, such elements as chromium, cobalt, and nickel also behave. Their content in all rock varieties is approximately the same and several tens of times lower than the clarke value; moreover, cobalt and nickel in individual rock varieties, except bereshites, are not detected at all.

Group of minor petrogenic elements

Lithium, as was noted above, is found in a noticeable amount only in bereshite, and its content is somewhat below the clarke value. The concentration of lithium in bereshites—products of an early stage of differentiation—is explained by its ability to substitute isomorphously for sodium, which is present in considerable quantity in this rock as part of nepheline. This is confirmed by spectral analysis of nepheline from bereshites, showing a lithium content of 0.03%.

The content of beryllium in all rocks of the massif is 0.001%, with the exception of rocks of the last phase, where it is 0.0008%, but this is also above the clarke value. It is difficult to speak of a connection of this element with any definite mineral, since it is found in equal amounts (0.001) in pyroxene, nepheline, and magnetite; this, evidently, explains its dispersed character and approximately equal content in all rock varieties.

Strontium in almost all rock varieties is contained in amounts above the clarke value; at the same time, a decrease in its content is observed from the rocks of the early stages of differentiation to the late ones. Thus, in bereshites and theralite-porphyries its content is 0.07–0.06%, with a clarke value equal to 0.044, while in the rocks of the last phase it is 0.03%. Such behavior of strontium is apparently explained by the closeness of its ionic radius to the radius of calcium, while the content of the latter also decreases sharply from the early products of differentia-

...to the late ones. Spectral analysis also gives a high content of strontium in the nepheline of bereshites, equal to 0.5%, which apparently indicates isomorphous substitution of sodium by strontium; and, possibly, strontium here is associated with calcium, whose content in nepheline reaches 8%.

The content of barium in all rock varieties is also higher than or equal to the Clarke value. This element behaves similarly to the preceding one—its content decreases from rocks of the early stage of differentiation to the late stage. In the rock of the first phase—bereshite—the barium content is 0.13%, whereas in the rocks of the last phase it is 0.03%. An interesting feature in the behavior of barium is that its content in the rocks of the late phases, which are richest in potassium, is considerably lower than in the early products of differentiation, although the literature indicates a constant concentration of barium in potassium-bearing minerals. Apparently, strontium and barium behave similarly here and are associated with the rocks richest in calcium. The similar behavior of barium and calcium is evidenced by the fact that, during the potassic alteration of bereshites, a large part of the calcium is removed from them. The same tendency is also observed with respect to barium: its content in potassically altered bereshites is 0.01% instead of 0.13% in the unaltered rock.

Group of metallic elements.

Lead and zinc behave similarly, and their content generally fluctuates around the Clarke value. Cases of reduced content are observed more often. No regularity in the behavior of these elements in rocks of different phases is observed.

The content of copper in all rock varieties is considerably below the Clarke value; moreover, a tendency is observed for its content to decrease from early products of differentiation to late ones.

Group of rare elements.

Of this group, three elements have been found in the rocks of the massif—yttrium, zirconium, and gallium. Yttrium, in general, is contained in the rocks of the massif in an amount close to the Clarke value. In individual rock varieties, fluctuations in its composition are observed in one direction or another. No regularity is observed in the change of its content in the successive series of products of differentiation. Spectral analyses show that its content is above the Clarke value—0.003—in pyroxene. In such minerals as nepheline, potassium feldspar, and magnetite, the element was not detected. Zirconium is characterized by the fact that in almost all rock varieties its content is above the Clarke value; moreover, a tendency is observed for its content to increase from early products of differentiation to late ones. Thus, if in bereshites and theralite-porphyrites its content is, respectively, 0.012 and 0.01%, then in nepheline monzonites and nepheline syenites it is 0.04 and 0.03%. This is explained by the fact that the element enters into the composition of the accessory mineral—zircon—the content of which increases toward the late products of differentiation. In the rock-forming minerals—pyroxene, nepheline, and potassium feldspar—the content of the element is 0.001–0.003%. Gallium is present in all rock varieties, except nepheline syenite, in amounts above the Clarke value; moreover, a tendency is observed for its content to decrease from rocks of the early phases to the late ones. This is possibly explained by the fact that, in terms of ionic radius, the element is close to aluminum, in which the rocks are richest.

early phases, owing to the presence in them of a large amount of nepheline and other aluminosilicates.

In addition to all the elements listed, traces of molybdenum and niobium are observed in the rock.

On the basis of the foregoing, the following principal conclusions may be drawn.

1. Attention is drawn to the small number of trace elements present in the rocks of the massif; moreover, the overwhelming majority of them are characteristic of rocks of basic composition.

All the elements characteristic of basic rocks—titanium, chromium, vanadium, cobalt, nickel, and copper—are present in amounts considerably below Clarke values. This is explained by the fact that the elements listed are characteristic of differentiates of magma of basic composition, namely effusives of predominantly basaltic composition, which formed earlier than the intrusive massifs described. In this respect cobalt and nickel are of interest, being characteristic of rocks of the early stages of crystallization.

1. Of the rare elements characteristic of alkaline rocks, only three are present—yttrium, zirconium, and gallium; moreover, their contents exceed the Clarke value for basic rocks only to an insignificant degree. The presence of these elements is all the less indicative if one takes into account that the parental magma of alkaline intrusive rocks is residual, a differentiate from magma of more basic composition, and that the accumulation of rare elements in the residue is entirely natural, even if their content in the parental magma was insignificant.

2. All the elements present in the rocks are through-going, with the exception of lithium, detected in appreciable quantity only in the rock of the first phase—bereshite.

3. Among all the elements present in the rocks, only barium and strontium are observed in amounts exceeding Clarke values, which is also a characteristic indication of the connection between intrusive and effusive formations, since these elements usually accumulate in the final products of crystallization, which in the present case are the described intrusive rocks formed from residual magma as a differentiate of a more basic one.