General Geochemical Features of the Alkaline Rocks of the Semenovsky Massif

A. A. Mityakin

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

The massif is located along the stream of the same name, a right tributary of the middle course of the Uryup River, in the northeastern part of the Kuznetsk Alatau. The area of the massif exposed at the surface is about 6 km². The massif occurs among effusive-sedimentary rocks of the Lower Devonian, which are replaced to the north by Middle Devonian formations. Here the Devonian rocks form a synclinal fold with an axis plunging to the northwest at an angle of 15–20°.

The Semenovsky massif is composed of rocks of varied composition, forming small bodies in the form of stocks, sills, and dikes of near-surface facies.

Taking into account the features of the mineralogical and chemical composition of the rocks, as well as their mutual relationships, five stages (phases) in the formation of the Semenovsky massif are distinguished:

1st phase — melteigites and bereshites,
2nd phase — theralite-porphyries and theralite-monzonites,
3rd phase — nepheline monzonites and nepheline syenites,
4th phase — dolerites,
5th phase — lime-alkaline and alkaline syenites.

The general geochemical features of the rocks of the massif may be summarized as follows:

Group of minor petrogenic elements. Lithium is found in appreciable quantities in this area in nepheline syenite—a rock formed during the penultimate stage. It is known from the literature that the behavior of lithium is determined partly by the behavior of magnesium, as well as by sodium and volatiles; in particular, as the magnesium content decreases, the lithium content increases. Nepheline syenites are leucocratic rocks with a low magnesium content and a significant sodium content. Spectral analysis shows a high lithium content in nepheline (0.03%), and this mineral evidently played a substantial role in the concentration of lithium in this rock. In absolute value, the lithium content in nepheline syenite is below the Clarke value.

Beryllium in all rock varieties is present in amounts above the clarke value; at the same time, there is a tendency toward a decrease in its content in rocks of the later phases, where it is equal to 0.0008–0.0009% instead of 0.001% in rocks of the early stage of differentiation. However, this difference in content is not so significant. The latter is explained, apparently, by the fact that this element is found in equal amounts in pyroxene, nepheline, and magnetite (0.001), i.e., it is dispersed in all minerals.

The content of strontium in rocks of different phases varies, and a clear tendency is observed toward a decrease in its content from rocks of the early phases to the late ones. Thus, in bereshites and melteigites its content is, respectively, 0.18 and 0.16%, whereas in nepheline syenites and alkaline syenites it is 0.036 and 0.017%. Such behavior of strontium is in complete agreement with the behavior of calcium, whose content decreases sharply in the same direction. It is also possible that such geochemical behavior of strontium depends on the behavior of sodium, whose role among the alkalis is especially high in rocks of the early phases. Confirmation of this is the high strontium content in nepheline, reaching 0.5%.

The content of barium in all rocks, except alkaline syenites, is twice the clarke value. In this respect the geochemical behavior of this element is similar to that of strontium. This is explained, apparently, by the closeness of the ionic radii of both elements and by the dependence of their behavior on that of calcium.

Elements of the iron group. Titanium in all rock varieties is present in amounts below the clarke values; at the same time, its lowest content was found in the alkaline syenites. According to spectral analyses, titanium is present in magnetite up to 2% and in pyroxene 0.3–0.5%. The absence of pyroxene in the alkaline syenites apparently explains the somewhat reduced titanium content in them.

The content of vanadium in rocks of all phases is below the clarke value. A tendency is observed toward a decrease in the content of this element from rocks of the first phase to the last. Thus, in melteigites its content is 0.003%, and in alkaline syenites 0.0009%. This is connected with the fact that vanadium is present, as usual, in ferromagnesian silicates, and the latter are richest in rocks of the early phases. The element was also found in magnetite in amounts up to 0.01%. The content of magnetite in the alkaline syenites is also considerably lower than in rocks of the early phases, which has caused its somewhat lower content in the syenites compared with the melteigites.

The geochemical behavior of chromium, cobalt, and nickel is analogous to the behavior of vanadium. The content of these elements in all rocks is considerably below the clarke values, and their highest content is noted in the melteigites. This is explained by the fact that cobalt and nickel, in particular, are usually associated with products of early crystallization, which here are the melteigites. The higher chromium content in the melteigites is due to the fact that this element, in terms of ionic radius, is close to iron and is capable of substituting for it in the crystal lattice of pyroxenes, in which the melteigites are richest.

Group of metallic elements. Lead and zinc behave identically—their content in rocks of all phases, with rare exceptions, is below the clarke values; at the same time, a tendency is observed toward an increase in content from rocks of the early phases to the late ones. Thus, the content of lead increases from 0.001% in the melteigites to 0.004% in the alka-

in alkaline syenites and becomes equal to the Clarke value, while the zinc content ranges from 0.005% in melteigites to 0.036% in nepheline syenites. Such behavior of these elements is apparently connected with their ability to accumulate in the residual melt; lead, in terms of ionic radius, is close to potassium, which promotes its accumulation in potassium-bearing rocks, such as the products of late stages of differentiation.

The content of copper in the rocks of all phases is below the Clarke value; moreover, it varies within the limits of 0.002–0.003% in all rocks. No regularity in the behavior of this element or connection with any particular mineral is observed. It is found in equal amounts by spectral analysis in pyroxene, nepheline, cancrinite, and magnetite, with which its approximately equal content in all rocks is associated.

Group of rare elements. Of this group, yttrium, zirconium, and gallium deserve attention.

Yttrium, except in melteigites, is contained in all varieties of rocks in amounts above the Clarke value. At the same time, there is a tendency for its content to increase from 0.0015% in melteigite to 0.008% in alkaline syenite, which is four times higher than the Clarke value. Such geochemical behavior of the element is connected with the fact that it tends to accumulate in potassium-bearing minerals, and also with the fact that its content usually increases as the calcium content decreases. It is precisely in this direction that the composition of the rocks of the Semenovsky massif changes. Zirconium in all varieties of rocks is present in amounts above the Clarke value. A clear tendency is seen for its content to increase from the early products of differentiation to the late ones. Thus, in melteigites the element is present in an amount of 0.013%, and in alkaline syenite, 0.072%. Such geochemical behavior of zirconium is explained by the fact that the element enters mainly into the composition of the accessory mineral zircon, the content of which sharply increases in the late products of differentiation. In the rock-forming minerals its content does not exceed 0.001–0.003%. The content of gallium steadily increases from the early products of differentiation to the late ones. Thus, in melteigite and ijolite it is equal to 0.001%, and in alkaline syenite—0.004%, which is more than twice the Clarke value. Its preferential accumulation in the late products of differentiation is caused, possibly, by postmagmatic processes, with which the redistribution of the element is connected. In addition to the elements characterized above, niobium, tin, and molybdenum are recorded in individual samples. From the foregoing the following main conclusions may be drawn:

1. All elements established in the rocks of the massif are through-going, except for lithium, which was found in appreciable amounts only in the nepheline syenite.

2. Of all the elements found in the rocks of the massif, the greater part consists of elements characteristic of basic or intermediate rocks. These include titanium, vanadium, chromium, cobalt, nickel, strontium, barium, zirconium, and zinc. It is noteworthy that elements characteristic of basic rocks (titanium, vanadium, chromium, cobalt, nickel, zinc) are contained in all rocks in amounts below the Clarke values; at the same time, their contents in the rocks of all phases fluctuate within narrow limits.

3. Strontium and barium are present in amounts above the Clarke values in almost all rocks. It is known that these elements tend to accumulate in late products of magmatic differen-

differentiation. Such a late differentiate is the parental magma of the alkaline rocks of the massif under consideration, as a derivative of a more basic magma, the early differentiation products of which are the numerous effusives with a predominance of basic varieties of the olivine-basalt type.

1. Of the elements characteristic of alkaline rocks, only three are present in the rocks of the massif—lithium, beryllium, and gallium; moreover, lithium was detected only in two varieties, and its overall content in the massif is insignificant.

2. Of the elements characteristic of acidic rocks, only lead is present, and its content is below the Clarke value.

In conclusion, it may be noted that the geochemical characteristics of the rocks of the Semenovsky massif make it possible to regard them as products of differentiation of a magma of basic composition.