Features of the Chemical Composition and Some Questions of the Genesis of the Alkaline Rocks of the Batayul Massif

A. A. Mityakin

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

The Batayul massif of alkaline rocks is located in the northern part of the Kuznetsk Alatau, along the Batayul spring, a left tributary of the middle course of the Uryup River. The total area of the massif exposed at the surface is about 5.5 km², with maximum dimensions of 1.2 km across strike and 5 km along strike.

The geological position of the region that includes this area is determined by its location within the marginal Devonian step of the Kuznetsk Alatau, bordering in the north on the Chulym–Yenisei depression. The structure of the region includes Upper Proterozoic and Lower Paleozoic deposits, overlain by a thick sedimentary–volcanogenic sequence of the Lower and Middle Devonian. All these sequences are intruded by a large number of intrusive massifs of alkaline nepheline-bearing rocks.

The nepheline rocks of the Batayul massif were first discovered in the form of bereshites by M. M. Grunin and K. V. Ivanov during geological mapping work in 1940. Subsequently it was visited by M. V. Voroshilov (1957) and Yu. D. Skobelev (1959). However, until recently the massif had been poorly studied. Bereshites were regarded as a constituent part of the Devonian sedimentary–effusive sequence. In petrographic descriptions only bereshites and nephelinites were noted.

As a result of field investigations carried out by the author in 1965, a great diversity of rocks participating in the structure of the massif was established. The earliest characteristic formations, if one does not count the few bodies of diabases and dolerites that arose first and in composition correspond almost exactly to the average types of such rocks according to Daly, are the bereshites. The peculiarity of their mineralogical composition consists in the paragenesis of diopside, basic plagioclase No. 70, and nepheline, which constitutes, on average, of the total mass of the rock ...

Figure 1. Variation diagram of the petrochemical composition of rocks of the Batayul massif, after A. N. Zavaritsky

about 50%. In bulk chemical composition the bereshites tend toward banakites. Their difference from the latter consists in the higher value of the value “C”—6.90 as against 3.90, and also “Q”—23.70. In Fig. 1 it is evident that the points of the chemical compositions corresponding to the bereshites show considerable scatter. The latter is caused by the zonal structure of the bereshite sill, observed in the direction from the footwall to the roof and arising as a result of gravitational differentiation in place. Chemical-composition point No. 13 from the chilled zone at the footwall of the bereshite sill most closely corresponds to the primary composition of the magma, since, owing to rapid cooling—the main mass is glassy—gravitational differentiation could not affect the composition of the rock here.

Above the chilled zone the bereshites are enriched in inclusions of pyroxene in the principal fine-grained mass. Owing to enrichment in femic components, the chemical-composition points of these bereshites—Nos. 5, 6, 7, 8—are located correspondingly below and to the left of point 13. In the roof of the sill, owing to the settling of femic components, the bereshites proved to be enriched in leucocratic components—the chemical-composition points Nos. 17, 20, located to the left and above point No. 13.

Analyses Nos. 15, 24, 25, 26, and 29 were taken from bereshites that had undergone the process of potassium feldspathization during the formation of potassium-feldspar nepheline-free syenites. The effect of the syenites on the bereshites was expressed in the acidification of plagioclase to albite and the decomposition of dark-colored minerals, with subsequent replacement of albite by potassium feldspar. In areas closely adjoining the syenites, the bereshites underwent complete

recrystallization with the formation of alkaline leucocratic syenites. The bulk chemical composition of the partially altered bereshites mentioned above shows a considerable decrease in their iron and calcium content and an increase in potassium, sodium, and silica, which is reflected in the variation diagram: whereas the unaltered bereshites tend toward banakite, the potassium-feldspathized ones tend toward umptekite. A distinctive feature of the mineralogical composition of the bereshites is the absence of olivine; in its place there is high-calcium diopside, in which the CaO content reaches more than 20%. Recalculation of the chemical analysis of the bereshites to a normative mineralogical composition gives up to 10% normative olivine. Such a discrepancy between the normative and the true mineralogical composition is explained by the oversaturation of the magma with calcium. The average chemical composition of the bereshites shows a CaO content almost two percent higher than the content of the latter in rocks of similar composition—banakites.

The rocks mineralogically characterized as tinguaite porphyries—chemical analyses Nos. 5 and 20—are, in chemical composition, close respectively to latite and banakite. They differ from the latter by elevated values of “C”—6.50–7.80 instead of 3.90–4.10—and “Q”—−10.60–22.70 instead of +1.10–11.90. In comparison with the average composition of tinguaite according to Daly, they show very similar numerical characteristics, except for the value of “в,” which in the rocks described ranges from 13.80 to 17.80 instead of 29.60 in Daly’s tinguaite; that is, these rocks are more leucocratic varieties, approaching trachybasalts in composition.

The nepheline monzonites and nepheline syenites formed after the tinguaite porphyries are close in composition, respectively, to alkaline-earth hornblende syenite and umptekite. The difference of both these varieties from the average rock types of analogous composition according to Daly consists in higher values of “C,” “Q,” and also “n.” If in the monzonites “C” is 6.70, “Q” is −9.20, and “n” is 75.50, then in rocks of similar composition according to Daly “C” is 3.20, “Q” is −7.00, and “n” is 48.20. Approximately the same relationships are maintained in the nepheline syenites as well. The alkaline-earth hornblende syenites have a composition varying from keratophyre to banakite. The difference between their composition and that of the latter, as in the preceding varieties, consists in higher values of “C,” “Q,” and “n.” Finally, the alkaline leucocratic nepheline-free syenites, among which there are quartz-bearing varieties, are very close in composition respectively to laurvikite or keratophyre and nordmarkite.

In general, the massif is characterized by a change in the chemical composition of the rocks from the early products of differentiation to the later ones toward a decrease in the content of the oxides of calcium, iron, and magnesium and an increase in the content of silica, which is reflected in the variation diagram, as well as by an increase in the role of potassium alkalis with a simultaneous decrease in the content of sodium alkalis. The latter is confirmed by the fact that monomineralic potassium-feldspar syenites formed in the final stage of differentiation. However, in percentage terms the increase in K₂O content is less pronounced.

Thus, if in the bereshites the average K₂O content is 1.98% (average of 16 analyses), in the nepheline-free alkaline syenites it is 2.69%; the maximum K₂O content in the indicated syenites reaches 5.5%. However, these figures do not reflect the primary content of potassium alkalis in the syenites, since the latter have been altered to a considerable degree by postmagmatic processes, as a result of which potassium feldspar for the most part underwent albitization, as indicated by the chemical analyses. Thus, in individual samples of potassium alkaline sy—

In melanocratic syenites the content of \(K_2O\) is only 0.10–0.30%, although the samples were taken from monomineralic feldspar rocks.

Taking into account that the formation of the massif took place under near-surface conditions, where, owing to rapid cooling, the possibility of any appreciable interaction of the magma with the enclosing rocks was excluded—apart from its loss of volatiles—as well as the interaction of later portions of magma with earlier products of differentiation, it may be concluded that the composition of the rocks of successive stages of differentiation closely corresponds to the composition of individual portions of magma that came from deeper horizons. On the basis of the foregoing, and taking into account the abundance of one or another rock variety participating in the structure of the massif, an average chemical composition has been calculated for the entire massif, giving an idea of the composition of the magma that produced the variety of rocks of the massif as a result of successive differentiation (Table 1).

As is evident from the comparative table, the average composition for the massif is close to the composition of trachyandesite according to Daly; the difference of the former consists in a lower silica content and a higher content of alumina, volatiles, and lime. On the variation diagram, according to A. N. Zavaritskii, the points of the chemical compositions lie close to one another. Was the magma that produced the variety of rocks of the massif independent, and what caused its depletion in silica with simultaneous enrichment in alumina, volatiles, and lime? In answering the first question, the alkaline rocks of the area cannot be considered in isolation from the widespread effusive formations of the same area. Among the latter, varieties are widespread ranging from olivine basalts through transitional alkaline varieties, including nepheline-bearing ones, and ending with trachytes (Fig. 2).

During areal geological mapping of the region at a scale of 1 : 5000, a periodicity was noted in the succession of effusive sequences; in each such sequence, alkaline varieties—trachyandesites, trachydolerites, phonolites, etc.—occur at the base, while at the tops occur basic, mainly olivine, basalts. From this it may be concluded that during each outburst of magmatic activity the alkaline differentiates of the magma, located in the upper part of the magmatic chamber, were the first to pour out onto the surface, whereas at the end of the outburst material from deeper horizons entered the vents of volcanoes, and therefore more basic material—in the form of olivine basalt.

If one also takes into account the structural features of the olivine basalts—olivine and pyroxene occur in them as phenocrysts, constituting up to 40% of the total mass of the rock—then it may be concluded that the olivine basalts, as well as all the alkaline varieties of the effusives—phonolites, tephrites, etc.—are products of differentiation of a single magma of basic composition. In terms of their structural and geological position, mineral composition, and content of rare elements, the effusive formations are comagmatic with the intrusive rocks, and the latter are the roots of the magmatic activity that was widely manifested in Devonian time.

Comparison of the variation diagrams of the effusive and intrusive formations shows that in the latter the lower part of the diagram, corresponding to olivine basalts, is absent, while the upper part of the diagram rises higher; i.e., the magma that produced the variety of rocks of the massif, in its composition, was to a considerable degree alkaline and was derived from magma of basic composition, just as were the derivative portions of magma that produced the alkaline varieties of the effusives. The presence of more finely divided products of differentiation among the rocks of the massif, corresponding to the upper part of the diagram, is explained by the fact that the formation

Table 1

Fig. 2. Variation diagram of the petrochemical composition of the effusives of the basin of the Urop River after A. N. Zavaritskii

the massif took place during the final stage of magmatic activity in a calmer tectonic setting. Thus, the magma of approximately trachyandesitic composition that gave rise to the diversity of rocks of the massif was, apparently, derived from a magma of more basic basaltic composition as a product of crystallization differentiation.

As noted above, a characteristic feature of the averaged chemical composition of the rocks of the massif is a reduced silica content, an elevated content of Al₂O₃, volatiles, and also CaO. The latter feature is expressed only weakly in percentage terms—the CaO content in the average chemical composition of the rocks of the massif exceeds its content in trachyandesite by 0.76% (Table 1). The calcium-oxide oversaturation of the magma is better judged from the features of the mineralogical composition. Thus, in bereshites, despite their considerable undersaturation with silica, olivine is absent; instead, high-calcium pyroxene of the diopside series is formed, with a CaO content of more than 20%. In these same rocks, at high alkalinity, plagioclase No. 70 is present. The percentage content of CaO in these rocks is almost 2% higher than its content in rocks of analogous composition according to Daly. Calcium-oxide oversaturation is also indicated by its high content in nepheline—more than 8%—as well as by the oversaturation of all the rocks with free carbonate.

Taking into account the listed features of the mineralogical composition, as well as the fairly high content of CO₂ for near-surface conditions,

stage of formation of the massif, when most of the volatiles had been lost, and also the reduced content of SiO₂ in the average chemical composition of the massif, it may be assumed that the primary magma underwent desilication in the course of interaction with the enclosing rocks and, in particular, with carbonate rocks. This conclusion is also supported by features of the mineralogical composition of the effusive rocks. Thus, in highly basic olivine basalts, in which the value “B” reaches 40 units, olivine is usually present in only a small amount; the bulk of the mafic minerals consists of monoclinic pyroxene, whose formation was promoted by an abundance of CaO. Supersaturation of the magma with CaO is also indicated by the fact of the high basicity of plagioclase in the basalts—its number is rarely lower than No. 80, and even in nepheline basalts containing up to 25% nepheline, plagioclase No. 80 is present.

Fig. 3. Comparative characterization of variation diagrams of the chemical compositions of alkaline rocks of the Batayul massif (1—1) and effusive rocks of the Uryup River basin (2—2)

Thus, on the basis of an analysis of the mineralogical and chemical composition of the rocks of the massif and their comparison, by these features, with effusive formations, it may be concluded that two factors had a substantial influence on the genesis of the alkaline rocks:

1. Crystallization differentiation, as a result of which the magma was separated into a more basic magma, corresponding in composition to olivine basalts, and a more alkaline magma of trachyandesitic composition.

2. Interaction of the primary magma with the enclosing rocks and, in particular, with carbonate rocks. Assimilation by the magma of a certain amount of carbonate led, first, to its desilication, and second, owing to supersaturation with CaO, made possible the formation of high-calcium monoclinic pyroxene instead of olivine, which additionally contributed to the formation of undersaturation of the alkaline mineral—nepheline.

In the process of magmatic differentiation, volatiles also apparently played a significant role; their content in the rocks of the massif, despite the near-surface conditions of their formation, is fairly high: CO₂—1.5%, H₂O—3.27%. The high content of volatiles promoted the separation of magma enriched in alkalis and alumina, owing to the possibility of the formation by the latter, upon addition of volatile, readily mobile compounds, which concentrated in favorable parts of the magmatic chamber.