PETROGENESIS OF THE SPILITE-DIABASE FORMATION IN THE SOUTHWESTERN PART OF THE KUZNETSK ALATAU

L. V. Peshekhonov

(Presented by Prof. A. G. Sivov)

In the southwestern part of the Kuznetsk Alatau, considerable areas of the watershed region of the Mrassu, Kondoma, and Pyzas rivers are occupied by volcanogenic formations of basic composition. Analysis of the factual material on the effusive rocks of the area enabled the author to assign the entire complex of volcanogenic rocks, in terms of composition and conditions of formation, to the spilite-diabase formation. The discovered remains of Lower Cambrian archaeocyaths and algae in the carbonate members of the formation indicate the formation, at least of its upper members, at the beginning of the Early Cambrian [6].

Structurally, the deposits of the formation are associated with zones of deep faults that were initiated in a eugeosynclinal type of structural-facies zone.

The spilite-diabase formation has a variegated composition—alongside the volcanogenic rocks, sedimentary deposits are also developed in it. Among the sedimentary rocks, siliceous and carbonate deposits are widely developed, and terrigenous deposits are rarer. Siliceous deposits usually replace lavas along the strike. Carbonate and terrigenous deposits are most often encountered in the upper parts of the formation and, in time, are associated with stages of the relative waning of volcanism. The volcanogenic formations proper are manifested predominantly as sheets of basic lava, the eruption products of which have been preserved as a number of petrographic varieties of diagenetically altered rocks. Among them, albitized basaltic and diabase porphyrites are typical. Spilites, albitized hyalobasalt porphyrites, and tuffs are less widespread. The presence of jaspers, siliceous shales, and spilites with a pillow type of texture indicates the submarine character of volcanism.

At present, as a result of petrochemical analysis of various associations of igneous rocks, the existence of three classes of magmas of basic composition may be considered established; in the opinion of Yu. M. Sheinmann [9], these cannot, by their chemistry, be reduced to one another. Yu. M. Sheinmann regards the andesite-basalt complex of rocks of island arcs, the olivine-basalt association of rocks of oceanic islands, and tholeiitic plateau basalts as reflections of independent classes of basic-type magma. To determine the initial class of magma for the rocks of the spilite-diabase formation of our area, we used the diagrams of Yu. M. Sheinmann [9], the construction of which is based on

using the alkalinity coefficient (Fig. 1) and the “femic index” (Fig. 2). In the diagrams presented, it is evident that most analyses for all rock types of the formation fall within the field of the olivine-basalt class of basic magma. On this basis, we are inclined to regard the rocks of the spilite-diabase formation of our area as differentiates of an olivine basalt magma. The olivine-free

Fig. 1. Silica–alumina–alkali index diagram for Pacific Ocean lavas (Yu. M. Sheinmann): 1—field of andesite-basalt complexes, 2—field of olivine-basalt complexes, 3—field of ultrabasic complexes, 4—Cambrian–Proterozoic effusive rocks of the spilite-diabase formation of the southwestern part of the Kuznetsk Alatau.
Fig. 2. Silica–“femic index” diagram for Pacific Ocean lavas (Yu. M. Sheinmann). Symbols are the same as in Fig. 1. $\alpha$—sum of sodium and potassium oxides; $\beta$—sum of oxides contributing to the formation of dark-colored minerals.

basaltic composition of the rocks of the formation described is explained by the fact that fractional crystallization of the parental magma (olivine) took place at a sufficiently high level of oxygen pressure, ensured by the introduction of water into the system [5], which is permissible under conditions of submarine volcanism.

The petrographic composition, as well as the chemical composition of the effusive rocks (Table 1), shows that, on the whole, volcanogenic rocks of the formation are characterized by sodium specialization in the composition of the alkalies. However, the sodium content in the rocks considered, as noted by A. F. Belousov et al. [1], and for basaltic effusives of the Upper Proterozoic

and Cambrian of Altai and Salair, only in exceptional cases reaches the average for high-sodium spilites. More than half of the analyzed rocks contain less than 3.5% \(\mathrm{Na_2O}\), whereas the average content of \(\mathrm{Na_2O}\) in spilites, according to H. Sundius [7], reaches 4.93%. At the same time, in the rocks of the formation described, with any content of \(\mathrm{Na_2O}\), plagioclase is usually represented by albite, in which relicts of more basic plagioclase are sometimes preserved. The latter circumstance indicates the secondary origin of albite during metasomatism of primary basic plagioclase. This process takes place with the introduction of Na and Si and the removal of Ca and Al according to the equation of F. Turner and J. Verhoogen [7]

\[ \underset{(\text{labradorite})}{\mathrm{NaCaAl_3Si_5O_{16}}} + \mathrm{Na^+} + \mathrm{Si^{4+}} \rightarrow 2\underset{(\text{albite})}{\mathrm{NaAlSi_3O_8}} + \mathrm{Ca^{++}} + \mathrm{Al^{3+}}. \]

In our opinion, Na and Si are introduced not from outside (seawater, later fluid solutions, etc.), but from the unstable basic mass of the solidified effusives. Usually this is the basic glassy groundmass of basalts or diabases, or the interstitial basic groundmass in spilites with pillow texture. In such rocks there occurs a process of redistribution and differential concentration of Na, Si, Al, and Ca between the crystalline phase of the effusives (plagioclase) and the final uncrystallized products of the solidified magma. A good example of the process of redistribution of the above-mentioned elements is provided by the results of analyses of the chemical composition of different parts of concentrically zoned pillow bodies, given in Table 2 (4, 7). From this table it is evident that the central parts of the spheres, in comparison with the peripheral ones, are characterized by an elevated content of Na and Si and a reduced content of Al. With respect to calcium, it may be said that it is carried out of the central parts of the spheres together with the calcium of the peripheral zones and is deposited as calcium carbonate in the amygdales of the rocks. The foregoing explains the fact that the basic mass in the effusives of the formation described has a chloritic composition, whereas in unaltered basalts the composition of the glass is richer in silica than its average content in the rock. This also explains why the content of \(\mathrm{Na_2O}\) in albitized basic effusives on average approximately corresponds to its average content in unaltered basalts—3.11% [2], which is concentrated mainly in crystalline plagioclase. The process of redistribution of the above-listed components during metasomatic albitization is expressed most clearly in rocks that have two forms of the state of the material of the solidified magma—crystalline and glassy. The content of \(\mathrm{Na_2O}\) in albitized rocks is then determined by the content of crystalline plagioclase in the rock. The fewer plagioclase crystals there are in the rock, the lower the content of \(\mathrm{Na_2O}\) in the bulk chemical analysis of the rock. At the same time, the amount of primary glass is of leading importance in this process. In well-crystallized rocks, albitization of crystals of basic plagioclase does not proceed to completion, since the insignificant amount of glass does not provide the necessary supply of Na and Si for the complete replacement of basic plagioclase by albite. This explains the presence in albite of relicts of basic plagioclase and of entire areas of primary rocks of basaltic composition within the general mass of volcanogenic rocks.

It is important to note that albitization begins after the solidification of the magma, but before its complete cooling, is possible, in part, in the solid state, and begins with the decomposition of volcanic glass. The decomposition of volcanic glass, in the opinion of M. A. Usov [8], is greatly assisted by

Table 1

Chemical composition of rocks of the spilite-diabase formation of the Talon-Kamzas region (in percent)

liquid and gaseous substances that are dissolved and occluded in the solidified magma. However, the decomposition of volcanic glass also takes place in effusives of continental eruption, but it never leads to albitization on a regional scale. Therefore it is natural to consider that the intensity of albitization depends on the influence of regional physicochemical factors (temperature, pressure, excess \(H_2O\) and \(CO_2\)), which are most favorably combined under geosynclinal conditions of volcanism—conditions of greater mobility. Tectonic movements occurring over the course of the long history of the devel-

Table 2

Chemical compositions of zones of spheroidal spilites. Analyses: 1, 2, 3, 4 are taken from the work of F. Turner and J. Verhoogen [7]. Analyses: 5, 6, 7 are taken from the work of G. D. Nichols [4]. 1 — Center of a sphere, Newborough, Wales. 2 — Chilled margin (variolitic) of the same sphere as the sphere of analysis 1. 3 — Groundmass surrounding the same sphere as the sphere of analyses 1, 2. 4 — Groundmass of spheroidal spilites, Alps, Champach, Switzerland. 5 — Central part of a zonal spilitic sphere from the Bilt volcanic series. 6 — Green material of the peripheral part of the same sphere as the sphere of analysis 5. 7 — Dark-gray material from the margin of the same sphere as the sphere of analyses 5, 6. 8 — Central part of an outwardly homogeneous spilitic pillow body of the spilite-diabase formation of the Lower Cambrian in the southwestern part of the Kuznetsk Alatau. 9 — Peripheral part of the same pillow body as the pillow body of analysis 8.

opment of geosynclines provide high and long-lasting mobility for liquid solutions, which, by their action at favorable temperature and pressure, lead not only to complete albitization of plagioclase but also to decomposition of pyroxene. In evaluating the end products of this replacement process, it is necessary to agree with D. S. Korzhinskii [3], who regarded the albitization of basalts as an ordinary metamorphic process arising at depths of not less than one kilometer.

REFERENCES

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