PETROCHEMICAL FEATURES OF DEVONIAN MAGMATIC FORMATIONS OF THE TALANOVSKY GRABEN

(KUZNETSK ALATAU)

V. Z. Mustafin, V. K. Cherepnin

(Presented by Professor A. M. Kuzmin)

The geological structure of the Talanovsky graben, the petrographic characterization of the effusive, intrusive, and dike formations distributed within it, and the history of their formation were considered earlier in the works of V. Z. Mustafin, V. G. Kryukov, and N. S. Lykhina [4, 5, 9]. According to current views, the following sequence of formation of effusive, intrusive, and dike rocks is established in the graben (from ancient to young), composing the Telbes series.

1. Effusives of basic composition, among which labrador-pyroxene, sometimes with olivine, labrador porphyrites and aphyrites with their lavas, clastolavas, and pelitic as well as psammitic tuffs are developed.

2. Effusives of intermediate composition, represented by plagioclase-hornblende porphyrites, among which lenticular bodies of tuff-conglomerates are noted. In the area described, the plagioclase-hornblende porphyrites form a dike-like body and are, apparently, the subvolcanic (vent) facies of the basic effusives, having both sharp and gradual contacts with the latter.

3. Effusives of alkaline composition, the principal representatives of which are trachyte porphyries and their lavas. Trachytoid phonolites are less widespread.

The alkaline effusives are cut by bodies of orthophyres and coarse-grained quartz-free syenites, which are evidently the vent facies of the alkaline effusive complex. By analogy with neighboring regions [1, 3, 8, 10], the age of all these formations is accepted as Lower Devonian.

1. The youngest member of the Telbes series, occurring in the very upper parts of its section, consists of lavas of pyroxene-olivine and later andesitic porphyrites. They are separated from the rocks mentioned above by a red-colored sedimentary sequence, which has a limited distribution within the graben. Correlation of the Talanovsky section with similar sections of Devonian effusives in neighboring regions makes it possible to regard the age of the pyroxene-olivine and andesitic porphyrites as Eifelian [4].

The effusive sequences in the graben are cut by dikes of labrador porphyrites, microsyenites, microdiorites, and also by bodies of quartz syenites. The latter intrude all the effusive sequences, character-

…characterized above; however, they themselves, in turn, are cut by the youngest dikes in the graben—diorite porphyrites and spessartites.

Thus, the volcanogenic formations of the graben are rather clearly separated into two independent complexes, composed of the 1st–3rd and 4th effusive strata. In time these complexes are divided by a prolonged period of formation of a red-colored sedimentary sequence. The petrographic and geological features of the complexes make it possible to assign the first of them to the trachyandesite formation, and the second to the basalt-andesite formations included among orogenic, chiefly geoanticlinal formations of mobile zones [7].

Below, the petrochemical features of the volcanogenic formations of the Talanovsky graben are considered (Table 1). Chemical analyses of the rocks were carried out in the Central Laboratory of the Novosibirsk Territorial Geological Administration and in the chemical laboratory of the MGRI. Recalculations of the analyses into the numerical characteristics of A. N. Zavaritskii [2] were performed at the Hydrougol Research Institute in Novokuznetsk on the “SETUN” computer. The recalculation results are given as vector representations of the rock composition. On their basis, variation curves were constructed for the volcanogenic Lower Devonian trachyandesite and Middle Devonian basalt-andesite formations (Fig. 1).

The trachyandesite formation is composed of a complex series of successively formed effusives of basic, intermediate, and alkaline composition. On the petrochemical diagram, the figurative points of the rocks composing the formation are grouped into a continuous band extending from bottom to top across its entire field.

The vectors of the rocks representing the lower parts of the formation are situated in the lower part of the diagram, relatively close to the SB axis, which is typical of varieties of basic composition enriched in femic components. On the ASB plane, the vectors of the labradorite and pyroxene-labradorite porphyrites (Nos. 27, 34, 45, 33) are rather large and oriented mainly steeply to the right, which is characteristic of rocks enriched in magnesian silicates. On the CSB plane, the figurative points of the basic effusives are located at some distance from the SB axis, which emphasizes the appreciable content of feldspathic lime in them. The orientation of the vectors and the high value of \(n\) (84.7–96.4) indicate the predominance of \(Na_2O\) in the sum of alkalies. In general, the alkalinity of these rocks, expressed by the ratio \(a : c\), is variable and changes from normal (0.6) for the labradorite porphyrites, according to R. Daly [2], to high (2.2) and very high (3.2).

The single chemical analysis of aleuritic tuff of basic rocks (No. 48) characterizes it as a variety oversaturated with alumina (the vector on the ASB plane has a left-hand orientation). The alkalinity of the tuff remains very high (\(a : c\) equals 2.6) for rocks of basic composition, and in the sum of alkalies \(K_2O\) sharply predominates over \(Na_2O\), which is emphasized by the gentle inclination of the vector on the CSB plane and the rather low value of \(n\) (25.0).

The figurative points of the plagioclase–hornblende porphyrites (Nos. 24, 25, 26), in comparison with the figurative points of the rocks considered above, are somewhat farther from the SB axis on the ASB plane, but remain at approximately the same distances from it on the CSB plane. This expresses an increase in the sum of alkalies in the rocks under consideration and the constancy in them of the amount of feldspathic lime. On the ASB plane, the vectors of these rocks acquire a steeper right-hand orientation, alternating with a left-hand orientation. The change in vector direction indicates variable saturation of the rocks with alumina.

Fig. 1. Diagram of the chemical compositions of magmatic rocks of the Talanovsky graben:
1—effusive rocks of the trachyandesite formation; 2—effusive rocks of the basalt-andesite formation; 3—intrusive and dike rocks; 4—variation curve of rocks of the trachyandesite formation; 5—variation curve of rocks of the basalt-andesite formation.

Table 1

Chemical compositions of magmatic rocks of the Talanovsky graben

Continuation of Table 1

Analysis numbers

Continuation of Table 1

Continuation of Table 1

The alkalinity of the plagioclase–hornblende porphyrites is not maintained ($a:c$ varies from 1.8 to 2.9) and is high in comparison with the average compositions of similar rocks, according to P. Daly ($a:c$ equals 1.7). In the sum of the alkalis, $Na_2O$ remains predominant, which is confirmed by the steep orientation of the vectors on the CSB plane and by high values of $n$ (80.2—87.6 as against 77.5 according to P. Daly).

On the diagram there are numerous representative points (Nos. 23, 28, 19, 58, 38, 39, 42, 51, 20, 46, 54, 2, 22, 55, 21, 1, 56, 49, 16, 17, 18, 41, 44, 4) of rocks representing the upper horizons of the trachyandesite formation. On the ASB plane they form a rather dense field situated at an ever greater distance from the SB axis. This corresponds to an increase in the total alkalinity of the rocks represented by them. At the same time, with a decrease of parameter $b$ (from 15 to 5), a branching of the variation curve of the effusives of the trachyandesite formation is outlined. One branch—the left one (Nos. 22, 55, 21, 49, 17, 16, 18, 44)—bends rather sharply toward the vertex S, which indicates an increase in the acidity of the rocks characterized by it. The other branch (representative points Nos. 39, 46, 2, 56, 1, 41, 4) preserves the general direction of the variation curve reflecting the compositions of the effusives of the lower beds of the formation. This branch continues to deviate from the SB axis, which testifies to increasing alkalinity of the rocks.

A large part of the effusives of both groups is strongly supersaturated with alumina, and the vectors representing them have a gentle to almost horizontal leftward orientation. Mineralogically, these features of the chemistry of the rocks are expressed by the appearance in them of mica or nepheline, manifested individually or potentially in their uncrystallized groundmass. Only individual analyses establish a normal alumina content in these rocks, and their vectors assume a rightward orientation. At the same time, the ratio of alkali feldspars to calcic feldspars ($a:c$) in the rocks of the left branch varies on average from 4.28 to 29.5 and reaches a maximum (104.2) where alkali feldspars predominate. The value of $n$ in them undergoes less abrupt fluctuations, which corresponds to the comparative constancy of the amounts of soda and potash alkalis, with the former usually predominating somewhat. Often, however, their quantities become close, and sometimes (No. 55) $K_2O$ predominates over $Na_2O$.

The rocks characterized by the right branch of the variation curve, distinguished by higher values of parameter A, are for the most part strongly supersaturated with alumina. The $a:c$ ratio in them is very variable (from 2.02 to 52.1) and reaches 87.2. The value of $n$ in rare cases (Nos. 4, 1) indicates almost equal amounts of $Na_2O$ and $K_2O$ in the rocks, more often testifying to the predominance of $Na_2O$ in their sum.

These variations in the quantitative ratios of $K_2O$ and $Na_2O$ are correspondingly reflected in the orientation of the vectors of both groups of the noted rocks on the CSB plane. From the position of the representative points on this plane, moreover, it is evident that the rocks characterized do not show any particular difference in the content of feldspathic lime. Their common variation curve steadily approaches the SB axis, thereby emphasizing a decrease in the amount of feldspathic $CaO$.

Comparison of the compositions of the alkaline rocks of the graben with the average compositions of the same rocks according to P. Daly shows that close types are found for the graben effusives. The observed differences in most cases should be attributed to hydrothermal alteration of the graben rocks. Thus, orthophyres and partly trachyte porphyries, frequently subjected to albitization, become saturated with soda alkali and therefore...

with respect to this feature show great similarity to the plagioporphyries of the Minusinsk intermontane trough [8].

The formation of the trachyandesite formation is apparently completed by alkaline syenites (Nos. 5, 10, 40), whose figurative points on the ASB plane spatially gravitate toward the right branch of the variation curve of the formation’s effusives. Their points deviate ever farther from the SB axis, emphasizing the increasing alkalinity of these rocks. On the CSB plane the plotted points lie very close to the SB axis, which is typical of rocks undergoing loss of feldspathic lime. The syenites are oversaturated with alumina (their vectors on the ASB plane have a left, gentle direction), and contain nearly equal amounts of sodium and potassium alkalis ($n$ from 48 to 62, while the vectors on the CSB plane are comparatively gentle). The ratio of alkaline feldspars to calcic feldspars ($a : c$) in the alkaline syenites of the graben (11.0–22.9) is close to its value in alkaline syenites and alkaline trachytes according to R. Daly (14.7–30.3). Accordingly, the amounts of sodium and potassium alkalis ($n$) are close to their contents in typical alkaline syenites and trachytes (55.8–62.1 versus 63.9, according to R. Daly).

Chemical analyses of pyroxene–olivine basaltic (Nos. 43, 37, 47) and andesitic (Nos. 30, 29, 32, 59) porphyrites of the younger basalt–andesite formation of the graben are few in number, but they illustrate rather fully the petrochemical features of this group of rocks. Having compositions close to the average compositions of basalts according to R. Daly, the pyroxene–olivine porphyrites of the graben, in particular, are distinguished by consistently high MgO contents. Only occasionally (No. 37) are they saturated with alumina, and their vectors on the ASB plane are oriented downward to the left. For the most part, however, these rocks are deficient in alumina, and their vectors on the ASB plane have a rightward orientation. The figurative points of the rocks described on this plane deviate moderately to the right, which expresses the increasing alkalinity of the rocks during their formation. The total alkalinity of the graben basalts ($a : c$), however, is lower than this value for typical basalts according to R. Daly and for olivine basalts (0.7–1.0 versus 1.5 and 1.17). The position of the plotted points of the characterized rocks on the CSB plane indicates a noticeably higher content of feldspathic lime in them than in the basic rocks of the trachyandesite formation. The orientation of the rock vectors on this plane reflects the stable predominance of Na₂O in the sum of alkalis.

Andesitic porphyrites (Nos. 29, 30, 31, 32, 35, 59), by their principal characteristics, are transitional from typical basaltic to andesitic porphyrites according to R. Daly, but with the former of these rocks they show great similarity. Their figurative points on the ASB plane continue to deviate from the SB axis to the right, and their vectors turn counterclockwise. These changes are caused, respectively, by an increase in alkalis and a decrease in the saturation of the rocks with magnesia and alumina. On the CSB plane the projections of their points remain at approximately the same distance from the SB axis, which confirms the constancy in them of the amount of feldspathic lime. The total alkalinity of the rocks deviates from average ($a : c$ — 1.2–1.3) to high (3.2 versus 1.75 for rocks according to R. Daly). Judging by the orientation of the vectors on the CSB plane, Na₂O predominates in the sum of alkalis; however, occasionally (No. 59) a substantial increase in the proportion of K₂O is noted.

The variation curves constructed for the formations considered allow some general conclusions to be drawn about the features of their petrochemistry.

The variation curve of the trachyandesite formation is traced across the entire field of the diagram and represents a prolonged evolution

of the magmatic center, which led to the formation of a complex assemblage of basic, intermediate, and alkaline effusives. In this case, the final product of the formation of this complex was quartz-free syenites.

The variation curve of the basalt-andesite formation is almost twice as short as the curve of the first formation. On the ASB plane, the lower half of this curve, representing basalt porphyrites, shows great similarity to the variation curve of the labradorite porphyrites of the basement of the trachyandesite formation. The upper half of the variation curve of the basalt-andesite formation, characterizing the composition of the andesite porphyrites, differs substantially from the latter. It undergoes a greater deflection to the right, corresponding to a greater saturation of these rocks with alkalis. Its upper end, however, bends sharply toward the SB axis, which expresses a tendency toward increasing acidity of the rocks of the formation. However, the development of this formation does not proceed beyond the formation of andesite porphyrites of increased basicity.

Still greater differences between the formations are manifested in the degree of saturation of their rocks with feldspathic lime, which is clearly visible on the CSB plane, where the variation curve of the basalt-andesite formation departs noticeably farther from the SB axis than the curve of the trachyandesite formation.

Thus, the volcanogenic formations considered differ not only in geological position, petrographic composition, and the character of the final products, but also in petrochemical features. All rocks composing the trachyandesite formation are characterized by increased alkalinity, with a predominant excess of Na₂O over K₂O, beginning with its earliest products—labradorite and labrador-pyroxene porphyrites.

The basalt-andesite formation has some petrochemical similarity with the trachyandesite formation; however, it is characterized by an incomparably shorter period of development, during which its distinctive features appear. The principal ones consist in a greater saturation of its rocks with feldspathic lime.

Volcanogenic formations are formed during the successive differentiation of magma, manifested differently for the trachyandesite and basalt-andesite groups. Both these formations, according to Yu. A. Kuznetsov, belong to the basalt-andesite-liparite group, the origin of which is undoubtedly connected with basaltic magma [7]. The petrographic and petrochemical features of the graben formations make it possible to suppose that they do indeed arise during the differentiation of basaltic magma having increased alkalinity, especially during the period of formation of the trachyandesite formation. At the same time, the increased alkalinity of the rocks may be explained by desilication of the initial magma as a result of its interaction with limestones [6]. In the territory described, as noted earlier, limestones of the Usin Formation of the Lower Cambrian are widely distributed; they underlie and border the graben.

REFERENCES

1. I. K. Bazhenov. Effusive and vein complex of alkaline rocks of the eastern slope of the Kuznetsk Alatau. In: Materials on the Geology of Western Siberia. Gosgeoltekhizdat, issue 64, 1963.

2. A. N. Zavaritskii. Recalculation of chemical analyses of igneous rocks and determination of their chemical types. 2nd ed., Gosgeoltekhizdat, 1960.

3. G. A. Ivankin. On the stratigraphy of the Devonian of the western margin of the Minusinsk depression (Devonian of the environs of Lake Itkul). Proceedings of Tomsk State University, vol. 146, 1960.

1. V. G. Kryukov, V. Z. Mustafin. Subdivision of volcanogenic formations of the Talanovskaya series on the area of the Talanovsky graben. Izv. TPI, vol. 165, 1969.

2. V. G. Kryukov, V. Z. Mustafin, N. S. Lykhina. History of the formation of the Talanovsky graben (northern spurs of the Kuznetsk Alatau). Izv. TPI, vol. 166, 1969.

3. E. A. Kuznetsov. Petrography of magmatic and metamorphic rocks. Moscow State University Press, 1956.

4. Yu. A. Kuznetsov. Principal types of magmatic formations. Nedra, 1964.

5. I. V. Luchitsky. Volcanism and tectonics of the Devonian depressions of the Minusinsk intermontane trough. Siberian Branch, USSR Academy of Sciences, Moscow, 1960.

6. V. Z. Mustafin, V. G. Kryukov, N. S. Lykhina. Principal features of the geological structure of the Talanovsky graben (northwestern slope of the Kuznetsk Alatau). Izv. TPI, vol. 151, 1966.

7. A. T. Siyov. On the Chidan Formation of the Devonian of the South Minusinsk Basin. In: Materials on the geology and mineral resources of Western Siberia. TSU Press, 1964.