PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV

Volume 252 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

INDICATIONS OF A GENETIC RELATIONSHIP BETWEEN INTRUSIVE AND EFFUSIVE ROCKS IN THE COMPOSITION OF THE GORYACHEGORSK ALKALINE VOLCANO-PLUTONIC COMPLEX

A. A. MITYAKIN
(Presented by the scientific seminar of the Department of Petrography)

The Goryachegorsk alkaline complex appeared over an area which, structurally, occupies a transitional position between the Chulym–Yenisei depression and the Kuznetsk Alatau in its northeastern part. The Kuznetsk Alatau, consolidated by the Salair cycle of tectogenesis, in Lower–Middle Devonian time became involved in active tectonic activity within the indicated transitional zone in connection with the establishment and development of the superimposed structure of the Chulym–Yenisei depression.

Tectonic activity contributed to an intense manifestation of magmatism. The products of magmatic activity are effusive rocks of varied composition, ranging from olivine basalts to phonolites, trachyandesites, and other alkaline varieties. Varieties of basic composition play the predominant role among the effusive formations. All effusive rocks account for 80% of the total mass of the Devonian effusive-sedimentary sequence in the indicated region.

At the final stage of magmatic activity, numerous massifs of alkaline nepheline-bearing rocks were formed; among them, in facies terms, two groups may be distinguished.

1. Near-surface subeffusive bodies in the form of sills, stocks, and dikes. The rocks of such bodies most often have a clearly expressed porphyritic texture, with phenocryst minerals several tens of times larger than the grains of the groundmass, which often has an aphanitic texture.

2. Intrusive massifs of deeper origin, most likely hypabyssal. The rocks of these massifs are holocrystalline, medium- or coarse-grained, often porphyroid. Examples of the former may be the Semenovsky, Batanayulsky, Bazyrsky, Inzyulsky, Andryushkina Rechka massifs; and of the latter—the Goryachegorsk, Belogorsk, Kurgusulsky, and other massifs.

Intrusive massifs have attracted the attention of many researchers, since rocks rich in nepheline are an important industrial raw material. In the Kiya-Shaltyr area, in particular, construction of a mine for the exploitation of urtites is being completed. One of the important questions in the study of alkaline massifs is their genesis. A correct solution to this question will facilitate the identification of new areas of nepheline-bearing rocks. In turn, in order to

to draw correct conclusions about the genesis of alkaline rocks, the establishment of the genetic connection of intrusive rocks with Devonian effusive formations is of great importance. The kinship of intrusive and effusive rocks was earlier assumed by I. P. Rachkovskii (1911, 1912), I. K. Bazhenov (1945), and I. V. Luchitskii (1959, 1960). However, no detailed studies in this direction had been carried out. During 1965–1968 the author carried out work on the study of alkaline rocks of the near-surface facies in the Batanaul, Semenovka, and Cheremushka areas. Detailed investigations showed that the nepheline-bearing rocks studied have a close genetic relationship with the effusive formations of the Lower–Middle Devonian, as well as with the deep-seated intrusive bodies of the hypabyssal facies.

The genetic connection between intrusive and effusive rocks can be judged from a number of features.

1. General geological features. These should include, first of all, the spatial connection of intrusive and effusive rocks. Most of the massifs occur directly within the field of development of Devonian effusives, especially the massifs of the near-surface facies. Structurally, the effusive and intrusive rocks are confined to the junction zone of the Kuznetsk Alatau with the Chulym–Yenisei depression, where tectonic activity was manifested most actively. The effusive rocks, formed in the initial period of magmatic activity, make up a thick sequence characterized by a regular internal structure. In carrying out areal geological mapping at a scale of 1:50000, a rhythmic character of effusive activity was noted, expressed in the structure of the effusive-sedimentary Devonian sequence. Its structure shows an alternation of packets of effusives with thin beds of sedimentary rocks. Two facts are noteworthy: first, in each packet of effusives, in its lower part, as a rule, more alkaline varieties of effusives occur than in the upper part, where olivine basalts usually predominate; secondly, in the overall section of the effusive-sedimentary sequence, alkaline effusives predominate in its upper parts. Thus, in the very upper volcanogenic suite, tuffites, trachybasalts, phonolites, and other alkaline rocks predominate. These facts indicate that effusives of basic and alkaline composition originated from a single parental magma. At the same time, the more alkaline melt, being lighter, accumulated in the upper parts of the magmatic chamber and, with each pulse of tectonic activity, reached the surface first. It was followed by a melt of more basic composition from deeper parts of the magmatic chamber; this accounts for the occurrence, in packets of effusives, of more alkaline varieties at the base and more basic ones at the top. Further, during the entire period of volcanic activity, the evolution of the magmatic focus proceeded toward the accumulation of alkaline melt. At the concluding stage of volcanic activity this led to the formation of substantially alkaline effusives occurring in the upper parts of the volcanogenic sequence. The formation of alkaline intrusive rocks at the concluding stage of magmatic activity confirms the general regular direction of the process of magmatism toward the formation of an alkaline magmatic melt.

2. Geochemical features. The effusives contain the same trace elements as the intrusive rocks—scandium, vanadium, chromium, cobalt, nickel, copper, zinc, gallium, barium, strontium, yttrium, zirconium, lead, and titanium. The only difference is that scandium has been found in appreciable amounts in the effusives, whereas lithium occurs only in the intrusives. This is explained by the fact that scandium tends to concentrate in ferromagnesian minerals, substituting in their

in the crystal lattice, divalent iron and magnesium. But since such minerals are richest in the early products of volcanic activity—effusives predominantly of basaltic composition—the concentration of scandium occurred in these rocks. Thus, this difference in the content of trace elements in effusive and intrusive rocks does not contradict, but rather supports, the genetic relationship of both groups of rocks.

The content of elements of the iron group in both groups of rocks is below the Clarke value; however, in effusives these elements occur in significantly larger amounts. This also confirms the genetic relationship of the rocks being compared, since the indicated elements are characteristic of basic rocks rich in ferromagnesian minerals, which effusives are in comparison with intrusive rocks.

The content of barium and strontium in effusive and intrusive formations is above the Clarke value, and in general their behavior is similar. A somewhat elevated content of these elements is observed in intrusive rocks. This is explained by the fact that these elements are characteristic of late products of differentiation.

Lead and zinc in both groups of rocks occur in quantities below Clarke values; however, in absolute terms their content is somewhat higher in intrusive rocks than in effusive rocks, which is explained by the ability of these elements to accumulate in residual solutions. Among geochemical features one should also note the behavior of yttrium, whose content is higher in intrusive rocks. This is connected with the fact that the geochemical behavior of this element is similar to that of the alkalis and especially potassium, and the content of the latter is significantly higher in intrusive rocks. All the noted geochemical features support the genetic relationship of both groups of rocks.

1. Petrochemical characteristics. When comparing the variation lines of effusive and intrusive rocks, it is evident that the line corresponding to the effusives generally has the same inclination as the line for the intrusive formations. In comparison with the latter it is closer to the \(SB\) axis and at the same time displaced downward. The latter circumstance indicates the presence among the effusives of more basic rock varieties, which are absent among the intrusives; moreover, these basic effusives account for the greater part of all effusives—about 60%. The displaced position of the variation lines of the effusives also indicates the absence among them of leucocratic rock varieties, to which the upper parts of the lines of the intrusive rocks correspond. Such a difference in the material composition of the two groups of rocks—the predominance of basic varieties among the effusives and of leucocratic varieties within the intrusives—can be explained by fractional crystallization of the primary magma. It is interesting to trace the course of magma evolution on the ternary diagram \( \mathrm{MgO}—(\mathrm{FeO}+\mathrm{Fe_2O_3})—(\mathrm{Na_2O}+\mathrm{K_2O}) \). In this diagram, most of the figurative points corresponding to the effusives tend toward the Fe—Mg line and toward the Mg corner, in comparison with intrusive rocks, which tend toward the side of the triangle \( \mathrm{Fe}—(\mathrm{Na_2O}+\mathrm{K_2O}) \) and toward the alkali corner. Magnesium, as a constituent part of the earliest minerals—olivine and pyroxene—was consumed mainly during the period of formation of the effusive rocks. The residual magma proved to be substantially depleted in this component. From the example of the effusives it is evident that differentiation of the magma proceeded toward depletion in magnesium in the initial period and enrichment in iron in some intermediate period, and in the final result there occurred the formation of residual magma rich in alkalis. If one compares the variation lines of magmatic differentiation

on these diagrams, it is clearly seen that the lines for the intrusive rocks are a direct continuation of the lines of the effusives. They reflect the direction of differentiation of magma already substantially depleted in magnesium toward a decrease in iron content and enrichment in alkalis. Such a course of magma evolution may be compared with other analogous examples, in particular with the Skaergaard intrusion (Wager and Deer, 1939), and also with the series of differentiated lavas of the Norilsk district (Zolotukhin and Vasil’ev, 1967) and the differentiated intrusions of Mount Zub (Godlevskii, 1959).

Another indication of the residual character of the magma that gave rise to the alkaline intrusive rocks is the position of the figurative points of the chemical composition of these rocks on the ternary nepheline–kaliophilite–silica diagram (after Bowen). When plotted on this diagram, all points for the intrusive rocks lie in the low-temperature region (trough) corresponding to a residual melt rich in alumino-alkaline components, produced as a result of differentiation of basaltic magma. The weighted-average chemical compositions for the massifs as a whole also lie in the trough region. The foregoing permits the conclusion that the magma that gave rise to the alkaline intrusive rocks was residual, approaching in composition a magma of alumino-alkaline composition having the lowest crystallization temperature.

1. Mineralogical features. Effusive and intrusive formations also reveal a genetic connection in their mineral composition. The rock-forming minerals in the basic effusives are pyroxene, most often augite, basic plagioclase, which in composition does not fall below No. 60 and more often corresponds to No. 70–80, and olivine. In the alkaline varieties of the effusives, nepheline, K-feldspar, and zeolites appear. The same minerals are also characteristic of the intrusive rocks. Their difference from the effusives consists in the presence, in certain rock varieties, of hornblende and aegirine-augite. However, these minerals are of limited distribution, and in almost all rocks the leading dark-colored component is pyroxene; even in the most leucocratic rocks—nepheline syenites—it is the predominant or the only one.

In the compared groups of rocks, the minerals are also similar in their composition. In both the effusives and the intrusive rocks, augite, titanaugite, and diopside predominate among the pyroxenes. A characteristic feature is the high basicity of plagioclases both in the effusives and in the intrusive rocks, which testifies to a common feature of the magma, namely, enrichment in calcium.

Thus, all the listed features—general geological, geochemical, petrochemical, and mineralogical—testify to a genetic connection between the intrusive and effusive formations as products of differentiation of a single parental magma. On this basis the Goryachegorsk alkaline complex should be regarded as volcano-plutonic, represented by hypabyssal intrusions, subvolcanic formations, and effusives. The latter constitute the main part of the complex, accounting for about 92% of the total rock mass of all three facies varieties.

The Goryachegorsk complex belongs to the trachyandesite formation according to Yu. A. Kuznetsov (1964) and to the gabbro formation according to Yu. M. Sheinmann, F. R. Apel’tsin, and E. A. Nechaeva (1961).

References

1. Bazhenov, I. K. Nepheline rocks of the eastern slope of the Kuznetsk Alatau. Problems of the Geology of Siberia, Vol. 1. Publishing House of the Academy of Sciences of the USSR, 1945.

2. Luchitskii, I. V. Nepheline ores and alkaline nepheline-bearing rocks of the south of Krasnoyarsk Krai. In: Mineral Resources of Krasnoyarsk Krai. Publishing House of the Academy of Sciences of the USSR, 1959.

3. Rachkovskii, I. P. On alkaline rocks of the southwestern Yenisei Governorate. Proceedings of the Geological Museum of the Academy of Sciences, Vol. 5, issue 4, 1911.

4. Rachkovskii, I. P. On the question of rocks of the alkaline series in the southwestern part of the Yenisei Governorate. Vol. 48, Notes of the Siberian Mineralogical Society, series 2, 1912.