THE ROLE OF GABBROIDS IN THE FORMATION OF VARIEGATED GABBRO–PLAGIOGRANITE COMPLEXES

S. S. Ilyenok

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

Within the Altai–Sayan folded region, batholith-type massifs containing rocks of varied composition are widely developed. Among these rocks we find gabbro, gabbro-diorites, diorites, quartz diorites, granodiorites, plagiogranites, and biotite–hornblende granites. Such intrusions are usually associated with Early Caledonian tectogenesis. They include numerous plutons in various parts of the Kuznetsk Alatau: the Verkhne-Kondomsky, Lebedskoy, Solgonsky, Saralinsky, Tsentralninsky, and many others. Such plutons have been described in the Eastern Sayan, Tuva, and other regions.

The relationship between gabbroids and granitoids in these plutons is manifested in different ways. Sharp intrusive contacts between them are encountered, as is the presence of xenoliths of gabbroids in granites, which indicates their connection with different intrusive phases. Among the gabbroids there occur characteristic hybrid formations with variable composition and taxitic textures. Petrographic features usually indicate, as is shown in the descriptions of the Main intrusion by V. N. Smyshlyaev (1958), alteration of early gabbroids under the influence of granite magma. Hybrid rocks are not infrequently associated with assimilation of enclosing rocks of varied composition. These facts often lead to statements about the origin of gabbroids in the indicated complex solely by hybridism due to the action of granite magma, and to denial of the presence of primary magmatic rocks of the gabbro series. However, the petrographic features of the rocks contradict this. If obviously hybrid formations are excluded, then the gabbroic rocks in the indicated complex have characteristic features of primary magmatic rocks, namely: they have gabbroic and ophitic textures, the presence of titan-augite and basic plagioclase as the main minerals, and a significant content of titanomagnetite. The latter, as well as titan-augite, do not appear in hybrid formations. Diopside and magnetite are usual for them; sphene and ilmenite may occur.

If one assumes the manifestation of independent intrusive phases for gabbroids and granitoids, we enter into a certain contradiction with the factual material. The point is that the occurrence of gabbroids in bodies of gabbro–plagiogranite associations is characterized by the presence of gabbro in near-contact zones and by their absence as independent bodies. At the same time, the development of an independent phase of gabbroidic

magmatism should have produced individual bodies of these rocks, subordinated to the corresponding tectonic plan. Undoubtedly, the superposition of the subsequent phase of granitic magmatism should have engulfed the bodies of gabbroids, and they would be better preserved beyond the boundaries of the granitoid fields, including in the near-contact zones; but they should also be preserved as independent bodies. This we do not see, for the gabbroids do not extend beyond the contact zones. All this compels one to recognize a very close connection and the closest succession in the formation of both groups of rocks.

Yu. A. Kuznetsov (1964) believes that the formation of such complexes is connected with the initial formation, at depth, of a basaltic magmatic chamber. The early phase of intrusive gabbroic magmatism is associated with it. Subsequently the magmatic chamber grows and moves upward into a sedimentary–granitic belt. In this process an independent plagiogranite magma develops. It may be the result of melting of acid roof rocks under the influence of large masses of basic magma, or the result of magmatic replacement under the influence of large flows of transmagmatic solutions of deep origin. However, this is contradicted by the strict subordination of the gabbroids to the contact zones of granitic bodies.

A feature of the close spatial relations between gabbroids and granitoids is also manifested in the fact that gradual transitions are often observed between them. Since this contradicts the concept of the multiphase character of plutons, many geologists do not attach significance to these facts. Thus, in the southern part of the Kuznetsk Alatau, the Lebed and Sadrinsky gabbro–plagiogranite massifs are situated in one belt with the Verkhne-Kondomsky massif, which belongs to the same complex. All the massifs have one and the same age, composition, and metallogenic features. In the first two massifs there are indications of intrusive contacts between gabbro and granitoids. In the Verkhne-Kondomsky massif, gabbroids are very widely developed. Among them diorites and quartz diorites predominate, while gabbro is poorly developed. Gradual transitions between gabbroids of different composition and granitoids are observed both in large and in small exposures. At the same time, the superposition of granitization upon the gabbroids is developed, as is a later potassic alteration, superposed in turn upon all the rocks—both acid and basic—as well as upon the skarns.

In the Lebed and Sadrinsky massifs, as in the majority of plutons of the gabbro–plagiogranite complex, intensive granitization is accompanied by the outflow and movement of granitic magma. Judging from geological data, the scale of displacement in most cases was insignificant. This displacement also creates intrusive contacts between gabbroids and granitoids. Under parallel processes of granitization, the gabbroic rocks in the places of these displacements undergo corresponding changes. Since gabbroids occur in the marginal and apical parts of massifs, the particular character of their development indicates the level of erosional truncation of the pluton.

Thus, the mechanism of formation of plutons of variegated composition, in connection with the features of their geology and the occurrence of gabbroids, may be outlined as follows. The magmatic chamber has a torch-like structure. In its upper part there is a high-temperature halo, strongly saturated with gases. In the zone where this torch meets the groundwater level, under conditions of intermediate depths, especially at the boundaries of geological formations, processes of oxidation and heating with melting of the enclosing rocks are sharply manifested. The real cause may be

...may be the known spontaneous processes of hydrogen oxidation. The average composition of the rocks enclosing the granitoids belongs to the intermediate–basic series, which determines the composition of the resulting melt. As a result of the indicated process, the gabbroids are a product of the advancing front of magmatic replacement. The gabbroid melt forms only the upper, narrow part of the magmatic chamber. Deeper down, intensive processes of granite formation take place; as they move upward, they subject the gabbro rocks as well to granitization, their formation occurring comparatively rapidly owing to the more intensive cooling of these horizons in comparison with the deep ones.

It should be emphasized that the formation of the hot cap of the chamber, with the appearance of gabbro magma, is accompanied by the spreading of the granitizing chamber to the sides, which also explains the frequent occurrence of laccolith-like and stock-like bodies of granite massifs.

References

1. Kuznetsov Yu. A. Principal Types of Magmatic Formations. “Nedra,” 1964.

2. Polyakov G. V. Paleozoic Magmatism and Iron-Ore Mineralization of the South of Central Siberia. “Nauka,” 1971.

3. Kuznetsov Yu. A., Bognibov V. I., Distanova A. N., Sergeeva G. S. The Early Caledonian Granitoid Formation of the Kuznetsk Alatau. “Nauka,” 1971.