THE ROLE OF MAGMATIC REPLACEMENT IN THE FORMATION OF GRANITOID COMPLEXES OF MOUNTAIN SHORIA

S. S. Il’enok

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

One of the most important tasks of petrology is to determine the conditions under which granitoid complexes are formed. The solution of this question cannot be unambiguous. It is necessary to take into account the entire complex of conditions—tectonic conditions, the nature of the environment, the thermodynamic setting, etc. All this leaves a definite imprint on the composition and ratios of the rocks composing the massifs, the features of dike complexes, the manifestation of contact metamorphism and metasomatism, and the character of metallogenic specialization.

In questions of determining the conditions of formation of granitoid complexes, as in the question of the origin of granites, there are various points of view. This is understandable, since in studying granitoid massifs diverse rocks are encountered, associated with one another; in the structures of the rocks one observes the duration and diversity of processes, with frequent development of replacement of early minerals.

The role of magmatic replacement in the formation of granitoids may be different. Undoubtedly, it cannot be thought that all granite massifs were formed only in this way. One should distinguish the place of origin of granitic magma from the areas of its crystallization with the formation of granitoid rocks. These areas do not always coincide. After its origin, a granitic melt may be squeezed out into higher horizons and may even be the source of acid extrusions and effusions.

The fact of the origin of granitic magma in the migmatite zones of the Precambrian does not raise serious objections. Here, as a result of anatectic processes, granitic melt may accumulate and complete magmatic replacement of the original rocks may occur, with the manifestation of large masses of granitoid magma.

As for the upper structural levels, where stock-batholithic formations of granitoids occur, the question of their origin is often debatable. Most researchers usually adhere to the concept of an intrusive origin, assigning a role to the phenomena of assimilation in the presence of hybrid rocks. The role of the process of magmatic replacement, to which D. S. Korzhinsky calls attention,

[1] and by Yu. A. Kuznetsov [2], little space is allotted to it, or it is not considered at all.

The granitoid complexes of Mountain Shoria that we have studied show, in most cases, clear features that should be recognized as a broad manifestation of magmatic replacement, and they are difficult to explain by any other processes.

First of all, it should be noted that all granitoid complexes are clearly confined to major deep-seated faults. At the same time, the granitoid bodies form elongated shapes along these zones. But they do not display sharp boundaries, as is characteristic of large dikes. The boundaries of the bodies are always uneven and indicate the spread of granites at the expense of the enclosing rocks, through their replacement. The gabbro–plagiogranite complexes, which are more common here, usually appear selectively in areas where effusive intermediate-basic formations are developed, while the gabbroids, encountered more often in the marginal zones of the massifs, bear traces of their intensive replacement by granitoids.

Certain regularities are observed in the composition of the massifs. Thus, the Asyr batholith, located on the watershed of the Tashtyp and Kobyrza rivers, is composed in its upper parts of plagiogranites, either without potassium feldspar or with a small amount of it, with transitions to granodiorite. The composition is very unstable, and the transitions between rock types are gradual. All of them form a single whole. At depth there are transitions into a comparatively stable rock type in the form of biotite–hornblende granites. Characteristic of the latter is a sharp zonality of plagioclase. In the central zones of the mineral, andesine is usual; in the marginal zones, oligoclase.

In the southern part the massif plunges beneath limestones. Here, in small windows among the limestones, rocks of the granitoid massif show transitions from granites to quartz and alkaline syenites and further to nepheline syenites. Such transitions are connected with a column formed during the magmatic replacement of limestones. In the remaining part of the massif, the distribution and composition of the rocks show the manifestation of magmatic replacement of the effusive sequence of the enclosing rocks. At the same time, the plagiogranites form a near-contact belt of rocks.

Another example is provided by a belt of massifs extending from the Lebed River to the upper reaches of the Kondoma River. These are the Lebed, Sadrin, and Verkhnekondoma massifs, as well as a series of smaller bodies situated between them. All of them form a single granitoid complex of Middle–Upper Cambrian age. The composition of the intrusive fields is varied. In the southern massifs—the Lebed and Sadrin—the principal part is made up of plagiogranites, granodiorites, and quartz diorites, and, to a lesser extent, tonalites and granites. In the northern Verkhnekondoma massif, diorites and quartz diorites occupy a large place, and gabbro is noted locally. Among the granitic rocks, plagiogranites, granodiorites, and granites are developed. Here too, in addition to the varied composition, gradual transitions between all rock varieties are observed. In the features of the petrographic composition and structure of the rocks, the process of rock transformation is expressed as a result of a progressive increase in the melts of silica and alkalis. Characteristic is a later manifestation, in comparison with the main stage of rock crystallization, of intensive potassium replacement with the formation of microcline. This process is regional and is superimposed both on the plutonites and on the enclosing rocks. In the former, it appears both as interstitial development of potassium feldspar and, locally, as its development in the form of porphyroblasts. Later stages are characterized by the development ...

veinlets of potassium feldspar, sometimes very dense, forming stockworks.

The contact zones of the massifs are characterized by the constant presence of broad zones composed of products of magnesian–ferruginous metasomatism. In the immediate contacts of the massifs, narrow bands of giant-grained hornblendites appear, passing into medium- and fine-grained amphibolites.

Thus, the formation of granitoids is divided into several stages. The earliest of these is the formation of gabbro, tending toward the marginal zones of the magmatic chamber. The upward rise in the cooling magmatic chamber of volatiles together with silica and alkalis led to the recrystallization of gabbro into diorites, quartz diorites, and, with a higher concentration of this light melt, to the appearance of more acidic rocks—tonalites, plagiogranites, granodiorites, and granites. The transformation of the rock occurred not only through the reactional adjustment of solid phases during their interaction with the liquid, but also with the active development of melt due to its penetration through interstices and an increase in its amount owing to the development of exothermic reactions and the transition of part of the solid phases into melt. This process apparently proceeds spontaneously in the form of a chain reaction. The granitization of the replaced material is clearly accompanied by the intensive removal of magnesium and iron, with the formation of amphibolites and, in the endocontact zone, hornblendites.

The formation of potassium feldspar and quartz belongs to the late stages of crystallization, but sometimes the mass appearance of potassium feldspar is manifested even later, as occurs in the massifs of the Kondoma and Lebed districts. This leads to the appearance of rocks enriched in potassium. The noted specificity of the formation of granitoid massifs is reflected also in the particular features of the manifestation of ore mineralization.

REFERENCES

1. D. S. Korzhinskii. Granitization as magmatic replacement. Izv. AN SSSR, ser. geol., No. 2, 1952.

2. Yu. A. Kuznetsov. The Main Types of Magmatic Formations. “Nedra,” Moscow, 1964.