ON THE PHENOMENA OF HYBRIDISM AND ASSIMILATION IN THE GRANITOIDS OF THE MUSTAG INTRUSION OF GORNAYA SHORIA

E. I. Lisenkov

(Presented by the scientific seminar of the Departments of Mineralogy, Petrography, and Mineral Resources)

The study of the phenomena of hybridism and assimilation is known to be of great importance for solving many petrogenetic questions. The phenomena of interaction between magmatic masses and the enclosing rocks at different stages in the evolution of a magmatic chamber have been widely discussed by many researchers [1—4, 6—8]. A brief review of the main viewpoints on this question is available in the works of I. Kh. Khamrabaev, V. F. Morkovkina [8], and other authors. Most petrologists associate the appearance of a diversity of granitoid types in the endocontacts of intrusions precisely with the processes of hybridism and assimilation, and not with the multiphase character of intrusion formation [1, 2, 4]. At the same time, special emphasis is placed on the role of the tectonic setting as “an important factor determining the scale of development of hybridism products” [6]. It should be noted, however, that despite the considerable study of the phenomena of hybridism, many aspects of the latter still remain debatable [8]. In this connection, the accumulation of new factual material on hybridism is of definite interest.

In the present work, the question raised is considered using the example of the Mustag intrusive body of Gornaya Shoria, within which various types of granitoids are observed, owing their origin to processes of hybridism and local assimilation.

The Mustag intrusion is represented by several massifs, the largest of which are the Mustag massif proper (area about 290 km²), the Sarlyk massif (25.4 km²), and the Yen Mountain massif (about 9 km²). The first and third massifs have, in plan, a linearly elongated form along the main structures of the region and are in contact with rocks of the Upper Proterozoic carbonate–siliceous sequence, metamorphic schists of the Lower Cambrian, and sandstone–siltstone deposits of Lower Ordovician age [5]. The Sarlyk massif, characterized by a rounded form in plan, cuts through Middle Cambrian effusive-sedimentary rocks. Contact metamorphism of the country rocks was expressed in the formation of various hornfelses and, very rarely, diopside-garnet and wollastonite skarns. In composition and structure the Mustag intrusion is fairly complex; its formation took place, at least, in three phases: 1) diorites and gabbro-diorites; 2) coarse-grained biotite granites; 3) fine- and medium-grained alaskite granites. Assimilation phenomena are most characteristic of the granitoids of the second phase, which make up about

80% of the exposed area of the intrusion (including the massifs of Mount Ien and Sarlyk).

The granitoids of the second intrusive phase display a clearly expressed zonal structure, recorded in an increase in the basicity of the rocks from the centers of the massifs toward the periphery. The central parts of the massifs consist mainly of biotite granites, whereas the peripheral parts consist of hornblende-biotite granites, plagiogranites, granodiorites, quartz diorites, and locally granosyenites. Such zoning is especially clearly manifested in the massif of Mount Sarlyk and in the northeastern part of the Mustag massif. The noted varieties of granitoids are connected with one another by mutual transitions and do not show mutual cross-cutting relationships, which makes it possible to regard them as members of one and the same intrusive phase. The width of the endocontact belt of granitoids of increased basicity is not constant and ranges from the first meters to 2 km. These rocks are most strongly developed in the endocontact belt at the contact of the granitoids with sandy-aleuritic Ordovician deposits (northeastern part of the Mustag massif) and with effusive rocks of different ages. It is noteworthy that, in contact with carbonate rocks, the biotite granites often do not show substantial changes in their composition. In the inner parts of the massifs, basified granitoids are also encountered locally, which is apparently connected with irregularities in the roof of the intrusion. In this respect the massif of Mount Ien is highly characteristic; it is only beginning to be exposed by erosion. The granitoids of this massif abound in xenoliths of effusive rocks and, in composition, are closest to the granitoids of the endocontact zones of the larger Mustag and Sarlyk massifs.

In mineralogical composition, the various types of granitoids are, on the whole, close to one another and differ in the quantitative proportions of minerals (Table 1).

Table 1

Comparative mineralogical characteristics of granitoids

Average quantitative mineral composition (in vol. %)¹

Note. 1) Average counts from 5–10 thin sections for each rock type are given. 2) Magnetite, apatite, orthite, sphene, etc.

Biotite granites, composing the central parts of the massifs, are the most widespread among the granitoids of the second phase of the intrusion. They are coarse-grained, weakly porphyroid rocks, distinguished by fairly consistent structural features and mineralogical and chemical compositions over a ...

over the entire area of their distribution. Among the accessory minerals, zircon, magnetite, orthite, and apatite are characteristic.

Biotite–hornblende granites differ from biotite granites by the presence of amphibole, the predominance of porphyritic structures, and higher contents of orthite and monazite. The amphibole is represented by ferruginous hornblende of the hastingsite series (with Ng = 20—25°, Ng = 1.704 ± 0.002, −2V = 46—48°, pleochroism: along Ng—dark green; along Nm—olive green; along Np—light, yellow-brown). Riebeckite is sometimes encountered in the form of small hexagonal crystals. The distribution of hornblendes in the granites is very uneven. In places where amphibole grains are clustered, pseudomorphs of sossurite after idiomorphic plagioclase crystals, a large amount of sphene, and magnetite are often found. The dimensions of such areas usually do not exceed 1.5—2 cm in diameter. Pseudomorphs of sossurite develop at the expense of plagioclases of a much more basic composition than the weakly sericitized oligoclase of the granites. Such areas stand out sharply in thin sections and represent incompletely assimilated disintegrated fragments of the country rocks. The quantitative mineral composition of the endocontact granites varies within very broad limits. Thus, the rarely occurring plagiogranites are distinguished by a sharp predominance of plagioclase over microcline-perthite (Table 1).

Granodiorites are characterized by a coarse-grained porphyritic structure and often contain small xenoliths of diorite-like fine-grained rocks. The usually hypidiomorphic-granular microstructures of the granodiorites sometimes show transitions to granoblastic ones.

Phenocrysts no more than 1 cm in size are represented by plagioclase No. 30—35, common hornblende (−2V = 80°), and, more rarely, microcline-perthite. In the groundmass, in addition to these minerals, quartz and biotite are present. Plagioclase is distinguished by a complex zonal structure. The number of zones usually does not exceed ten. Alternation of zones of now more acidic, now more basic composition is observed, although in general

Fig. 1. Change in the composition of plagioclase zones from granodiorite.

the basicity of plagioclase in the zones decreases in the direction from the cores of the grains to the periphery (Fig. 1). The accessory minerals present are apatite, sphene, magnetite, and zircon.

Quartz diorites are noted in the immediate contact of the granitoids with the country rocks. These are porphyritic coarse-grained rocks of gray appearance, containing a large amount of var-

varying in size (from 5 cm to 1.2 m) xenoliths. In the characteristics of their mineralogical composition, the quartz diorites are similar to granodiorites. The form of the xenoliths is usually oval, rarely angular; their boundaries are sharp, while in small xenoliths they are diffuse. In composition, among the xeno-

Table 2

Average chemical composition of the granitoids of the Mustag intrusion

Numerical characteristics of the granitoids (after A. N. Zavaritsky).

liths the most widespread are amphibolites, consisting of 70–80% ordinary or actinolitic hornblende and andesine, and fine-grained quartz diorites. In the endocontact of the northeastern part of the massif of Mount Mustag there occur xenoliths of hornfelsed fine-grained sandstones, in many features very similar to those of the exocontact zone.

Quartz syenites and granosyenites occur sometimes in direct contact of the granitoids with limestones.

The rocks consist of microcline-perthite, plagioclase No. 25–30, ferruginous biotite, hornblende of the hastingsite series, and quartz. Accessory minerals are zircon, apatite, orthite, sphene, and magnetite.

The petrographic features of the granitoids briefly described above are in full agreement with the data of the chemical analyses (Table 2).

The change in rock composition is illustrated especially clearly by a variation diagram constructed for the corresponding types of granitoids of the northeastern part of the Mustag massif (Fig. 2). Toward the contact, the role of calcium, magnesium, aluminum, total iron, phosphorus, and titanium increases. In the same direction the contents of silica and potassium decrease.

Fig. 2. Diagram of the change in composition of granitoids of the northeastern part of the Mustag massif during assimilation of the enclosing rocks (after T. Barth): I—granite, II—plagiogranite, III—granodiorite, IV—quartz diorite.

Analysis of the vector diagram of numerical characteristics, calculated by the method of A. N. Zavaritskii (Fig. 3), makes it possible to identify the following features of the chemistry of the granitoids:

1) biotite and hornblende-biotite granites are close in chemical composition to P. Daly’s average granite type, differing from the latter by a somewhat lower lime content;

2) the granitoids of the endocontact zone are distinguished by some heterogeneity of composition (scatter of the representative points), and by a certain oversaturation in alumina in comparison with P. Daly’s average rock types;

3) toward the contact, the melanocratic character of the granitoids increases, which is distinctly recorded by an increase in the parameter “\(b\)”;

4) the position of the representative points in the \(csb\) plane indicates an increase of feldspathic lime in the rocks of the endocontact;

5) as the contact is approached, the role of sodium and trivalent iron increases (the parameters “\(n\)” and “\(g\)” increase).

On the basis of the factual material presented above, the following conclusions may be drawn:

1. The granitoids considered above are facies varieties of a single intrusive phase.

2. The zonal structure of the Sarlyk massif and of the northern part of the Mustag massif is due to complex processes of interaction of granitic magma with the enclosing rocks at the level of emplacement of the intrusion (local assimilation).

3. The granitoids of the endocontact zone bear traces of hybrid origin; this is indicated by the heterogeneous composition of the granitoids, the presence in them of xenoliths of the enclosing rocks reworked to varying degrees, and the uneven distribution of dark-colored

minerals, complex zoning of plagioclases, and a large amount of sphene and apatite.

1. Assimilation was selective in character, depending on the chemical-mineralogical composition of the rocks in contact with the intrusive.

2. The most characteristic feature is enrichment of the initial granitoid magma with calcium, magnesium, iron, titanium, and phosphorus.

3. The processes of hybridism and local assimilation were thus the cause of the diversity of granitoid types in the endocontact zones of large massifs and of the increased basicity of the granitoids of small intrusive bodies, similar to the Mount Iyen massif.

Fig. 3. Vector diagram of the chemical compositions of granitoids of the Mustag massif. The numbering of the rocks is the same as in Fig. 2. 4, 42, 45—average rock types according to P. Daly: granite, quartz diorite, granodiorite, respectively.

REFERENCES

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2. A. N. Distanova. The Martaiga granitoid complex. In: Magmatic formations of the Altai-Sayan folded region, Moscow, 1965.

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4. I. M. Isamukhamedov. The role of assimilation processes in petrogenesis. In: Magmatism and associated mineral resources. Gosgeoltekhizdat, 1960.

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