On the Origin of Gabbro–Plagiogranite Complexes

S. S. Il’enok

Among the plutonic associations in the Altai–Sayan region, such associations are widespread in which both rocks of gabbro and diorite composition and acidic rocks, represented by various varieties of granodiorites and plagiogranites, are observed. Such combinations of rocks are characteristic of gabbro–diorite–plagiogranite complexes, assigned to the gabbro–plagiogranite formation according to Yu. A. Kuznetsov. But analogous associations also occur in batholithic granitoid formations of variegated composition (Olkhovsky, Tannuolsky, Tigirtyshsky, Martaiginsky, and others). The latter are also often called gabbro–plagiogranite complexes.

Complex associations of rocks have long attracted attention and have received various explanations from their investigators (Yu. A. Kuznetsov, E. P. Izokh, G. L. Dobretsov, G. V. Pinus, G. V. Polyakov, A. N. Distanova, V. N. Smyshlyaev, A. D. Shelkovnikov, and others). Thus, most investigators believe that the basic rocks of these complexes are the result of the intrusion of an earlier intrusive phase, after which granitic magma was subsequently emplaced. At the same time, Yu. A. Kuznetsov and E. P. Izokh believe that this combination is difficult to explain by crystallization differentiation of magma, and that the most probable phenomenon is the enlargement of the magmatic chamber with its transition from the basaltic zone into the upper horizons, which have a sedimentary–gneissic composition. In this case, early fractures lead to the appearance of intrusions of gabbroic magma, while later ones, which arose during the period of enlargement of the magmatic chamber and the development of acidic magma, lead to subsequent intrusions of acidic magma.

To explain the proximity and sequence in a sufficiently contrasting intrusive process, E. P. Izokh considers it probable that a magmatic chamber originated in the region of the boundary between the basaltic and granitic layers. At the same time, geological and geophysical data obtained in areas of modern volcanism indicate that such chambers originate deep in the mantle, from 50 to 150–200 km.

Another group of investigators connects the formation of the main mass of gabbroids with the assimilation of enclosing rocks by granitic magma and with processes of its hybridism. Some investigators, considering the appearance of gabbroids as a result of the intrusion of early portions of basic magma, also do not deny the appearance of a number of rocks as a result of assimilation processes; this especially concerns diorites.

Recently G. L. Dobretsov (“Geol. i geof.,” No. 5, 1971) expressed

the idea of the jump-like displacement of nascent magmatic chambers. Following the appearance of a basaltic melt in the deep parts of the mantle and the movement of part of the magma toward the surface as a result of powerful heat flows along a magma-conducting conduit, granite formation occurs in the region of the acidic Earth’s crust. Mixing of the two types of magmas can yield a variety of rocks of intermediate-acid composition.

The geological material on the occurrence of gabbro–plagiogranite associations does not confirm the view of independent intrusions of basic and acidic magmas, nor the assimilation theory, although phenomena of successive movements of magma did take place, as did phenomena of magmatic replacement and, at times, hybridism. The movements of magma were local in character and were connected with a single magmatic chamber.

First of all, it should be noted that, if gabbroic and granitoid magmatism occurred as independent intrusive processes, their independent development in space should have been evident. Yet these were separate stages of tectonic activity, separated in time. Differences in the tectonic position of the products of the early and late phases of magmatism should inevitably have appeared. This is all the more so because, during the interval from the manifestation of the first phase to the second, displacement of the magmatic chamber occurred. Of course, superposition of the products of magmatism could have taken place, but at the same time it could not have been a completely exact coincidence. With an imprecise spatial coincidence of these products, independent bodies of gabbroids not connected with granitoids should be encountered. However, this is not observed. Gabbroic rocks are everywhere spatially tied to granitoids. They compose the upper apical parts of massifs. Usually gabbros are situated in the very uppermost part of the latter. These rocks pass into the more widespread diorites and quartz diorites.

In many massifs, intrusive boundaries between gabbroids and granitoids are described. At the same time, several intrusive phases are often established. The manifestation of these intrusive phases depends to a considerable extent on the depth of the erosional section of the massif. Thus, in the Verkhne-Kondoma massif (Gornaya Shoriya), belonging to the indicated complex, gabbros are poorly developed, while large areas of the massif’s exposures are composed of diorites and quartz diorites. Between the indicated rocks and granitoids gradual transitions are observed everywhere. The large bodies situated to the south, in the form of the Lebed and Sadrin massifs, are exposures of the same specific magmatic complex. But within the composition of these massifs granitoids predominate, while basic rocks form small fields in the marginal parts of the massifs. Here distinct intrusive contacts between gabbroids and granitoids are observed. Geological data show that the latter massifs are deeper erosional sections in comparison with the massif in the Verkhne-Kondoma region.

The study of gabbro–plagiogranite massifs everywhere shows their zonal structure. At the same time, individual rock zones have different thicknesses. Thus, in the Asyr massif of Gornaya Shoriya, gabbros and diorites form a thin outer zone. They are replaced by plagiogranites, and the latter by biotite–hornblende granites. In individual massifs, late injections, or in deeper sections, manifest themselves as normal granites.

Petrographic study of rocks of all varieties shows the manifestation of features of magma differentiation. Crystallization of gabbro, which is located in the apical parts of bodies, proceeded

earlier than the formation of other rock varieties. Following the appearance of gabbro, further differentiation of the magma took place in the underlying part of the magmatic source. It was manifested under conditions of differential mobility of the components of the melt, with the separation of mobile constituent parts, their penetration into the gabbroids, and the local development of phenomena of magmatic replacement. In the deeper parts of the magmatic source, as is evident in the Lebed and Sadrin plutons, granitoid magma advanced and cutting contacts with gabbro and diorites appeared. The petrographic features of gabbros of different origin are clearly distinct. Normally magmatic and assimilation-hybrid rocks differ not only in structural and textural features, but also in the character of the rock-forming minerals. There is no doubt that the ordinary near-contact gabbros of the massifs of the southern part of the Kuznetsk Alatau that we studied are normal gabbroids. They contain labradorite, titanaugite, and ordinary high-temperature titanomagnetite with traces of solid-solution exsolution in the form of ilmenite segregations.

Thus, in the massifs there is non-simultaneous crystallization of apical gabbroids and deeper granitoids. The effect of granitic magma on gabbroids was observed, along with partial displacement of this magma with the manifestation of intrusive contacts, but this displacement was local in character. In places the granitic magma bordered on limestones. At the same time magmatic replacement of limestones also took place. But this process was not of a mass character. The products of such replacement are developed in the area of the contact zone over a thickness of several tens of meters. Such examples have been described by us for the southern part of the Asyr pluton. In such replacement, following the plagiogranites in the direction toward the contact there appear quartz syenites, then syenites, nepheline syenites, overlain by marbles. Owing to the removal of silica, tremolitic rocks develop in the marbles near the contact.

The close association of gabbroids and granitoids and their spatial arrangement make it possible to suppose that, before the crystallization process, the magmatic source had a torch-like structure. The upper part of this torch was high-temperature. This is undoubtedly connected with the formation of a gas cap above the magmatic source. The high temperature of the outer layer was maintained by the constant influx of hydrogen and other gases from the magmatic chamber, as well as by exothermic processes of their oxidation. The role of the oxidizing agent was most probably played by water in the enclosing substrate. In the high-temperature layer of the torch, gabbroic magma was formed. A favorable factor for this was the basic composition of the enclosing effusive-sedimentary rocks.

In the transition zone from the upper gabbroic layer to the deeper portion of granitic magma there was a zone of magma mixing. This portion, apparently, corresponds to the zones of dioritic rocks that usually compose large parts of the massifs.

The high temperature of the outer layer of the magmatic source determines a certain migration of alkalis and silica in the opposite direction to the composition of basaltic magma. Thus, in the Kuzbass burnt rocks, formed from argillites rich in alkalis and silica, newly formed coarse-grained ophitic gabbros were encountered, instead of granites, which should have formed from these rocks.

Judging from geophysical data, confirmed by scanty fragmentary information on individual massifs of the gabbro-plagiogranite association, the shape of their bodies is horizontal, with considerable thickness.

These bodies have separate feeder channels. The thickness of the caps of gabbroid rocks is sometimes insignificant—40–50 m—but may reach 1 km. In individual massifs, the gabbroic rocks have undergone granitization and magmatic replacement, with the formation of transitional rocks of the monzonite series. In this case, gabbro occurs only as remnants in the very near-contact parts of the bodies.

In the process of magmatic replacement of the gabbroids, a powerful removal of magnesium, iron, and other components takes place, with the manifestation of metasomatism zones, most often in the form of amphibolites, culminating in the formation of iron-bearing skarns.