ON THE GEOLOGICAL-STRUCTURAL POSITION OF THE OB GRANITOID MASSIF

A. M. KUZMIN, P. N. PARSHIN

The Ob granitoid massif is a granitoid complex, intricate in composition and structure, developed in the area of the Novosibirsk Cis-Ob region, within the western exposed part of the boundary of the Kolyvan–Tomsk folded zone. Earlier ideas about this complex as consisting of separate, comparatively small massifs isolated from one another, exposed by the Ob River and its tributaries (known as the Verkhne-Chiksky, Senchansky, Novosibirsky, Mochishchensky, Kolyvansky, Barlaksky, Orlovsky, and Ob massifs), are not confirmed at present. Geophysical investigations of recent years have shown that the named massifs are parts of a single enormous granitoid body, traced at varying depths from the ground surface—from 4–5 km (the Novosibirsk “massif”) to 13 km (the Barlaksky and Kolyvansky “massifs”) (Moiseenko, Puchkov, Borozdin, 1966). These same authors express the view that, with depth, the Ob granitoids, as magmatogenic formations, acquire the character of granodiorites and diorites.

By individual mapping boreholes the granitoids described have been traced for almost 250 km in the meridional direction (from the town of Kamen-na-Obi in the south to the village of Baturino in the north) and for approximately the same distance in the latitudinal direction (as far as the area of Barabinsk station).

The composition of the Ob massif in its exposed part may now be considered to have been studied in sufficient detail (Bellitterli, 1932; Gusev, 1934; Nuvar’eva, 1968; Kozlov, 1971, and others). As a result of all these studies it has been established:

1. The granitoid complex is a complex, multiphase, intrusive magmatogenic formation, represented by individual stocks exposed by erosion. Rocks from gabbroid (diabases, augite porphyries) to acid composition of the type of alaskite granites take part in their structure.

2. Two principal varieties of granites are distinguished rather clearly: hornblende-biotite granites, developed mainly in the area of the city of Novosibirsk, and lighter-colored varieties—biotite and, in places, two-mica granites, most characteristic of the Kolyvan and Barlak areas. The former, by A. L. Matveevskaya and some other geologists, are regarded without serious grounds as more ancient and are compared with the Zmeinogorsk complex, while the latter are compared with the Kalba granitoid complex of Rudny Altai.

1. The absolute age of different parts of the granitoid complex, determined by the potassium-argon method and by the method of comparative dispersion of birefringence (which give good agreement of results), for the various parts of the complex ranges from 274–285 (Kolyvan area) to 583–590 million years (Mochishche quarry); nevertheless, without sufficient grounds their age is accepted as Late Hercynian.

2. The different parts of the complex have different metallogenic specialization, which is regarded as a criterion of different depths of erosional truncation in light of the views of the group of workers at SNIIGGiMS on the vertical petrogeochemical zonality of granitoid formation, confirmed by the work of a number of other researchers (Kuzmin, 1963, and others).

Let us dwell on a brief analysis and on some comments regarding these basic views of the massif under consideration. First, it seems to us that geologists who have the opportunity for a more detailed and comprehensive study of this massif should substantiate the age of the massif more reliably than has been done up to the present.

At the initial stage of study of the age of the Priob granitoid formations there were two views. Thus, A. A. Inostrantsev (1898), I. D. Chersky (1885), and G. G. Petz (1896) assigned them to the Archean, on which, in their view, rest unconformably the clayey schists developed east of the massif described. A. N. Derzhavin (1898) considered them younger than the enclosing schists. Those gneisses, gneiss-granites, and micaceous schists that G. G. Petz assigned to the Archean group, according to A. N. Derzhavin, are nothing other than clayey schists metamorphosed in contact with the Kolyvan granite. M. E. Yanishevsky (1915) considered that, of these two views, the second was the more probable, although he noted that this view is contradicted by certain facts on which the advocates of the first view rely. Thus, the clayey schists occurring east of the Kolyvan area strike from southwest to northeast, whereas the schistose rocks (granito-gneisses and gneisses) in the massif itself strike from north to south (Petz, 1896) or even from northwest to southeast (Chersky, 1885), that is, they intersect the strike line of the Lower Carboniferous clayey schists at a considerable angle. These data, as A. N. Derzhavin noted, may indicate that the origin of the Kolyvan massif is not connected with Hercynian folding.

The position now accepted by the majority of researchers regarding the Late Hercynian age of the Ob granitoid complex is based on conceptions of the close connection of its formation with Late Hercynian folded formations. According to A. L. Matveevskaya, “...the age of the Novosibirsk intrusive complex under consideration must be recognized not only as post-Carboniferous but also as post-Permian, since the Balakhon and Kuznetsk series are undoubtedly affected by the main tectonic movements” (Matveevskaya, Ivanova, 1960, p. 71). This statement essentially confirms the conclusion of A. I. Gusev (1934, p. 44) that “the lower age limit of the entire intrusive body as a whole is determined by the fact that it cuts through the youngest of the deposits known to us” (referring to the presumably Lower Carboniferous deposits, the age of which has not yet been precisely established).

It seems to us that the conclusion about the Late Hercynian age of the entire Ob complex requires revision. It is inconsistent both with a number of already known factual data, noted above, and with newly appearing factual data. Thus, the enormous dimensions of the massif lead one to expect more substantial effects on the sediments enclosing this massif de-

of Devonian and Carboniferous (and even Permian) age, represented by argillaceous shales, sandstones, effusive rocks and their tuffs, which are especially susceptible to processes of contact metamorphism. However, analysis of geological maps and natural sections shows that no such appreciable effect on the reliably Devonian and Carboniferous sediments is observed. The sediments here are crumpled into rather gentle folds and, as a rule, are weakly metamorphosed—mainly dynamically metamorphosed. Phyllitized shales of the same character can be traced eastward as far as the city of Tomsk and Tutalsk.

The age of the rocks intruded by the granites also remains unclear. The nearest occurrences of fauna (Lower Carboniferous) are in the section along the Poros River near the village of Galinskoe, approximately 5 km east of the granitoid outcrops along the Ob River. But here, too, according to geological-survey data, the rocks enclosing the fauna gravitate toward the center of a large synclinal fold and bear no traces of contact or regional metamorphism. To this it may be added that T. I. Khublydikov is inclined to distinguish, in the roof of the granitoid massif, metamorphic complexes bearing evidence of deep regional metamorphism (Khublydikov et al., 1962), although other geologists do not yet agree with this (Mikhailovskii et al., 1964, and others).

At present, analysis of new geological and geophysical data, as well as a number of absolute-age determinations, inclines one to the idea of a more ancient age for the Ob massif.

The different levels of erosional truncation of different parts of the massif, established by geologists of SNIIGGiMS, are explained by them in different ways. Thus, Yu. A. Nuvar’eva (1968) suggests that, during the formation of the Kolyvan and adjacent “massifs,” the magmatic melt was displaced from the region of its generation (the level of Upper Devonian deposits) into higher horizons of Carboniferous and Permian sediments, by 9 km relative to the coeval (Upper Paleozoic) granitoids of the Novosibirsk “massif” that originated at similar depths.

A. M. Kozlov (1971), in order to explain the coevality he accepts for all the granite bodies of the Novosibirsk region, exposed at different levels of erosional truncation of the granitoid complex (which he regards as Lower Permian), assumes that the various granitoid bodies were displaced to different degrees in the vertical direction during the manifestation of block tectonics after the formation of the granitoids. The amplitude of vertical displacement of the melt is assumed to have been already a maximum of up to 4 km, from the level of “magmatic replacement of Devonian rocks” to the level of Middle–Upper Carboniferous deposits (the Balakhon suite).

It should be noted that at present, in particular by the calculations of F. S. Moiseenko (1969), the impossibility of the existence in the Earth’s crust of regionally developed zones of granitoid magmas is being demonstrated, as is the unreality of ideas about significant displacements of magma in the Earth’s crust, except for small injections. It is also difficult to accept the proposition of post-Tertiary, many-kilometer vertical displacements of individual granite blocks relative to one another, which the authors mentioned above required in order to explain the causes of the geochemical heterofacial nature of separate, erosionally exposed parts of the granitoids. The maximum amplitudes of faults in the region do not exceed tens (at most the first hundreds) of meters. Moreover, these faults are connected not with the area of development of the granitoids but mainly with the Bugotak–Mitrofanovo horst-anticlinal zone developed farther east. The sandy-argillaceous shales adjoining the granitoids experienced only a manifestation of insignificant fault tectonics and are folded

with a fairly gentle dip (along the Oyash River, possibly in places of emplacement onto the granites) or fairly steep (the mouth of the Inya River, in places of their most probable tectonic boundary with the granitoids)*. This fact, incidentally, makes it possible to raise the question of whether the Ob massif, as a rigid structure, was the cause of the formation of Kolyvan folds in the sediments of the Middle and Upper Paleozoic.

Finally, a continuous and comprehensive analysis is needed of the available and newly appearing data on the absolute age of individual rocks and minerals and on the sequence of their formation. By way of example, some of them may be cited.

In 1965, 1 km southeast (azimuth 130°) of the Novosibirsk-Yuzhnyi station (between the Borok quarry and the railway), granitoid rocks were exposed by a ditch to a depth of up to 3 m. Here it was possible to observe how granite (sample 99) is cut by dikes of pyroxene–plagioclase porphyrite, and at the same time both the granite and the porphyrite are cut by quartz–biotite—

Fig. 1. Thin section No. 95, magnification 40, crossed nicols. 1 — granite, 2 — pyroxene–plagioclase porphyrite, 3 — quartz–biotite syenite-porphyry.

* B. F. Speransky, who knows the geology of the Novosibirsk region well, in a conversation with L. M. Kuzmin, when discussing the question of the relationship of the Novosibirsk granites to the sandy-shaly rocks enclosing them, was inclined to the view that the contact of the granites with the rocks enclosing them is tectonic, since neither he nor other geologists found signs of contact metamorphism. The Dubrovino mica schists and gneisses are of more ancient origin.

by syenite-porphyry. The mutual relationship of these three varieties is shown in the photograph of a thin section from a sample of the contact zone (Fig. 1).

As can be seen, the oldest of the three rocks described is granite, composed predominantly of quartz, plagioclase No. 35, a small amount of potassium feldspar (grid microcline), and biotite. Among the ore minerals, magnetite is present in the form of very fine grains, distributed rather uniformly through the rock. The intrusion of a dike of pyroxene-plagioclase porphyrite into the granite caused weak contact alterations in the latter: the quartz is cataclased; in places it is completely crushed, i.e., the intrusion of this dike occurred along a tectonic zone of crushing. The main mass of the dike of pyroxene-plagioclase porphyrite is fine-grained and consists of plagioclase, brown hornblende, and an ore mineral. In the near-contact part the rock is not recrystallized (Fig. 1).

The youngest of the observed rocks is quartz-biotite syenite-porphyry. In the porphyritic segregations there is zonal plagioclase (andesine), weakly albitized. Quartz occurs in rare melted grains. The groundmass is plagioclase and potassium feldspar. Magnetite and zircon are present.

The results of absolute-age determinations show that the age sequence (granite → diabase porphyrite → syenite-porphyry) corresponds to the average figures of absolute age: 394 → 372 → 371 million years (Table 1). The absolute-age figures refer to biotite.

Table 1

Results of absolute-age determinations on biotite by the optical method

(Determination by O. A. Kukharenko on collections by P. N. Parshin)

According to A. M. Kuzmin, in the Kalba type of granitoids biotite develops later than alkaline feldspars and albite, which themselves bear features of metasomatic formations. Alkaline feldspar replaces plagioclases and quartz, relicts of which are preserved in it. Biotite replaces the mafic minerals of granitoids and is readily distributed along fractures. Accessory minerals are predominantly in biotites or

in proximity to it [10]. Similar fragments of granite, cut by aplite veins, were observed by A. I. Gusev in the “Borok” quarry (100 m west of the trench described by us) as “fused into diorite” (Gusev, 1934, p. 43).

Preliminarily, the time of albitization and biotitization of the original granitoid rocks and the emplacement of separate diabase dikes and aplite veins may be correlated with the formation of the Lower–Middle Devonian porphyrite–diabase–keratophyre formation of the Bugotak horst-anticline, whose pyroxene porphyrites intrude the granitoids in the area of the “Borok” quarry.

Granites close in composition to those described have been exposed in the Novobibeevo quarry, where their age, based on individual biotite grains, ranges from 396 to 447 million years.

Separate apophyses of granites (samples 63–65) are traceable in dark-colored, dense andesitic porphyrites developed in the eastern part of the granite massif. The age of a granite vein 10 cm thick, cutting pyroxene andesite-basalt, averages 403 million years. In other parts of the quarry, pyroxene porphyrites cut rocks of granosyenite composition. The age of the granosyenite by biotite is 424 million years.

The reliability of the absolute-age determinations we have presented for the granitoid formations of the Novosibirsk and Bibeevo massifs (especially the Novosibirsk massif) is also confirmed by data from absolute-age determination by the potassium–argon method on biotite, which for the “Borok” quarry, according to Yu. A. Nuvar’eva, is 365 million years. Moreover, 6 km north of the city of Novosibirsk, in the Moishche quarry, the age of granite by biotite is, according to Yu. A. Nuvar’eva (1968), 583–590 million years (respectively on the southwestern and northwestern walls of the quarry).

The clastic material of Upper Devonian and Lower Carboniferous sediments filling the Kolyvan–Tomsk fold zone east of the area of development of granitoid formations (arkosic sandstones, etc.) often indicates the presence of granitoids in the source area located west of the sedimentation area (Korovin, 1933; Tyzhnov, 1941, and others). B. F. Speranskii (1924) also explained in this way the presence of abundant mica in the Upper Devonian sandstones developed east of the city of Novosibirsk. The source of the terrigenous material of these sediments may have been some of the above-described outcrops of more ancient granitoids relative to the Upper Devonian and Lower Carboniferous sediments.

For the Kolyvan and Orlov massifs, the potassium–argon method on biotite has yielded, respectively, absolute-age values of 274–285 million years and 229, 238–244, 252, 258, 275 million years (Verigo, Mareev, 1965), confirmed by determinations of our samples carried out by O. A. Kukharenko by the birefringence-dispersion method. The large scatter in the determination of the absolute age of the granites described cannot at present be explained unambiguously. The presence, for them, of a large number of determinations within the range of 390–590 million years indicates that the age of the Ob granitoid massif is older than the age attributed to it by a number of researchers. The “rejuvenation” of the absolute-age determinations of individual parts of the massif may be caused by various reasons.

The loss of radioactive argon and uranium by granitoids occurs on a broad scale: first as a result of autometamorphic processes associated with the emplacement of the massif, then in the process of cryptomorphism with an increase in pressure to 10 or more bars and temperature to 300°C, and with mo-

the moment of its exposure by erosion—as a result of the leaching of radiogenic formations by migrating waters.

Within the massif described, waters enriched in radon are confined to areas having the most “young” absolute age. The same regularity is manifested east of the massif in the Bugotak–Mitrofanovka horst-anticlinal zone, composed of volcanogenic-sedimentary formations of the Lower–Middle Devonian. The absolute age of the latter is determined within the range of 155 (Mokhnataya sopka)–245 (Chumakov sopka) million years (Amshinskii et al., 1966).

Thus, it is possible that the granitoids of the Novosibirsk Ob region are characterized not only by a multiphase, but also by a long-lasting history of formation. Similar conclusions are now being drawn for some granitoids of other regions. For example, for Kazakhstan it has been established that intrusion and metamorphism of granitoids occurred in 14 tectono-magmatic phases with ages from 514 to 253 million years (Ivanov, 1967). In structural terms such granitoids may correspond, as noted by N. A. Shtreis (1968), to geoanticlines and median massifs associated with geosynclinal troughs filled with formations of the spilite–keratophyre group with intrusions of basic composition (under our conditions, the Bugotak suite of the Lower–Middle Devonian).

For more definite conclusions, additional materials and a special analysis are necessary, one not constrained by any one-sided theoretical conceptions.

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