Experience in Applying the Method of Formational Analysis in the Western Sayan

S. S. Kumeev

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

The application of the method of formational analysis in areas where repeated intrusive and effusive magmatism is manifested makes it possible not only to systematize the basic regularities in the history of the geological development of a region, but also to resolve correctly the key questions of genetic connection and inheritance among individual epochs of magmatic activity. A direct consequence of this, naturally, is both the obtaining of general recommendations concerning the prospects of certain areas and the development of the most productive criteria for prospecting for particular minerals.

The present work considers the possibilities of applying the method to the area of the northeastern part of the Western Sayan, situated in the region of junction of the Western and Eastern Sayans and the Tuva intermontane depression. The intrusive and effusive activity of this node was controlled by the Kandat and Kurтushibin (Sayan–Tuva) deep faults, which meet in the area of the headwaters of the Kandat River. These faults border from the north and east a rigid block of Proterozoic–Sinian formations, which represents the eastern termination of the Dzhebash anticlinorium. Accordingly, the northern and eastern margins of this block are regions of the North Sayan and Kurtushibin mobile zones and are characterized by the development of magmatic formations of the geosynclinal stage proper.

Both zones began their development in Lower Cambrian time and are composed of formations of the spilite–keratophyre group. At the same time, the rocks of each zone reveal their own specificity of petrographic composition: in the North Sayan zone (its northeastern termination) there is a quartz–keratophyre formation, whereas in the Kurtushibin zone (the northern part of the zone) a tendency toward diabase-porphyrites is observed. All effusive formations of the region are at the stage of considerable greenschist alteration. Both the quartz–keratophyre and the diabase-porphyrite formations are not strictly uniform in composition and are characterized by facies transitions and petrographic variability. Especially in the diabase-porphyrite formation, transitions to the ordinary petrographic members of the spilite–keratophyre formation, as well as local bodies of andesite porphyrites and dacites, can often be observed.

In direct dependence on the effusive magmatism of the geosynclinal stage is also the diversity of intrusive formations of the region. Here a gabbro–plagiogranite formation is distinguished (the West Sayan ...).

the Lower Cambrian complex of small intrusions) and the plagiogranite formation (the Maina complex), whose intrusions are distributed exclusively among the effusive formations. At the same time, the northern termination of the Kurtushiba zone is characterized by the presence of a gabbro–plagiogranite formation, whereas the North Sayan zone is characterized by a plagiogranite formation; this apparently reflects certain features of the geosynclinal regime of the zones [6]. The intrusions of the complex of small intrusions, in the form of dikes and small stocks of linear character, are elongated in accordance with the direction of the structures of the enclosing effusives, have cross-cutting relationships with them, and, most probably, are intrusive equivalents of the diabases and diabase porphyries—the predominant rocks of the Kurtushiba zone. The petrographic composition of the rocks of the complex, which is of two-phase nature, is rather varied: gabbro, hornblende gabbro, gabbro-norites, diorites, granodiorites, plagiogranites, granites. A sharp predominance of rocks of the basic phase over those of the acid phase is characteristic.

The Maina plagiogranite complex is represented by the large Malokandat massif and by a series of smaller massifs, also of sublinear character, situated in the field of the quartz–keratophyre formation, and may be regarded as the intrusive equivalent of the latter [9]. Formation of the massif took place in two phases, but the basic constituent—gabbro, hornblende gabbro, diorites—is expressed much more weakly: more than 80% of the rocks are represented by plagiogranites, tonalites, and quartz diorites. It is necessary to note the extremely wide development of hybrid rocks of the tonalite and quartz diorite type, which indicates an intensive qualitative transformation of the gabbroids under the influence of plagiogranite magma.

The formation of a number of hyperbasite massifs is associated with the period of revived activity of the Kandat and Kurtushiba deep faults; their intrusions appeared after the initial stage of development of the geosyncline. The example of the hyperbasite formation also shows a certain difference in the conditions of life of the North Sayan and Kurtushiba zones. Whereas the former, within the limits of its eastern termination, is characterized by the distribution of small bodies, predominantly of serpentinite composition, within the latter the hyperbasite formation is distinguished as consisting of two complexes, and some of their massifs occupy areas of up to 50 sq. km. The earlier one—the gabbro-peridotite complex of Lower Cambrian age, whose rocks cut the deposits of the effusive formation (\(\mathrm{Cm}_1\)) and the Precambrian formation of ortho- and paraschists (the Dzhebash suite). The massifs of the complex, subparallel to the controlling faults, are composed of apoperidotite serpentinites and gabbroids cutting them (allodinites, hornblende, ilmenite, and titanomagnetite gabbro). The Aktovrak complex of ultrabasic rocks (\(\mathrm{Cm}_{1-2}\)) is represented by two large massifs—the Ergak and Kalnin massifs—which are composed of apoperidotite and apodunite serpentinites, harzburgites, lherzolites, wehrlites, dunites, and pyroxenites. Within the massifs, processes of hydrothermal metamorphism are widely manifested.

The material presented makes it possible to interpret the beginning of the development of the geosynclinal zone of the northeastern part of the Western Sayan as follows: a rigid block of Precambrian rocks is bordered on the north and east by narrow mobile zones of eugeosynclinal (Kurtushiba) and epieugeosynclinal (North Sayan) type. Both zones are characterized by a broad development of effusive magmatism and by the presence of differentiated intrusive formations genetically connected with it (gabbro–plagiogranite and plagiogranite). After this effusive–intrusive cycle—apparently concluding it—ultrabasite magmatism develops within the zones.

The stage of inversion of the geosyncline and subsequent geoanticlinal development in the northeastern part of the Western Sayan is expressed very weakly. In the present case it is recorded by the development of a formation of differentiated gabbroic and noritic intrusions, which cut through deposits of the effusive formation \((Cm_1)\) and the flysch formation \((Cm_3)\), and which have no effusive analogues of their own. The intrusions of Mt. Bulka and Mt. Sydyg, assigned to the Bulka complex, belong to this formational type. Both intrusions are deeply differentiated, although the degree of differentiation is somewhat unequal. The massif of Mt. Sydyg is composed of peridotites, olivine gabbros, gabbro-norites, norites, and biotite gabbros. The Bulka massif, by contrast, consists of gabbros, gabbro-norites, norites, trachytolites, hornblende and titanomagnetite gabbros, hornblendites, gabbro-diorites, and anorthosites. Crosscutting bodies of acid rocks of granitoid composition are also noted. The structural and age position of the formation of differentiated intrusions falls somewhat outside the scheme proposed by Yu. A. Kuznetsov (1964), and may be explained by the complex deep-seated transformations undergone by the Proterozoic basement in the area where two mobile zones join.

Very characteristic for the region is the absence of typical orogenic magmatic formations, with the exception of the formation of batholiths of post-folding origin. The existence of the orogenic stage of development itself is established only indirectly, by the presence of these batholiths among Precambrian rocks, whereas the rocks of the other formations may possibly have been completely denuded. An analogous formation of batholiths is also established in the zone of the Khemchik–Sistig-Khem outer geosynclinal trough, inherited by the Tuvinian intermontane depression [2]. This zone is characterized by thick accumulation of flysch- and molasse-type sediments in Late Cambrian–Ordovician time.

The above-mentioned formation of granitic batholiths should include the Bolshie Porogi complex, whose massifs are often the apical parts of large batholiths. The age of the complex is Silurian [3]. The petrographic composition of the rocks is extremely uniform—biotite and two-mica granites.

Silurian time marks the completion of the development of the Caledonian geosyncline of the Western Sayan.

A further revival of tectonic and magmatic activity in the northeastern part of the Western Sayan took place in the Lower Devonian and was due to the renewal of the Kandat and Kurtushiba deep faults. Here too, however, features characteristic of both mobile zones are visible. In the area of the Kurtushiba zone, Devonian magmatism is manifested in a number of troughs inherited from the Silurian and laid down upon a Caledonian folded basement. In the troughs there accumulated effusive deposits of very variegated composition, with more basic varieties prevailing in the lower parts of the section (labradorite and andesine porphyrites) and acid varieties in the upper parts (oligophyres, albitophyres, quartz albitophyres), but with an overall acid specialization. The intrusive equivalent of the effusive formation is the formation of subvolcanic granites, distinguished as the subintrusive Lower Devonian complex. The small massifs of the complex are closely associated spatially with the effusives (the liparitic formation) and show petrochemical similarity to them. In all likelihood, these massifs belong to the root system of the Devonian effusives. The rocks of the complex are characterized by broad variations in petrographic composition: microcline granites, granosyenites, and granodiorites predominate; quartz monzonites, syenodiorites, and gabbro-diorites occur more rarely.

On the basis of the nature of the interrelations of the rocks, one may assume a multiphase character for the intrusions of the complex.

Devonian magmatism in the North Sayan zone therefore appears differently, where the superposed Minusinsk intermontane trough in its eastern termination is represented by narrow troughs elongated in accordance with the trend of the zone and filled with effusive formations of the Imir suite, which, despite its generally acid composition, is distinguished by increased alkalinity. The rocks of the suite rest unconformably on the Cambrian basement. They lack an intrusive root system in the form of a subintrusive complex, but the system itself is, in rare cases, established by the presence of effusive-intrusive formations of the volcanic-vent facies.

The Lower Devonian system of activation of deep-fault activity is completed by the formation of a granite-batholith formation, to which belongs the Dzhoi granitoid complex of Lower–Middle Devonian age. The massifs of the complex are widely developed in the central part of the Western Sayan and, in general, are confined to the folded framing of Devonian intermontane depressions [5]. Because of this, within the region they are rather often associated with the Devonian effusive formation. Among the rocks of the Dzhoi complex, alaskites, porphyritic microcline granites, and granophyres predominate.

Thus, two stages of consolidation of the North Sayan and Kurtushibin mobile zones are indicated, which follow the formation of granite batholiths. The first stage—the Late Silurian—marks the completion of the development of the Caledonides and the establishment of subplatform conditions. The second stage—the Middle–Late Devonian—characterizes the final completion of folding after a short-term activation of deep faults in the Lower Devonian, leading to the displacement of rigid blocks of the Caledonian basement. As a result of the latter, already stable, consolidation, the northeastern part of the Western Sayan is transformed into a young platform.

On the whole, the sequence of magmatism outlined for the northeastern part of the Western Sayan, in volume, periodicity, and content, corresponds to the systematics of magmatic formations of Yu. A. Kuznetsov, which makes it possible clearly to distinguish individual stages in the development of the region. The regular succession of formations testifies to the directed character of the tectono-magmatic process and to the unified character of the general geological transformations.

LITERATURE

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2. L. P. Zonenshain. Tectonics of the Western Sayan. Gosgeoltekhizdat, 1963.

3. T. N. Ivanova. Magmatic formations of the central part of the Altai–Sayan region and the ore complexes associated with them. In: “Magmatic Formations.” Proceedings of the 3rd All-Union Petrographic Conference. Nauka, 1964.

4. I. N. Kazakov. A brief outline of the stratigraphy and magmatism of the Western Sayan. Information collection of VSEGEI, issue 27, 1960.

5. A. N. Ken. Geological formations of the central part of the Altai–Sayan folded region and their ore potential. In: “Materials on the Geology and Mineralogy of Ore Deposits of the USSR,” VSEGEI, vol. 103, 1964.

6. Yu. A. Kuznetsov. Principal Types of Magmatic Formations. Nedra, 1964.

7. D. M. Orlov. A brief outline of the magmatism of the Western Sayan. In: “Materials on the Regional Geology of the Altai–Sayan Folded Region,” VSEGEI, vol. 58, 1961.

8. G. V. Pinus, V. A. Kuznetsov, I. M. Volokhov. Hyperbasites of the Altai–Sayan Folded Region. Publishing House of the Academy of Sciences of the USSR, 1958.

9. V. N. Smyshlyaev. The plagiogranite intrusive complex of the northern slope of the Western Sayan. In: “Magmatic Complexes of the Altai–Sayan Folded Region,” Transactions of the Institute of Geology and Geophysics, issue 33, Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1963.