ON THE RELATIONSHIPS BETWEEN METAMORPHIC AND INTRUSIVE ROCKS OF THE YENISEI–OYA INTERFLUVE

(WESTERN SAYAN)

V. P. Korobeinikov, A. P. Shcheglov

(Presented by Professor A. G. Sivov)

The area studied is located at the junction of several heterogeneous structures having a block character. Here the Dzhebash salient, the Kizas–Oya anticlinorium, and the Borus graben-synclinorium are distinguished. The last two structures are composed of effusive-sedimentary rocks of the Chingin Formation. Below we consider the mutual relationships of rocks in the Dzhebash salient, which is composed of metamorphic and crystalline schists, amphibolites, gneisses, quartzites, and marbles of the Dzhebash Formation, granitoids of the Kogdar massif, and also ultramafic rocks of a hyperbasite intrusion (Fig. 1).

The chemical composition of some metamorphic rocks of the Dzhebash Formation is shown in Table 1.

The most widespread among the rocks of the Dzhebash Formation are formations of the greenschist facies, which extend in a broad belt from Lake Teletskoye to the junction of the West Sayan structures with the East Sayan structures. Within the area described, from the Yenisei River to the Oya River, the greenschists belong to the quartz–albite–muscovite–chlorite and quartz–albite–epidote–almandine subfacies, according to the classification of F. Turner and J. Verhoogen (1961).

The listed subfacies of greenschists are in complex mutual relationships. In general, a regular transition of subfacies is observed from west to east, from low-temperature to high-temperature ones within the greenschist facies.

On the watershed of the Malaya and Srednyaya Shush rivers, we observed a gradual replacement of greenschists developed west of the watershed by amphibolites and crystalline schists extending into the basin of the Kogdar River.

The amphibolites are characterized by the amphibole–plagioclase association, which is often accompanied by garnet and epidote. The amphibolites are derivatives of basic igneous rocks and belong to the staurolite–almandine subfacies, which corresponds to the weakest metamorphism within the almandine–amphibolite facies.

Garnet schists, gneisses, and also crystalline schists, which in the successive change of facies from west to east replace the amphibolites, belong to the granulite facies, which reflects a metamorphic temperature of about 700–800°C.

Crystalline schists and amphibolites pass into a zone of layered migmatites. The substrate most often displays a lepidoblastic

and nematoblastic structures, whereas the granitic material has a granoblastic structure. Orthoclase makes up about 40% of the total amount of granitic material. It forms porphyroblasts and grains of sinuous shape, which corrode quartz and plagioclase. Elements of a poikiloblastic structure are sometimes observed: small corroded grains of plagioclase and quartz are enclosed in large orthoclase grains. Oligoclase constitutes about 20% of the granitic material. The remaining part of the granitic component of the migmatites is composed of quartz and single small grains of biotite or hornblende. Among the layered migmatites, augen migmatites composed of the same components are not uncommon. Their characteristic feature is the absence of traces of introduction of granitic material along fractures or along bedding, and the absence of magmatic structures in the vein material. On the contrary, replacement structures are very often observed. Orthoclase corrodes quartz and plagioclase; elements of poikiloblastic and porphyroblastic structures are characteristic. Both in the crystalline schists and in the migmatites, biotitization of hornblende is observed, but the content of the latter in these rocks is considerably lower than in the amphibolites. It is often present together with biotite.

The amount of granitic material in the migmatites increases eastward, while the amount of substrate correspondingly decreases. The homogenization that begins in the migmatites leads to the formation of diorite-gneisses, granodiorite-gneisses, and granite-gneisses. In these rocks, a typical granoblastic structure with traces of replacement structures is still observed, leading to an increase in the alkaline components. The granite-gneisses are replaced by massive gneiss-granites, sometimes with elements of gneissic appearance. An extremely important feature of their structure is the combination of granoblastic and hypidiomorphic-granular structures, and of poikiloblastic and poikilitic structures. Finally, closer to the central part of the Kogdar massif, massive rocks of magmatic appearance appear, among which granites are the most widespread. In these rocks, too, traces of metasomatic transformation processes are found, expressed in the microclinization of plagioclases. The Kogdar granitoids are characterized by variability of composition and structure, by the presence of numerous poikilitic inclusions of plagioclase, mafic mineral, or quartz in potassium feldspars. The latter are often present in two generations: 1) as idiomorphic or hypidiomorphic, often pelitized grains; 2) as fresh, irregularly shaped, large grains (up to 8 mm) with poikilitic inclusions.

Chemical analyses of the Kogdar granites differ from granites of normal composition by a somewhat elevated alumina content, approximately equal contents of Mg and Fe, or a slight predominance of magnesium, as well as by a predominance of Na over K.

Gradual transitions of granites into the enclosing rocks, the presence in the latter of structures of replacement of sodium-calcium feldspars by microcline and orthoclase and of hornblende by biotite, as well as inherited gneissic textures in the granites, lead to the conclusion that the granitoids of the Kogdar massif, at least in its marginal parts, are products of metasomatic granitization.

When comparing the formulas of amphibolites, crystalline schists, granodiorites, and granites, compiled by the oxygen method of T. Barth (Table 2), it is easy to see that in the process of metasomatic reworking of metamorphic rocks into granites, K, Na, and Si are introduced, while Ca, Mg, Fe, Al, and Ti are removed from them; this apparently also leads to the formation of amphibolites closely spatially associated with the granitoids of the Kogdar massif.

Table 1

Chemical analyses of certain metamorphic and intrusive rocks and their numerical characteristics
(according to A. N. Zavaritskii’s method)

Notes: 1 and 2 — amphibolites; 3 — crystalline schist; 4 — granodiorite; 5, 6 and 7 — granites; 8 — diorite-gneiss; 9 and 10 — serpentinites.

Fig. 1. Geological-structural map of the Koldar River basin.

1–6 — Dzhebash Formation: 1 — marbles and crystalline schists; 2 — garnet schists; 3 — gneisses; 4 — quartzites; 5 — marbles; 6 — green metamorphic schists; 7 — clay shales and phyllites; 8 — diabases, diabase and pyroxene porphyrites; 9 — tuffaceous rocks; 10 — limestones; 11 — microquartzites; 12 — Quaternary deposits; 13–15 — rocks of the Koldar gabbro-gneiss massif: 13 — granites, granodiorites; 14 — gneiss-granites; 15 — gneiss-granites, granodiorites; 16 — gabbroids; 17 — pegmatites; 18 — granites of the Bolshinen pluton of the Mana complex; 19 — migmatites; 20 — boundaries between rock units; 21 — rock boundaries; 22 — inferred boundary of the Koldar massif; 23 — faults; 24 — bedding: a) inclined, b) vertical; 25 — schistosity; 26 — crystallization schistosity: a) inclined, b) vertical; 27 — banding: a) inclined, b) vertical; 28 — gneissose structure.

It has been established by many researchers (N. A. Eliseev, 1959; Yu. A. Kuznetsov, 1941, 1955; F. Turner, J. Verhoogen, 1961, and others) that phenomena of regional metamorphism are accompanied by metamorphic differentiation. In our area, phenomena of metamorphic differentiation are manifested in the formation of banded textures in amphibolites and crystalline schists, in the segregation of rocks of granitic composition among crystalline schists and amphibolites, in the growth of porphyroblasts of almandine in garnet schists of the kinzigite type, in the presence

Table 2

Formulas of the principal rock types (after T. Barth)

Note: 1 — amphibolites; 2 — crystalline schists; 3 — granodiorites; 4 — granites.

of numerous epidote veins in amphibolites and schists, and in the formation of augen migmatites. Since there are entirely gradual transitions between amphibolites and crystalline schists, on the one hand, and layered migmatites, on the other, it is possible that these migmatites are likewise products of metamorphic differentiation. In this case, the application to these formations of the term “migmatite” in its genetic sense is incorrect, and we use this term descriptively, adhering to Sederholm’s original definition.

As an extreme product of metamorphic differentiation and alkaline metasomatism, gneiss-granites and rocks of magmatic appearance may also be regarded; however, it is quite probable that some part of the rocks composing the Kogdar massif arose by a magmatic route, with the formation of melt, according to the concepts of Yu. A. Kuznetsov (1955), N. G. Sudovikov (1955), F. N. Shakhov (1956), and some other researchers, occurring in place, when under conditions of rising temperature the composition of the granitized material approached the composition of granite. The possibility that part of the granitoids of the Kogdar massif originated from a melt is indicated by clearly expressed hypidiomorphic-granular textures, zoning of plagioclases in the rocks of the central part of the massif, and the presence of a large number of pegmatite veins, which cut both the “enclosing” rocks and the granitoids.

The intense manifestation of ultrametamorphism is confined to the tectonically stressed junction zone of three heterogeneous structures, a zone of development of deep-seated or, in any case, regional faults. The fact that the Maloshushensky fault and the faults of the Borus structure spatially control the distribution of deposits of the Chingin Formation, and that most bodies of hyperbasites are confined to these faults, indicates the existence of these faults in Dochingin time. The inception precisely here of a center of granitization indicates the high permeability of the Earth’s crust in this sector, which may be regarded as evidence for the existence of these faults already in Dzhebash time. It is beyond doubt that in the basin of the Kogdar River the present denudational

the section exposes such parts of the earth’s crust as were situated at depths with temperature and pressure sufficient for the extreme metamorphic transformation of the rocks of these parts.

There is a view that the bodies of hyperbasites in the area described are xenoliths among the rocks of the Kogdar massif (D. M. Orlov, 1961; L. P. Zonenshain, 1963). The age relations of the ultrabasic and granitic rocks are interpreted accordingly. Our observations confirm the conclusion of K. V. Radugin and A. G. Sivov (1948) that the hyperbasites are younger than the Kogdar granitoids. The following testifies in favor of this:

1. The bodies of hyperbasites occur both among rocks of magmatic appearance and among granite-gneisses, migmatites, and crystalline schists of the Dzhebash Formation. Large bodies of hyperbasites most often have a massive texture without any traces of schistosity or banding, whereas the granitic rocks are endowed with these features.

2. Some linearly elongated bodies of hyperbasites are oriented at an angle to the strike of the gneissic and schistose textures of the enclosing rocks.

3. There is a complete analogy in the structure and chemical composition of the ultrabasic rocks of massifs situated among rocks of the Dzhebash Formation and granitoids of the Kogdar massif, on the one hand, and in the field of development of rocks of the Chingin Formation, on the other.

4. A linearly elongated body of hyperbasites, traced along strike in an east-northeastern direction in the area of the sharp bend of the valley of the Kogdar River to the southeast, is situated both among granitoids and among amphibolites and thus has a cross-cutting contact.

5. Four kilometers below the confluence of the Right and Left Kogdars, crystalline schists are exposed at the water’s edge on the left bank. The schists are cut by veins of medium-grained pegmatite, and quartz veins cut both the schists and the pegmatites. The crystalline schists are intruded by hyperbasites, which in turn are cut by quartz veinlets with a thickness of fractions of a centimeter. In the near-contact part of the hyperbasites, small crystals of plagioclase are observed. Since the temperature at the contact even of large hyperbasite bodies hardly exceeded 500°C (F. Turner, J. Verhoogen, 1961), this fact may be regarded as mechanical capture of enclosing, partly disintegrated material during the intrusion of the hyperbasites into granitized schists.

In the same outcrop, a cross-cutting contact of serpentinites is observed with respect to the crystallization schistosity of the enclosing rocks (Fig. 2). In addition, at the contact with serpentinites in the crystalline schists, the linear-parallel texture becomes obliterated. They are transformed into massive and intensely silicified rocks.

1. Five hundred meters north of the confluence of the Left and Right Kogdars, a trench exposes the contact of diorite-gneisses and serpentinites. The contact surface is very sharply expressed and dips to the southwest at an angle of 80°. At the contact of the serpentinites with the diorite-gneisses, the latter contain three zones of amphibolization, situated at distances of 30, 50, and 75 cm from the contact. In these zones there is an accumulation of large crystals of greenish-black and black hornblende. An accumulation of hornblende at the contact of gneisses with younger serpentinized peridotites was described by Ch. Porro in the Swiss Alps, as noted by V. N. Lodochnikov (1956). In the opinion of these authors, accumulations of hornblende have a “hydrothermal origin in connection with the eruption of peridotites.”

All these facts, from our point of view, testify quite convincingly that the hyperbasites are younger than all the formations of the Dzhebash Formation and the granitoids of the Kogdar massif. Thus, the following sequence of geological events is indicated in the area described:

1) formation of the rocks of the Dzhebash Formation;

2) emplacement of sublatitudinal faults, folding, regional metamorphism, and granitization of the rocks of the Dzhebash Formation with the formation of the Kogdar granitoids;

3) formation of the rocks of the Chingin Formation;

4) folding and formation of the hyperbasites.

Fig. 2. Nature of the relationship of serpentinites with crystalline schists.
1 — quartz–feldspar–biotite–amphibole crystalline schist; 2 — conformable quartz veins; 3 — serpentinite; 4 — quartzization.

The events that preceded deposition of the Chingin Formation are so significant that they could not have fit into a short interval of time; moreover, there are data indicating the presence of a break in sedimentation before deposition of the rocks of the Chingin Formation. The age of the Chingin Formation corresponds to the greater part of the Lower Cambrian, but it is not excluded that the lower part of the formation already corresponds to the Late Precambrian. If the age of the Chingin Formation is considered only Lower Cambrian, then in this case as well the assignment of the Dzhebash Formation to the Upper Proterozoic raises no doubt. Equally obvious is the presence in the Western Sayan of Proterozoic granitoids.

REFERENCES

1. N. A. Eliseev. Metamorphism. L., 1959.

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

3. Yu. A. Kuznetsov. Granite intrusions of the South Yenisei Range. Proceedings of the Tomsk Industrial Institute, 62, issue 1, 1941.

4. Yu. A. Kuznetsov. Origin of igneous rocks. “Magmatism and the associated useful minerals,” 1955.

5. V. N. Lodochnikov. Serpentines and serpentinites of Ilchir and others. Transactions of TsNIGRI, issue 38, 1936.

6. D. M. Orlov. A brief outline of the magmatism of the Western Sayan. Transactions of VSEGEI, new series, vol. 58, 1961.

7. A. G. Sivov. Cambrian and Precambrian of the Western Sayan. Transactions of GGI ZSFAN USSR, issue 2, 1948.

8. N. G. Sudovikov. On the question of a possible connection between hydrothermal mineralization and granitization. Magmatism and the associated useful minerals, 1955.

9. F. Turner, J. Verhoogen. Petrology of Igneous and Metamorphic Rocks. IL, 1961.

10. F. N. Shakhov. On the question of the origin of granitic magmas. Mineralogical Collection of the Lvov Geological Society, No. 10, 1956.