METAMORPHISM AND SOME FEATURES OF TITANIUM LOCALIZATION IN THE SHINDA-DERBA FAULT ZONE (EASTERN SAYAN)

O. M. GLAZUNOV

(Presented by Professor A. M. Kuzmin)

The Shinda-Derba fault, with which the metamorphic formations described here are associated, is located on the southwestern wing of the East Sayan anticlinal uplift. It extends parallel to the leading geological structures of the region from the headwaters of the Derbina River (a tributary of the Yenisei River) to the upper reaches of the Kyzyr River in the central part of the Eastern Sayan.

According to areal investigations by F. Ya. Pan and V. V. Khomentovsky [16], the fault bounds the structure of the Protero-Sayan from the Paleozoic folded region.

In the basin of the Sisima and Shinda rivers, the fault is a system of conjugate longitudinal dislocations cutting the deposits of the effusive-sedimentary (Kuvai) suite of the Upper Proterozoic at the boundary of their junction with more ancient carbonate strata. The general tectonic picture of one of the segments of the fault is presented in Fig. 1.

The width of the zone in plan is 3–5 km; the predominant dip of the displacement planes is to the southwest at ∠65–80°. The most weakened portions of the dislocation are marked by a chain of complexly banded massifs of gabbro-pyroxenite-serpentinite composition, united into the Lysansk complex [2].

Within the zone, the Proterozoic rocks are strongly dislocated and schistose, and selectively cataclased. It is characteristic that they bear traces of more intense metamorphism than analogous formations situated at some distance from the fault. This circumstance, together with the broad linear development among the fields of effusive rocks of such rocks as amphibolites and migmatites, has long attracted the attention of researchers and has received various explanations in their works. The main attention in this connection was usually given to the origin of the amphibolites.

Thus, D. A. Vasiliev, who in 1942 was the first to briefly describe the metamorphic rocks of the upper Sisima, considered the amphibolites to be a product of deep regional metamorphism and on this basis assigned them to the most ancient Archean complex of the Eastern Sayan.

F. Ya. Pan linked the amphibolites with the manifestation of contact metamorphism.

Later, Z. I. Ikonnikova proposed regarding them even as derivatives of gabbro and gabbro-amphibolites of the Lysansk complex. As will be shown, this is completely contradicted by the factual data.

All the conclusions presented here were based on incidental observations and, naturally, cannot be regarded as definitive.

Meanwhile, the poor degree of study of the rocks within the tectonic zone under consideration—in particular, the absence of information on the degree of their metamorphism, the primary substrate, and consequently their genesis—has given rise to lively debate among researchers. Interest in these formations has recently increased in connection with the discovery by F. Ya. Pan of titanomagnetite mineralization in serpentinites, and by us of interesting concentrations of rutile in amphibolites.

The present materials do not claim to be complete, but, in our opinion, they make it possible to approach more closely the solution of the question of metamorphism

Fig. 1. Fragment of the tectonic pattern of the fault.
Fold axes: 1 — synclinal, 2 — anticlinal. Predominant directions: 3 — schistosity, 4 — layering, 5 — gneissosity. Faults: 6 — principal, 7 — secondary, 8 — inferred, 9 — established. Fractures: 10 — rupture, 11 — shear; 11a — zones of intense schistosity. Lower Proterozoic: 12 — marbles with packets of graphitic schists. Upper Proterozoic: 13 — greenstone effusives, 14 — amphibolites, 15 — schists, 16 — gneisses, 17 — limestones, 18 — massifs of the Lysanskii gabbro–pyroxenite–serpentinite complex.

in the fault zone, the genesis of the amphibolites and the titanium mineralization associated with them in the western part of the Eastern Sayan.

One of the most characteristic features of the Shinda-Derba tectonic zone is that it contains rocks belonging to various metamorphic facies.

In accordance with the classification of F. J. Turner [14], we distinguish the following types of metamorphic formations:

a) effusives at the greenstone stage of alteration.

b) albite–actinolite–chlorite schists of the greenschist facies,

c) hornblende and epidote–albite–actinolite schists of the epidote–amphibolite facies,

d) amphibolites of the amphibolite facies proper,

In addition to the rocks indicated, migmatites are found, as well as bodies of gabbro, pyroxenites, and serpentinites, on the characteristics of which we shall not dwell. The study of the metamorphism of the latter may constitute the subject of an independent work.

The zone under consideration as a whole has a complex geological structure (Fig. 2). In individual areas, rocks metamorphosed to varying degrees are so closely intermingled with one another that at first glance it is impossible to identify any regularities in their distribution and relationships.

They do not occupy a broad area, but form separate linear belts in the most weakened parts of the fault.

A detailed study of the sections as a whole reveals a zonal structure of the zone.

First of all, attention is drawn to the close spatial association of the most strongly metamorphosed rocks and of the products of the processes of biotitization and albitization with the central part of the zone, which is marked by chains of gabbro-pyroxenite massifs and by areas of migmatite development.

Fig. 2. Geological section through the fault zone. 1—effusives; 2—metamorphic schists at the expense of effusives; 3—marbles with interbeds of graphitic schists; 4—amphibolites; 5—migmatites; 6—intrusive massifs of gabbro-pyroxenite-serpentinite composition; 7—dislocations.

There is also an association of amphibolites with zones of increased schistosity and fracturing.

With distance from the center of the zone in a direction transverse to the strike, a decrease in the degree of metamorphism of the effusives is observed. Amphibolites are replaced by hornblende schists. The latter pass into epidote-actinolite-albite schists, which, in turn, give way to chlorite-actinolite-albite schists and to effusives in the ordinary greenschist facies state.

Below we shall consider the petrographic characteristics of the rocks, arranging them successively from weakly altered to more metamorphosed.

Let us first note the principal features of the original effusives. We should also indicate that each metamorphic facies is characterized by its own typomorphic mineral associations, “the first appearance of which—

… (in the transition from a low to a higher grade of metamorphism) marks the outer boundaries of the zone under consideration” [14]. For example, the greenschist facies is characterized by chlorite and albite, and the epidote-amphibolite facies by blue-green aluminous hornblende.

I. Effusive rocks far from the zone described have a greenish-gray color, an aphyric, more rarely porphyritic or amygdaloidal structure. In outcrops, spheroidal jointing and interbedding with beds of limestone and microquartzites are often observed.

The rock consists of elongated laths of plagioclase (No. 5–7), immersed in a scaly-fibrous groundmass of chlorite and actinolite with an admixture of calcite, epidote, zoisite, and sphene. The named minerals crystallize during diagenesis and are hysterogenic for the greenschist facies.

Metamorphism of the effusive rocks leads to the formation, at their expense, of rocks (albite-actinolite-chlorite schists) with schistose, ocellar texture and nematoblastic or pseudoporphyroblastic microstructure. Chlorite and albite play a substantial role in the composition of the rock, with actinolite subordinate (\(2V = -70^\circ,\ CNg — 16^\circ\)); sphene, rutile (needles in plagioclase), quartz, and epidote are also present.

II. According to Turner, the more metamorphosed rocks of the epidote-amphibolite facies include hornblende and epidote-albite-actinolite schists.

In appearance these are schistose-banded rocks consisting of green hornblende and albite.

Depending on mineral content, hornblende proper and albite-hornblende schists are distinguished.

The first group includes nearly monomineralic formations of blue-green amphibole with admixtures of epidote and albite. Their distinctive feature is high rutile content. The amphibole of epidote-albite-actinolite and amphibole schists pleochroizes in green and light-green tones and forms prismatic, acicular crystals. At a magnification of 12,000 times in the electron microscope, short columnar sections are observed.

By its optical constants (\(2V = (-)82—83^\circ,\ CNg = 16—19^\circ,\ Ng - Np = 0.018—0.021\)), the mineral belongs to actinolite or occupies an intermediate position between actinolite and grammatite [15]. The results of chemical analysis of the “bulk” amphibole in wt. % are as follows:

Table 1

In rutile-bearing hornblende schists several varieties of amphibole are distinguished. The most widespread is blue-green alkaline hornblende (\(2V = (-)64—68^\circ,\ CNg = 34—37^\circ\)), corresponding in composition to eckermannite. Of subordinate importance is an amphibole with green-brown pleochroic colors (\(2V = (-)40—56,\ CNg = 20—23^\circ\)), apparently close to barkevikite. According to Turner [14], blue-green amphibole is a sensitive index mineral of the epidote-amphibolite facies.

Albite is clearly xenomorphic in relation to amphibole, penetrates into sections of the latter, and forms bands alternating with bands…

of amphibole, or accumulates in the hinge parts of microfolds. Together with these and other minerals, later-segregated biotite and quartz occur everywhere. The presence of epidote and zoisite is characteristic of rocks of this facies.

The structure of the rocks described is predominantly nematoblastic, caused by the development of elongated prisms of hornblende. In places where amphibole contains inclusions of plagioclase, it approaches poikiloblastic. The texture is schistose, more rarely—augen, owing to the presence of relics of primary effusive rocks.

A characteristic feature of epidote amphibolites is their rutile-bearing nature, which is confined to rocks composed of blue-green hornblende. Albitic amphibolites, as well as the gabbro and gabbro-amphibolites “interbedded” with them, are poor in rutile or contain none at all. The rutile-bearing schists are contoured into a band extending for more than 20 km, with an approximate width in plan of 200 m.

The distribution of rutile in the rocks is generally uneven. It develops in the form of isolated acicular crystals and elongated grains up to 3 mm in size at the boundary between plagioclase and amphibole, more rarely as a rim around ilmenite grains. Sometimes the finest needles of the mineral are arranged along the cleavage of plagioclase. Almost always, large rutile grains are grouped into chains parallel to the schistosity. In some thin sections a clear spatial relationship between rutile and epidote is indicated.

III. Epidote amphibolites show gradual transitions to amphibolites proper, which tend toward the central part of the zone.

In outward appearance the amphibolites proper are dark-green, massive, with transitions to banded varieties of the rock. In their composition, as is usual in amphibolites, green hornblende predominates \((2V = (-)80—83^\circ; CNg = 16—20^\circ)\); a subordinate place belongs to andesine (No. 35—40). Among the accessory minerals, epidote, zoisite, chlorite, and sphene are noted.

All the minerals are distinguished by considerable freshness. The microstructure of the rock approaches granoblastic.

Rocks analogous in composition and structure have been described in many regions of the Altai-Sayan area. They are most widely developed within the Precambrian blocks of the Eastern Sayan and Northern Tuva [3].

Migmatites represent the final product of metamorphism of effusive rocks in the tectonic zone. They occur near gabbro, forming separate bands within the body of amphibolites and effusive rocks.

The morphological features of the rocks are fairly uniform. All of them belong to the type of banded migmatites according to N. G. Sudovikov [13] and K. A. Shurkin [17]; they are characterized by a banded texture due to the parallel arrangement of light plagioclase bands against the green background of amphibolite, or amphibolitized effusive rock.

The dark bands are composed of grains of common hornblende of varying coarseness \((2V = (-)76, CNg = 18)\), and the light bands—of albitized and saussuritized plagioclase (No. 30—40) with inclusions of amphibole and quartz.

In the dark bands of some samples, relics of effusive rocks are observed, enveloped by streams of plagioclase and amphibole. The boundaries of the bands are sharp and rectilinear. In contact with plagioclase, amphibole crystals are deformed with rupture and without rupture of continuity; they are partly replaced by chlorite and penetrated by plagioclase. Individual amphibole crystals are detached from the substrate and seem to float in

in the mass of plagioclase, and sometimes are stretched out into strings, creating a boudinage pattern.

In chemical composition the migmatites are similar to amphibolites, but differ from them only in their relative poverty in alkalis and titanium and in a higher content of calcium and aluminum (Table 2). We regard the migmatites as typical injection formations genetically associated with the final products of gabbro–pyroxenite magma.

To illustrate the mineral parageneses of the metamorphic facies, we used composition–paragenesis diagrams in the form of an equilateral triangle—the base of a tetrahedron with the virtual components \((\mathrm{Mg}, \mathrm{Fe})\mathrm{O}\), \(\mathrm{CaO}\), \(\mathrm{Al}_2\mathrm{O}_3\). Mobile components and impurities, in accordance with D. S. Korzhinskii’s mineralogical phase rule [6], are not shown on the diagrams.

Taking into account the constancy of the mineralogical associations, we limited ourselves to constructing one diagram for each facies. In Fig. 3, in addition to the basic data, figurative points are plotted both for the initial rocks and for the products of their metamorphism, as well as analyses of various gabbroids. The arrows show the direction of change of the rocks in the process of metamorphism.

As can be seen from the diagram (Fig. 3), the paragenetic association of minerals of effusives and green schists (actinolite–albite–epidote–chlorite) falls within the closed equilibrium field I; carbonate–chlorite–actinolite schists fall within equilibrium field II.

The mineral complex formed as a result of deeper metamorphism is shown in Fig. 4. Here the most stable assemblage is hornblende–epidote–albite I. Hornblende schists with epidote and biotite fall within equilibrium field II.

Fig. 3–4. Composition–paragenesis diagrams for rocks of the greenschist and epidote-amphibolite facies.

Within this same facies an epidote–quartz–sphene association is possible.

The amphibolite facies proper of the rocks is represented by the paragenesis hornblende–andesine (Fig. 5). Along with this, low-temperature minerals are also present here, such as chlorite and calcite, whose appearance should evidently be connected with the process of diaphtoresis.

To resolve the question of the genesis of the amphibolites, let us dwell on a comparison of the chemical characteristics of the effusives, schists, and amphibolites. We shall also draw a parallel between gabbro and gabbro-amphibolites, on the one hand, and amphibolites, on the other, since a number of geologists working in the area still adhere to the view that the amphibolites formed at the expense of gabbroids rather than effusives. It should be emphasized that in field investigations no transitions from amphibolites to gabbro are observed anywhere, whereas all stages in the transformation of effusives into amphibolites are recorded everywhere.

In chemical composition, and first of all in the high alkali content (A. I. Zavaritskii’s parameter “a” equals 8.8), the effusives are close to Daly’s spilites, although they differ from the latter in their elevated

Fig. 5. Composition-paragenesis diagram for rocks of the amphibolite facies.
I — effusives; II — hornblende and epidote-albite-hornblende schists (average of 10 analyses); III — amphibolites; IV — migmatite (sample 1274); 1 — gabbro-amphibolite (sample 2404); 2 — gabbro (sample 11); gabbro (average of 58 analyses).

content of magnesium, iron, phosphorus, and by a relative poverty in silica. Spectroscopic study (Fig. 6) indicates the presence in their composition of vanadium, chromium, nickel, cobalt, manganese, and a number of other components.

The rocks of the amphibolite facies (Table 2) are situated closer to the effusives than to the gabbro. This similarity is reflected in Fig. 5, where the position of the figurative points of the amphibolites and effusives is seen to form an independent and distinct variation line, removed from the corresponding line of the gabbroids.

Moreover, the closeness is confirmed by the presence of the same trace elements (Fig. 6) in equal quantitative amounts.

The position of the series indicates a marked change in the composition of the effusives during metamorphism, consisting in their enrichment in the ferruginous component and calcium, with a simultaneous decrease in the aluminum content.

Table 2

Results of chemical analysis

1—effusives, average of 10 analyses; 2—effusive (sample 1505); 3—amphibole schist (sample k—51—2); 4—the same (sample k—52—21); 5—amphibolite of Lysaya Mountain (sample 3001); 6—gabbro-amphibolite (sample 2404); 7—gabbro (sample 11); 8—gabbro, average of 58 analyses; 9—migmatite of Lysaya Mountain (sample 1274).

Analyses by the laboratory of the Krasnoyarsk Geological Administration.

Fig. 6. Diagram of average contents of various elements in metamorphic rocks.
1—effusives, 2—schists formed at the expense of effusives, 3—amphibolites, 4—gabbro, 5—gabbro-amphibolites.

As can be seen from Table 2 and Fig. 5, the gabbro-amphibolites, in chemical terms, tend toward gabbro rather than effusive rocks. They are more highly saturated with alkalies (“a” = 8.5–12, as against 5 in the amphibolites), and contain somewhat less magnesium.

The amounts of chromium, nickel, and vanadium in them are one order of magnitude lower than in the effusive rocks and amphibolites. It is very important that in the gabbro such elements as scandium, beryllium, zirconium, and lithium have been determined; these are entirely uncharacteristic of amphibolites and of their substrate—effusive rocks.

Thus, the study of the chemistry of the rocks convincingly confirms the direct field observations on the origin of amphibolites in the Shinda-Derba fault zone from effusive rocks, and not from gabbroids, as was previously believed.

Let us recall that G. V. Pinus, V. A. Kuznetsov, and I. M. Volokhov give an almost analogous explanation for the genesis of amphibolites occurring in zones of deep faults of Western Sayan and Tuva [11].

The possibility of the transformation of effusive rocks into amphibolites in other regions is indicated by F. Yu. Levinson-Lessing [10], E. A. Kuznetsov [9], and a number of other researchers.

Using the available results of rock analyses, we attempted to take into account the material balance of the effusive rocks and to trace its redistribution during metamorphism. For this purpose, a series of samples was recalculated by the oxygen method and, on the basis of the results, a diagram was constructed (Fig. 7). To exclude the influence of contact metamorphism from the gabbro-pyroxenite intrusion, analyses from the contact zone were not included in the calculation.

The observed chemical transformations are correlated with changes in mineralogical composition.

Fig. 7. Diagram of the change in the content of components during metamorphism. Explanation for Nos. 1 to 5 is in Table 2.

In the process of progressive metamorphism in the fault zone, that is, as the degree of amphibolitization of the effusive rocks increases, the content of magnesium and iron in them increases, with an insignificant increase in calcium and potassium, while at the same time the amount of aluminum, sodium, titanium, and silicon decreases.

In rocks of the amphibolite facies, amphibole is formed at the expense of the femic part of the effusive rocks and of the anorthite component, which under these conditions proves unstable. Part of the plagioclase was bound into epidote and zoisite. In the process of desanorthitization, albite apparently remains unchanged, which explains its considerable percentage in rocks of the epidote-amphibolite facies.

As a result of the weak activity of basaltic magma, the exocontact...

changes in the effusives, for their part, were expressed in the development of a narrow reaction selvage of chloritite with ilmenite and carbonate, more rarely with epidote, and of anorthite stringers. The formation of this mineral complex takes place with mobile behavior of calcium, sodium, potassium, ferric iron, titanium, and with inertness of magnesium, ferrous iron, and silica (Fig. 8). However, the indicated migration is of insignificant extent and is unrelated to the process of dynamothermal metamorphism leading to the transformation of effusives into amphibolites.

Especially noteworthy is the high activity of calcium, which promotes the appearance of migmatites and anorthite veinlets.

At the same time, the origin of the rutile-bearing hornblende schists described can be associated only with the postmagmatic activity of basaltic magma, in particular with considerable migration of sodium. This phenomenon is well explained from the theoretical standpoint

Fig. 8. Diagram of changes in the composition of effusives under contact influence of gabbroid magma. 1 and 2 — augite-pyroxenites, 3 and 3a — gabbro, 4 and 5 — effusives.

of D. S. Korzhinskii [8], who considers that in zones of intense metamorphism changes are possible not only in the ratio of water and carbon dioxide, but also in an increased mobility of the alkalis.

In the words of D. S. Korzhinskii, here normal metamorphism passes into metasomatic metamorphism [8].

In our example, it is precisely “metasomatic,” and not regional, metamorphism that accounts, in certain favorable areas, for the appearance of blue-green hornblende at the expense of green amphibole and for the concentration of practically important accumulations of rutile. Localization of titanium in the zone under consideration does not occur during regional metamorphism. On the contrary, a general depletion of the effusives in titanium is observed. Following N. G. Vertushkov [1], one may assume only a partial and very slight increase in the titanium content in amphibolites due to the release of isomorphic titanium impurities from silicates and oxides and their subsequent binding into minerals of simple chemical composition. A certain share of titanium may become concentrated in newly formed amphibole, which may be promoted [18] by an excess of magnesium and a deficiency of aluminum.

In conclusion, let us emphasize the following principal points:

1. In the Shinda-Derba zone, the most highly metamorphosed rocks belong to the amphibolite facies and are characterized by the equilibrium association hornblende–plagioclase.

2. Rocks of different degrees of metamorphism are distributed quite regularly. An increase in the depth of metamorphism is observed with approach to tectonic dislocations.

3. The rocks corresponding to the greenschist facies are undoubtedly connected with regional metamorphism (in its classical sense), which took place under the conditions of the Early Proterozoic geosyncline of the Eastern Sayan.

As for the amphibolites, the character of their alteration within the tectonic zone cannot be assigned either to regional metamorphism, which usually embraces large areas, or to contact metamorphism, manifested in direct contact with an intrusive rock.

Probably it corresponds to the metamorphism of zones of shearing and deep faults, recently described by V. A. Reshitko for the Kochkonar district of the Urals [12]. Many similar features are found in comparing the metamorphic rocks of the characterized belt and the Irtysh folded zone in the Rudny Altai, as well as the formations of the zones of deep faults of the Western Sayan and Tuva [11].

1. The amphibolites in the Shinda-Derba fault zone apparently arise as a result of so-called local dynamothermal metamorphism, their substrate being Proterozoic effusives.

According to N. A. Eliseev, metamorphism of this type is manifested in narrow linear zones where there are favorable conditions for intense unilateral pressure and an increase in temperature, and where “a close connection is often observed between the intensity of dynamothermal metamorphism and proximity to intrusive bodies” [4].

The factors listed above can be attributed almost completely to the zone studied. However, during intrusion the basaltoid magma did not exert any substantial contact influence on the regionally altered effusives, apart from manifestations of migmatization. At the same time, active epimagmatic migration from the magmatic source of alkalis, in particular sodium, creates in certain areas a favorable environment for recrystallization of the amphibolites and concentration of titanium in the form of rutile.

REFERENCES

1. G. N. Vertushkov. Ilmenite-magnetite ores from a dolomite bed in the Urals. Proceedings of the Sverdlovsk Mining Institute named after V. V. Vakhrushev, vol. XXVI, 1956.

2. O. M. Glazunov. The Lysansk gabbro-pyroxenite-serpentinite complex in the western part of the Eastern Sayan. Geologiya i geofizika, No. 3, 1961.

3. G. M. Drugova, M. L. Lur’e, S. V. Obruchev. The Precambrian of northeastern Tuva. Proceedings of the Laboratory of Precambrian Geology, vol. 5, 1955.

4. N. A. Eliseev. Metamorphism. Leningrad State University Press, 1959.

5. A. N. Zavaritskii. Some fundamental questions of the geology of the Urals. Izvestiya AN SSSR, geological series, No. 3, 1941.

6. D. S. Korzhinskii. Mobility and inertness of components during metasomatism. Izvestiya AN SSSR, geological series, No. 1, 1936.

7. D. S. Korzhinskii. An outline of metasomatic processes. In: Fundamental Problems in the Doctrine of Magmatogenic Ore Deposits. 2nd ed., USSR Academy of Sciences Press, 1955.

8. D. S. Korzhinskii. Problems of the physicochemical theory of petrological processes. Izvestiya AN SSSR, geological series, No. 1, 1962.

9. E. A. Kuznetsov. Tectonics of the Tagil district. In: Geology of the USSR, vol. 12, part 1, 1944.

10. F. Yu. Levinson-Lessing. Petrography, 1940.

1. G. V. Pinus, V. V. Kuznetsov, I. M. Volokhov. Hyperbasites of Tuva. Publishing House of the Academy of Sciences of the USSR, 1955.

2. V. A. Reshitko. Metamorphism of zones of shearing and deep-seated faults in the area of the Kochkonar gabbro-peridotite massif in the Urals. Soviet Geology, No. 12, 1959.

3. I. G. Sudovikov. Migmatites, their genesis and methods of study. Transactions of the Laboratory of Precambrian Geology, Publishing House of the Academy of Sciences of the USSR, issue 5, 1955.

4. F. J. Turner. Evolution of Metamorphic Rocks. Foreign Literature Publishing House, 1951.

5. B. E. Treger. Tables for the Optical Determination of Rock-Forming Minerals. 1958.

6. V. V. Khomentovsky. On the tectonics of the Eastern Sayan. Bulletin of the Academy of Sciences of the USSR, Geological Series, No. 7, 1957.

7. K. A. Shurkin. On the question of the classification of ultrabasic rocks in general and migmatites in particular. Transactions of the Laboratory of Precambrian Geology, Academy of Sciences of the USSR, 7, 1957.

8. V. V. Shcherbina. On the geochemical conditions for the separation of oxide compounds from silicates. In: Physicochemical Problems of the Formation of Rocks and Ores, vol. II, Publishing House of the Academy of Sciences of the USSR, 1963.