SCAPOLITES FROM THE EXOCONTACT ZONES OF TRAP BODIES ON THE WESTERN MARGIN OF THE SIBERIAN PLATFORM

B. V. OLEINIKOV

(Presented by Prof. A. M. Kuzmin)

The literature contains indications that scapolite in the contact zones of trap bodies plays an insignificant role [2, 3, 4, 7]. A detailed study of the geological features of the exocontact zones of trap intrusions of different ages allowed the author to establish, in a number of places, the widespread development of scapolite. Areas of intensive manifestation of the scapolitization process were observed at many points located within the territory bounded in the south by the basin of the Bakhta River and in the north by the Kulumba River.

In speaking of the development of scapolite in the contact aureoles of trap intrusives, it must be emphasized that the process of scapolitization is manifested intensively in connection with quite definite trap bodies of different ages. Thus, scapolite is noted in the exocontact zones of amphibolitized varieties of rocks of the trap formation, forming independent bodies in the basins of the lower courses of the Northern and Letnyaya rivers (tributaries of the N. Tunguska River). The most extensive zones of scapolitization were observed in connection with differentiated trap intrusions of the Kuzmovka phase [6]. Sometimes scapolite rocks were recorded in the outer zones of massifs of porphyritic gabbro-dolerites. In contrast to the above, the emplacement of the extremely widespread bodies of poikilitic dolerites of the Tunguska-Lena phase, as well as of a number of other trap intrusions distinguished within the western part of the Siberian Platform [6], was not accompanied by a process of scapolitization of the enclosing rocks. This applies equally to the emplacement of differentiated nickel-bearing trap intrusions of the Norilsk phase, in the contact zones of which scapolite is encountered very rarely.

The presence of three generations of scapolite has been established. Scapolite I is a constituent of pyroxene-scapolite, scapolite-epidote-pyroxene, amphibole-scapolite, and other contact-metasomatic rocks. Rocks composed almost entirely of scapolite I occur much more rarely. The thickness of the zones of scapolitization varies from the first tens of centimeters to the first tens of meters. In the latter cases the scapolite formations alternate with other metasomatic and metamorphosed rocks.

Scapolite of the second generation is observed much more rarely; together with epidote, amphibole, sphene, magnetite, calcite,

composes veins that fill joint cracks in trap bodies and in contact-metasomatic rocks. The appearance of scapolite together with later analcime in cavities of garnet skarns has also been established, where scapolite overgrows rhombododecahedral crystals of andradite. This is the final, third generation of scapolite. The two latter generations of the mineral described are of limited distribution.

Among scapolite-bearing rocks, pyroxene-scapolite varieties, with variable contents of the principal rock-forming minerals, are the most widespread. As a rule, however, scapolite in them predominates over pyroxene of diopsidic composition. Pyroxene-scapolite metasomatites have a gray, greenish-gray, or bluish-gray color and a banded or spotted texture. Microscopic study of pyroxene-scapolite rocks shows that the most characteristic structures for them are poikiloblastic and sieve-like. Small (0.01–0.05 mm) isometric and short-prismatic grains of diopside and, more rarely, sphene occur as inclusions in scapolite. These pyroxene crystals are somewhat earlier formations than the oikoblasts of scapolite. At the same time, individuals of monoclinic pyroxene are not infrequently observed that are larger (0.1–0.3 mm) and approximately equal in idiomorphism to the prismatic grains of scapolite. These crystals probably formed simultaneously with scapolite as a result of the collective crystallization of small pyroxene prisms. Pyroxene-scapolite rocks usually arise at the expense of aleurolites, polymict sandstones, and basic tuffs.

Amphibole-scapolite metasomatites are less widely developed. They have a greenish-gray color and a prismatic-granular composition. Scapolite is the dominant mineral in them. Amphibole is represented either by pale-green hornblende or by a blue-green, weakly alkaline variety. Scapolite-amphibole varieties arise through the transformation of clastic sedimentary rocks and, more rarely, tuffs.

Scapolitites have a white or gray color and a prismatic-granular composition. Elongated prismatic crystals of scapolite, reaching sizes of up to \(3.0 \times 0.5\) cm, are associated with small (0.2–0.3 mm) grains, which leads to the development of a prismatic-granular structure of heteroblastic type; more rarely, an equigranular granoblastic structure is observed. In limited amounts, pyroxene, sphene, apatite, and calcite have been established in the composition of the scapolitites; together they account for 5–15% of the rock volume.

Occasionally, the intrusive traps themselves undergo scapolitization. Thus, in the area of Mt. Tiptur (right bank of the Kureika River), scapolite-actinolite-calcite rocks were observed, forming replacement veins along joint cracks in the body of porphyritic gabbro-dolerite. In the middle course of the Deltyula River, metamorphosed rocks were noted consisting of pyroxene—30, scapolite—45, muscovite—5–7, phlogopite—10%, and small amounts of hornblende, sphene, calcite, and chlorite. They arose at the expense of porphyritic gabbro-dolerites, with inheritance of the porphyritic structure of the latter. Porphyritic segregations of intratelluric plagioclase of labrador-bytownite composition are replaced by scapolite, with the formation of scapolite pseudomorphs after feldspar. In turn, as a result of later metasomatism, scapolite is partly replaced by phlogopite and muscovite.

In all the formations listed above, scapolite occurs as prismatic crystals ranging in length from fractions of a millimeter to 15 cm. Not infrequently, prismatic individuals are arranged more or less pa-

parallel to one another and thereby emphasize the orientation of the bedding of the primary sedimentary rock. More rarely, radial-fibrous and sheaf-like aggregates of scapolite crystals are recorded. The color of most scapolites is white (colorless) or gray, sometimes with a greenish or bluish tint. The luster is vitreous, with a pearly sheen on cleavage planes. Hardness 5.5–6.0. Cleavage medium along the prism {100}.

The optical properties of the studied scapolites are given in Table 1. Comparison of the obtained values of the refractive indices of the scapolites (the determinations were made with an accuracy of ±0.001) with data published in the handbooks of A. N. Winchell and H. Winchell [1] and W. E. Tröger [9] shows that they have somewhat lowered values. This can be explained by features of the chemical composition of the minerals studied, which will be considered below. All the measured scapolites are uniaxial and optically negative.

Table 1

Optical properties of scapolites

¹ All scapolites belong to the I generation, with the exception of sample 110—II generation and sample 114—III generation.
² Determined from the diagram of A. N. Winchell and H. Winchell [1], on the basis of the magnitude of the birefringence.

An X-ray study of two scapolites was carried out by M. F. Sokolova using the Debye method in the X-ray laboratory of SNIIGGIMS. The obtained values of interplanar spacings and intensities (Table 2) show that both minerals are identical in X-ray characteristics. In both samples the presence of quartz was noted; therefore, in addition to those listed in the table, reflections of 2.45 kX, 2.29 kX, 2.13 kX, 1.813 kX, 1.668 kX, and 1.378 kX were also observed,

Table 2

Results of the X-ray measurement of scapolites. Recording conditions: Co—anticathode, Fe—filter (thickness 0.10 mm), D = 57.3 mm, d = 0.5 mm, V = 45 kV, I = 10 mA. Corrected from a separate exposure with quartz

which could belong both to scapolite and to quartz. Comparison with the X-ray metric constants of marialite and meionite [5] permits the assumption that the minerals studied are situated in the scapolite series closer to marialite than to meionite.

Thermal analysis (Fig. 1) was carried out both for monomineralic scapolite—curve I (sample 58)—and for scapolitite—curve II (sample 0-1). The first endothermic effect on the heating curve of scapolite and on the analogous thermogram of scapolitite is due to the release of molecular moisture. The nature of the remaining effects, without additional chemical or X-ray metric investigations, is as yet unclear. In general, the thermograms of scapolite and scapolitite are close to one another. Their difference consists in the presence on the heating curve

Fig. 1. Heating curves of scapolite (I) and scapolitite (II).

of scapolitite of an exothermic effect at \(t = 630^\circ\), which is probably caused by the presence of accessory minerals.

The results of chemical analysis of scapolite and of the scapolite rock are given in Table 3. In recalculating the chemical analyses of scapolites to the generally accepted constituent molecules, it was necessary to encounter certain ambiguities, conventional structural schemes, and assumptions requiring further study. Thus, in the composition of scapolite sample 58 there is a considerable deficiency of silica, as compared with its theoretical content calculated from the value of the mineral’s birefringence. At the same time, a certain excess of the \(R_2O_3\) group and of sodium is observed. Taking into account that the main mass of the scapolites studied are high-temperature minerals, and that the crystal structure of framework silicates entails the presence in it of large cavities, one may, in order to explain the deficiency of silica, assume the existence of heterovalent isomorphism according to the scheme \(Na^{+1} + Al^{+3} \rightarrow Si^{+4}\). Such isomorphism, with the introduction of additional cations, shown by V. S. Sobolev [9] for a series of minerals, is probably possible in the present case as well. In addition, an excess of calcium has been established in the scapolite. Both illustrated analyses are characterized by the absence, among the volatile components entering into the additional molecules, of \(F\), \(CO_2\), and practically \(SO_3\). The amount of chlorine in them is insufficient for the formation of an appendage molecule in the marialite componen-

Table 3

Chemical composition of scapolites. Analysts A. V. Krivets and A. S. Lozovaya

At the same time, the presence of a large amount of water is recorded. This makes it possible to suppose that the group of additional anions includes OH\(^{-1}\), i.e., the scapolites we studied are, to some extent, hydroxyl-bearing minerals, which apparently was also reflected in their optical properties. The possibility is not excluded that zeolite-type moisture is present in the studied scapolites, as indicated by the prolonged character of the first endothermic effect (Fig. 1).

In interpreting the results of the chemical analysis of scapolitite (sample 0-1), it should be noted that the analyzed rock contains about 1% apatite, 3% sphene, 6% talc, 7% kaolinite, and approximately 80% scapolite of composition Ma\(_{76}\)Me\(_{24}\). In addition, the calculation reveals an excess of silica, due to the presence of quartz relics.

The ratios of the marialite and meionite components of scapolites of different generations are illustrated in Table 1.

Spectral analysis was used to study 15 scapolites from various localities on the western margin of the Siberian Platform. As a result, the presence in all scapolites, in the form of strong lines (1% and more), of Na, Ca, Al, and Si was established. A number of minerals contained about 1% K, Sr, and Mg. Tenths of a percent were established for Mg, Fe, Sr, K, and Ce. In hundredths of a percent, Mg, Ti, Sr, Zr, Mn, V, Ba, and Y are present. Contents in thousandths of a percent were noted for Cu, Zn, Ni, Co, Ga, B, Sr, Ba, Cr, Ti, Zr, V, Mn, Y, and Sc. Traces of Be, Li, and Pb were recorded.

Among the secondary minerals that arise during the destruction of scapolite, kaolinite is the most widespread. It develops in the form of separate brownish spots, often confined to cleavage cracks.

During the formation of later mineral associations, replacement of scapolite I by garnet, epidote, phlogopite, talc, and a number of other minerals is established. Scapolite II is replaced by epidote, and scapolite III by analcime.

From the brief review of paragenetic associations given above, it is evident that scapolite formation took place over a considerable period of the contact-metasomatic process. However, it reached its maximum in the initial period of this process, and only part of the scapolite formed in the post-skarn stage.

Thus, on the basis of the factual material presented above, the following main conclusions may be drawn.

1. The processes of scapolitization of the country rocks accompany the emplacement of quite definite trap intrusions. They are very widely manifested in the exocontact zones of some trap bodies and are completely absent in the contact aureole of other coeval intrusives. This is apparently connected with changes in the quantitative and qualitative content of volatile components in the magma of different phases [6].

2. Most of the scapolites studied have a sharply expressed sodium composition.

3. Scapolite formation took place over a considerable temperature interval. Later generations of the mineral contain more of the meionite component than the rocks participating in the composition of scapolite-bearing metasomatites.

REFERENCES

1. A. N. Winchell, H. Winchell. Optical Mineralogy. Izd. in. lit., 1953.

2. A. P. Lebedev. Trap Formation of the Central Part of the Tunguska Basin. Tr. IGN AN SSSR, issue 161, 1955.

1. M. L. Lur’e, V. L. Masaitis, L. A. Polunina. Intrusive traps of the western margin of the Siberian Platform. In: Petrography of Eastern Siberia, vol. I, Publishing House of the Academy of Sciences of the USSR, 1962.

2. V. L. Masaitis. Petrology of the Alamdzha trap intrusion. Proceedings of VSEGEI, vol. 22, new ser., 1958.

3. V. I. Mikheev. A Röntgenometric Determinative Guide to Minerals. Gosgeoltekhizdat, 1957.

4. B. V. Oleinikov. Trap intrusions and questions of metamorphism of the enclosing rocks. In: Geological Structure and Prospects for Oil-and-Gas Potential of the Northwestern Siberian Platform. Proceedings of SNIIGGiMS, issue 28, Gostoptekhizdat, 1963.

5. V. S. Sobolev. Petrology of the traps of the Siberian Platform. Proceedings of the All-Union Arctic Institute, vol. 43, 1936.

6. V. S. Sobolev. Introduction to the Mineralogy of Silicates. Publishing House of Lviv State University, 1949.

7. V. E. Treger. Tables for the Optical Determination of Rock-Forming Minerals. Gosgeoltekhizdat, 1958.