Albite-Bearing Crystalline Schists of the Borus Range

M. I. Yudin

(Presented by Professor A. M. Kuzmin)

On the southeastern side, adjoining the hyperbasite massif of the Borus Range, there are crystalline schists whose characteristic feature is the presence of a fairly large number of albite porphyroblasts. Questions connected with the origin of these schists have, in our opinion, great theoretical and practical significance, since

Fig. 1. Diagram of the geological structure of the Borus Range: 1 — dunites, 2 — albitites, 3 — talc-carbonate rocks, 4 — granites of the Dzhoy type of intrusion, 5 — serpentinites and serpentinized peridotites, 6 — phyllitized clayey and carbonaceous-clayey schists with interbeds of quartzites and effusives (Chinginskaya suite), 7 — crystalline schists of the Dzhebashskaya suite, 8 — albite-bearing crystalline schists (albite-epidote-chlorite gneisses and albite amphibolites).

as the correct determination of the nature of their origin may in many respects help geologist-stratigraphers engaged in subdividing the ancient sequences of the Western Sayan.

The schists of interest to us occur along the southeastern contact of a hyperbasite massif elongated in a northeastern direction (Fig. 1). The strike of the schistosity in the schists coincides with this same direction; the dip is no less than \(70^\circ\), sometimes northwestward, sometimes southeastward. The width of the belt of crystalline schists is not everywhere the same. It is greatest in the headwaters of the M. Abdyr River (1 km). From there, in northeastern and southwestern directions, the albite-bearing schists gradually pass into green chlorite-sericite and phyllitized clayey schists.

In places, concordant lenses and thin veinlets of quartz occur in the schists studied. More rarely, crosscutting quartz veins are observed, from a few millimeters to 5 centimeters thick.

The two mineral associations revealed by microscopic study (albite–muscovite–chlorite–zoisite–quartz and albite–hornblende–clinozoisite) make it possible, respectively, to distinguish two groups of rocks: albite–muscovite–chlorite schists and albitic amphibolites, containing epidote and chlorite in varying amounts.

Below is a petrographic description of the rocks mentioned and some considerations on their origin.

Albite–muscovite–chlorite schists

According to their structural-textural features and the quantitative content of albite in them, they may be subdivided into muscovite–chlorite schists, in which albite is either not identifiable under the microscope or is present as an admixture in amounts of less than 10%, and albitic gneisses with an albite content from 10 to 30 percent and more. Between the varieties mentioned, as is easy to see, there are gradual transitions, and their division into the indicated groups is to a certain degree arbitrary.

Albitic gneisses are macroscopically greenish-gray, usually coarsely schistose rocks with a clearly expressed porphyroblastic structure. On the planes of schistosity, chlorite and muscovite are usually readily distinguishable. Albite porphyroblasts from 1 to 6 mm across are clearly visible in a fracture transverse to the schistosity. Here, against the gray-green background of the groundmass, they appear as pinkish or pinkish-brown isometric-rounded grains, giving the rock a mottled coloration. Under the microscope the principal minerals present are albite (10–50%), chlorite (10–25%), muscovite (7–10%), quartz (5–20%), zoisite (or clinozoisite) (5–15%). As an admixture, sometimes in considerable quantities, carbonate occurs; more rarely biotite and actinolite are found. Accessory minerals are sphene, apatite, tourmaline, rutile, and ore grains; garnet is rare.

Most of the albite grains (\(\mathrm{No.}\ 0\text{—}5; 2V = +76^\circ\)) are represented by fresh single crystals; twins are rare. It is twinned more often according to the Carlsbad law, less often according to the albite law. Optical constants based on the curves of Tröger [9] make it possible to assign it to the low-temperature type. A characteristic feature of the albite porphyroblasts is the presence in them of numerous inclusions, among which tourmaline, needles of rutile, zoisite, and sometimes small grains of sphene, carbonate, and quartz are often encountered. In almost all thin sections the porphyroblasts show signs of cataclasis in the form of crushed grains, bending of twin lamellae, and undulatory extinction. Crushing of gra-

albite grains, as a rule, is accompanied by recrystallization with the formation of a mosaic aggregate of second-generation albite grains. As a result, the porphyroblasts often have the form of relicts. Separate parts of the crushed grains are divided by thin streams of muscovite or of muscovite and quartz. In examining such thin sections it is not difficult to be convinced that most of the muscovite and quartz arose after the formation of albite and chlorite. The indicated interrelations of the minerals also testify that the albite of the porphyroblasts appeared at the very earliest stages of folding, and that under conditions of strong stress its crystal lattice proved to be mechanically unstable.

The minerals of the groundmass—muscovite, chlorite, zoisite, and flattened lenticular quartz grains—are arranged with their elongation parallel to one another, forming a typical crystallization-schistose texture. In varieties with large albite porphyroblasts, a compact shell is observed around the latter, consisting of chlorite or muscovite flakes flowing around them; quartz often accompanies muscovite and in places forms monomineralic small lenses and thin bands stretched along the schistosity.

Actinolite, as an admixture, was found in two thin sections (50-2 and 332-5a). Its thin elongated-prismatic crystals, no more than 0.7 mm long, are usually present as idioblasts in albite and chlorite. The mineral is weakly colored and shows weak pleochroism from colorless to pale bluish green. The maximum extinction angles give values of 15–20°.

It is characteristic to note that the actinolite-bearing schists are very poor in muscovite, whose content is reduced to isolated flakes sparsely scattered through the rock.

The occurrence of certain accessory minerals is, in our view, of considerable interest. Thus, tourmaline, in the form of isolated well-formed crystals measuring 0.3–0.4 mm along their elongation, was encountered in all the thin sections studied. It should be emphasized here that these crystals occur only as inclusions in albite, and not a single grain of this mineral was found in the quartz-chlorite-muscovite aggregate of the groundmass. The light-blue color of the tourmaline, passing into brown tones, indicates that it belongs to schorl, i.e., permits it to be regarded as a sodium-bearing aluminum borosilicate. If there are every grounds for connecting the appearance of albite with the introduction of sodium, then the formation of tourmaline can hardly be explained by the simultaneous introduction of boron together with sodium. Such an assumption is contradicted above all by the uniform distribution of tourmaline in the rocks and by its extremely low content. Taking these features of tourmaline behavior into account, there are grounds for assuming the presence of small amounts of boron in the original rocks. Goldschmidt and S. Peters [8] consider that marine clayey sediments may contain about 0.1% \(B_2O_3\), which, in their opinion, is sufficient for the appearance of tourmaline as an accessory mineral in metamorphic rocks.

Apatite occurs frequently. It has the form of short-columnar crystals, partly or completely, densely, almost to complete opacity, filled with inclusions, owing to which it has a yellow-brown color.

Garnet was encountered in two or three thin sections: its small grains (0.06–0.08 mm) are for the most part arranged in the form of chains along the boundary of quartz-muscovite and chlorite bands; more rarely they occur in albite porphyroblasts. In all thin sections where it could be seen from the periphery of the grains and along cracks, replacement by chlorite is observed.

Indications of analogous transformations of garnet in crystalline schists are encountered fairly often [3; 7]. A. A. Kadenskii explains its replacement by chlorite by the introduction of alkalis, of which potassium, in his opinion, combining with part of the alumina of the garnet, forms muscovite.

Muscovite–chlorite schists

The rocks of this group differ from the closely related gneisses only in textural features and in the absence or insignificant content of albite porphyroblasts. Macroscopically these are finely schistose, often crumpled into small folds, gray-green rocks. On a fracture transverse to the schistosity, a fine banding is usually clearly expressed, caused by the alternation of greenish-gray chlorite bands and yellowish-white discontinuous bands composed of quartz and muscovite.

In thin sections, as in the gneisses, where appreciable amounts of albite are present the structure appears porphyroblastic. Albite porphyroblasts not more than 1 mm in size are almost always crushed; as inclusions they contain needles of rutile and zoisite, elongated in a number of cases parallel to the schistosity.

The main mass consists chiefly of grains of quartz and zoisite and flakes of chlorite and muscovite, which form a lenticular-banded texture. Even at a cursory glance under the microscope one obtains the impression that the rock consists now of more or less parallel, now of interlacing bands and veinlets composed either essentially of chlorite, or of muscovite and quartz. In places quartz forms lenticular aggregates of small grains with a typical granoblastic structure.

Of the accessory minerals, apatite occurs, and more rarely sphene. The mineral relationships are exactly the same as in the albite gneisses.

Albite amphibolites and albite–epidote–hornblende rocks

The rocks of this group occur in close association with albite–muscovite–chlorite schists. Unlike the latter, in the area studied the rocks described are extremely limited in distribution. With respect to mineralogical composition, characteristic of them is the complete absence of muscovite and the constant presence of hornblende, the content of which in a number of specimens reaches 50–60%. In no thin section was muscovite observed together with hornblende. The paragenetic relation noted here is reflected in the composition–paragenesis diagram (Fig. 2). Hornblende in the rocks of this group occurs in association with epidote and chlorite. Most of the albite in all cases is an additional phase.

Macroscopically the rocks described usually do not have a clearly expressed schistosity, and therefore appear massive, sometimes even hornfels-like. In rare cases a coarse schistosity and barely perceptible banding are observed, caused by the presence of narrow, 1–3 mm alternating bands: gray-green ones, composed essentially of hornblende with some chlorite and epidote, and pink ones, in whose composition almost only albite participates. In those varieties in which banding is not manifested, the albite in the general mass of the rock is distributed unevenly, usually in the form of streaks and nest-like accumulations (Fig. 3). Even within a single specimen, in some areas it makes up about 50% of the rock volume, while in others its content decreases to isolated grains, and the rock proves to consist of amphibole alone.

Their texture is more or less distinctly granular. In hand specimens, usually only the greenish-black patches of amphibole and the white or pinkish-white grains of feldspar can be distinguished with the naked eye.

Under the microscope it can be seen that the rock under consideration consists of albite, hornblende (15–60%), epidote (15–30%), and chlorite (10–15%). As impurities, carbonate and biotite are sometimes encountered; in two thin sections isolated grains resembling diopside were found. Of the accessory minerals there is usually much sphene and a few grains of ore mineral.

Fig. 2. Paragenetic diagram for rocks formed under conditions transitional from the greenschist facies to the albite-epidote-amphibolite facies, from pelitic rocks with excess SiO₂ and deficiency of K and Mg.

The texture is usually clearly porphyroblastic, with a nematolepidogranoblastic groundmass. The porphyroblasts are represented by fresh albite, very similar to quartz, forming isometric grains, often with uneven lace-like outlines. They usually do not

Fig. 3. Photograph of a specimen of albite amphibolite (natural size): dark—hornblende aggregate; white and gray—albite, forming accumulations of grains in the form of spots and vein-like aggregates. Isolated grains of it are rarely scattered among the hornblende aggregate.

twinned. In an entire thin section one sometimes finds with difficulty one or two grains with simple twins and approximately the same number of grains with one or two thin, tapering twin lamellae. Study of the twins gave the following results.

Thus, the optical properties of the plagioclase, irrespective of whether we find it in association with hornblende or with muscovite, convince us that its composition is essentially one and the same and, according to Tröger’s diagram [9], belongs to the low-temperature variety of albite.

Fig. 4. Photomicrograph of albitic amphibolite. A — parts of an albite grain stretched along the schistosity, crossed nicols, magnification 30.

Signs of cataclasis of the albite porphyroblasts are not manifested in all thin sections. However, in a number of cases a strong fragmentation of albite grains with numerous microdisplacements is observed. Some grains, as a result of strong lateral pressure, have been torn into parts and stretched along the schistosity (Fig. 4). In places where such grains were ruptured, thin needles of hornblende appeared at the expense of albite. The next feature of albite is that almost all grains of this mineral are filled with poikiloblastic inclusions, among which there are many needles of hornblende, grains of epidote, zoisite, apatite, and sphene. Let us note in passing that tourmaline, constantly present in associations with muscovite, was not found in any thin section in rocks with hornblende. This convinces us that boron was not introduced during the process of metamorphism. The appearance of tourmaline in some rocks and its absence in others indicates that boron was contained in the original rocks that, during metamorphism, gave rise to albite-muscovi-

...gneisses, and is absent in rocks that were transformed, as a result of metamorphism, into albite amphibolites.

Hornblende is represented by a greenish-blue, sometimes bluish variety, pleochroic according to Ng—bluish-green or bluish-blue, according to Nm—pale bluish-green, according to Np—pale yellow, almost colorless or pale greenish-gray. The absorption scheme is thus \(Ng>Nm>Np\). The extinction angles with the Ng axis range from 11 to 14° in varieties with a bluish tint, and from 14 to 20° in bluish-green ones. \(2V\) varies from \((-)\ 50\) to \((-)\ 68^\circ\). According to Winchell’s diagram [1], the optical properties given here make it possible to assign it to the group of common hornblendes with a composition varying from the pure pargasitic variety—\(\mathrm{NaCa_2(Mg,Fe)_5[AlSi_7O_{22}](OH)_2}\), in which Mg and Fe are present in approximately equal amounts—to pargasite, in which, with a sharp predominance of iron over magnesium, about 30% consists essentially of the ferro-tremolite molecule.

Thus, in composition the hornblende described belongs among ferruginous, low-alumina, magnesium-poor varieties. Similar hornblendes, judging from their optical properties, were described by Baklund and A. N. Zavaritskii [2] in the albite amphibolites of the Polar Urals. The latter of the authors mentioned supposed that the composition of the hornblendes contained a considerable amount of the “glaucophane molecule.”

In our rocks, hornblende always occurs as elongated, unbounded prisms or spindle-shaped grains, the length of which is 5–7 times greater than the thickness; in transverse sections one observes the cleavage typical of hornblendes in two directions. The faces of the prism in these sections are usually poorly expressed; more often these sections have irregular-oval outlines, the dimensions of which along the maximum diameter are 0.02–0.04 mm.

The elongated form of the hornblende grains and their more or less parallel arrangement in the rock actually determine the structure of the groundmass, which in areas enriched in this mineral may be defined as nematoblastic, and, in the presence of appreciable amounts of chlorite and epidote, as nematolepidogranoblastic.

Epidote is usually present in an amount not exceeding 10–15%. In some thin sections, however, its content reaches 40–50%; moreover, individual bands are almost entirely composed of epidote. It occurs for the most part as small elongated grains, whose dimensions range from 0.06 to 0.2 mm. Individual grains reach 1–2 mm. In some thin sections its large grains are crushed and, like albite, are stretched out along the schistosity.

Among the accessory minerals, sphene is very characteristic, and its content in a number of thin sections is fairly high. It occurs as grains of various sizes, having the oval or lenticular form typical of sphene. They are usually distributed more or less uniformly in the rock, and only in places can one see small grains forming small groups or accumulations elongated along the schistosity.

Calcite is not present in all specimens; however, its content may be rather substantial. It usually occurs as comparatively large and small xenoblasts forming lens-shaped aggregates and indistinctly expressed bands. Its appearance, judging from the vein-like and lens-shaped form of the aggregates, should be associated with a later stage of the hydrothermal process.

Origin of the Crystalline Schists

Establishing the genesis of crystalline schists, as of any other rocks of metamorphic origin, is essentially connected with resolving two fundamental questions: 1) what is the nature and composition of the original rocks? 2) as a result of what processes were these original rocks transformed into metamorphic ones?

The geological setting in which the crystalline schists occur is sufficiently clear from the appended geological map (Fig. 1). Its main features may be reduced to the following. The schists of interest to us compose a comparatively small field and are confined

1 and 2 — albite gneisses; 3 — phyllite-like schist; 4 — average of 12 analyses of quartz–albite–muscovite–chlorite schists of New Zealand [8].

to the contact zone of the hyperbasites. In the northeastern and southwestern directions (along the strike of the sequence) the degree of metamorphism decreases, albite porphyroblasts disappear, and the crystalline schists we studied gradually pass into chlorite–sericite schists and phyllite-like schists of the Dzhebash Formation. This circumstance alone makes it possible to suppose that, if not for all the rocks described, then in any case for the schists containing muscovite in their composition, the original material was argillaceous rocks. This, in our opinion, is also indicated by the chemical analyses given below, carried out in the Department of Silicate Technology of the Tomsk Polytechnic Institute.

In considering the chemical analyses, the following compositional features attract attention. In all three analyses a considerable excess of alumina over the sum of lime plus alkalis is observed. MgO predominates over CaO. These features, in the opinion of many researchers, are characteristic of sedimentary rocks of argillaceous composition. If one proceeds from the generally accepted view that the bulk chemical composition of a rock during regional metamorphism remains, in general, unchanged, then the chemical features noted here to some extent support the supposition that the original

the rocks were sedimentary in origin and highly aluminous in composition. And, finally, the following feature, which at first glance contradicts the assumption made, is expressed in the fact that the content of Na₂O predominates over K₂O, whereas S. M. Shvetsov [10], Rosenbusch [6], V. N. Lodochnikov [4], and others, on the basis of numerous chemical analyses, have shown that in clayey sediments K₂O predominates over Na₂O. It should also be added that the Na₂O content in clayey rocks is generally small. Thus, according to nine analyses of shaly clays given by Rosenbusch, the maximum Na₂O content is 1.45%, and of eighteen analyses of argillaceous schists by the same author, only in one does Na₂O amount to 2.29%. Even if this maximum value is taken as the amount of Na₂O contained in the original rocks that served as the material for the formation of the schists we studied, then for the albitic gneiss (No. 1) we must assume an influx of Na₂O during metamorphism from an external source in an amount of about one-third of that established by analysis.

The introduction of alkalies from crystallizing magmas into pelitic sediments during regional metamorphism has been demonstrated by many investigators. F. D. Turner notes that Goldschmidt, in one area of Norway, described a gradual transition from quartz–muscovite–chlorite schists, formed as a result of normal regional metamorphism of pelitic sediments, through porphyroblastic albitic schists with a much higher Na₂O content, to augen gneisses in which the K₂O content is also increased. It is well known that in green and chloritic schists albite is a typical mineral. It may, of course, also be a primary mineral, as V. N. Lodochnikov states. Nevertheless, in a whole series of cases the introduction of sodium during the formation of albite in analogous rocks is beyond doubt. The introduction of sodium in this case is associated by investigators both with acid and with basic and ultrabasic magmas. The spatial association of albite-bearing crystalline schists in our area with hyperbasites, in the absence of nearby acid intrusions, leaves us no doubt that the source of the sodium emanations is the hyperbasite intrusion itself. This supposition will evidently be more convincing if we mention in passing to the reader that here, in connection with the hyperbasites, we also know of small bodies of albitites and rocks rich in glaucophane. The genetic connection of rocks of the same composition with hyperbasites has been proved by V. N. Lodochnikov [4] in numerous examples known in abundance both in Russian and in foreign literature. E. S. Larsen asserts that “the usual type of contact metamorphism of peridotites is the development of schists rich in albite, sodic mica, and sodic amphibole” (quoted after [4]). All that has just been said, as well as the porphyroblastic character of the albite, indicates that the albite in the crystalline schists we studied, including the albitic amphibolites, is in the main metasomatic and genetically connected with the hyperbasites. The fragmentation of the albite grains and their elongation along the schistosity testify, moreover, to crystallization of the porphyroblastic albite in the very earliest stages of folding.

The question of what the original material was that, in the process of metamorphism, gave rise to the albitic amphibolites and rocks close to them in composition is resolved rather definitely on the basis of the analysis available to us: sample 321-1. In chemical composition, the albitic amphibolite (sample 321-1) proved to be very similar to the albite–epidote amphibolite from the Baksan River (Northern Caucasus), the analysis of which is given in the work of S. P. Solov’ev [7].

In this analysis the following features immediately stand out: 1. With a rather high alumina content (14.08%), the sum of alkalis is slightly more than 1%. 2. MgO = CaO. Such ratios of the indicated oxides are not observed in any igneous rock.

The comparatively low silica content, small amounts of alkalis, and high contents of Al₂O₃, MgO (10%), and CaO (10%) give grounds for assuming that the original material of the albite amphibolites was sedimentary, highly aluminous rocks, whose composition approaches that of a dolomitic marl.

321-1 — albite amphibolite, Borus Range; 1c — albite-epidote amphibolite (Baksan River, Northern Caucasus [7]).

The presence in the crystalline schists studied of two characteristic mineral associations, in one of which muscovite is always present, and in the other hornblende, allowed us to divide all the schists into two groups. If we were to follow this further, and strictly adhere to the classification of metamorphic-rock facies developed by F. J. Turner, we would have to assign them respectively to two facies (the greenschist facies and the albite-epidote-amphibolite facies), thereby ascribing to them rather different physical conditions under which they formed. In reality, however, it seems to us that we are dealing with isofacial rocks. In support of what has been said, we shall cite the following considerations.

1. Field observations showed that albite amphibolites are rare. They occur as interbeds in albite gneisses and muscovite-chlorite schists. It is hardly possible to imagine that, at the sites where albite amphibolites arose, the values of temperature and pressure during metamorphism were noticeably different from those in the enclosing rocks.

2. The appearance of magnesian-ferruginous garnet in muscovite-bearing albite gneisses, as is known, is not characteristic of the greenschist facies. “The greenschist facies,” writes F. J. Turner [8], “is defined here as a facies including all rocks formed at grades of metamorphism lower than that at which almandine appears in pelitic rocks.” The appearance in our muscovite-bearing schists of garnet, whose composition, judging from its replacement by chlorite, is magnesian-ferruginous, indicates the emergence during metamorphism of physical conditions approaching, in their values, those under which rocks of the albite-epidote-amphibolite facies are formed.

Thus, the association muscovite-chlorite with albite and zoisite is evidently stable over a considerable part of the albite-epidote-amphibolite facies distinguished by F. J. Turner and, in this connection, cannot be considered critical for the greenschist facies.

Conclusions

1. The schists studied are products of regional metamorphism of rocks of clayey and marly composition.

1. The chemical composition of the schists, as well as the interrelations of the minerals composing them, indicate the participation, in the process of metamorphism, of solutions containing alkalis and, possibly, silica.

2. The sodium metasomatism that manifested itself in the early stages of the folding process, in connection with the hyperbasite intrusion, led to the formation of porphyroblastic albite in the clayey sediments.

3. The two paragenetic associations established in the course of the study—albite–muscovite–chlorite (zoisite–quartz) and albite–blue-green hornblende–epidote (chlorite)—do not provide grounds for assigning them, respectively, to two facies according to F. D. Turner’s classification. It is most likely that the schists studied should be regarded as isofacial formations, and that the physical conditions under which the original material was transformed into crystalline schists were transitional from the greenschist facies to the albite–epidote–amphibolite facies according to F. D. Turner.

REFERENCES

1. Winchell A. N. Optical Mineralogy. IL Publishing House, 1949.

2. Zavaritskii A. N. The Rai-Iz Peridotite Massif in the Polar Urals. VGRO, 1932.

3. Kadenskii A. A. Magmatic Geology of the Front Range of the Northwestern Caucasus. Publishing House of the Academy of Sciences of the USSR, 1955.

4. Lodochnikov V. N. Serpentines of Ilchir and others. Transactions of TsNIGRI, vol. 38, 1936.

5. Lodochnikov V. N. A Short Petrography without a Microscope. Gosgeolizdat, 1956.

6. Rosenbusch G. Descriptive Petrography. 1934.

7. Solov’ev S. P. Origin of the crystalline schists of the upper reaches of the Baksa River (Northern Caucasus). Proceedings of the Béres Mineralogical Society, part LXVII, no. 2, 1932.

8. Turner F. D. Evolution of Metamorphic Rocks. IL Publishing House, 1951.

9. Tröger W. E. Tables for the Optical Determination of Rock-Forming Minerals. Scientific-Technical Publishing House of Literature on Geology and Protection of Mineral Resources, 1958.

10. Shvetsov M. S. Petrography of Sedimentary Rocks. State Publishing House of Geological Literature, 1958.