PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION
AND ORDER OF THE RED BANNER OF LABOR
POLYTECHNIC INSTITUTE NAMED AFTER S. M. KIROV

Vol. 236　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1976

ON THE PETROGRAPHY OF THE CHEREMUSHKINSKY MASSIF OF ALKALINE ROCKS (KUZNETSK ALATAU)

A. A. MITYAKIN

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

The massif is located along the stream of the same name, a right tributary of the Kurgusul River, on the eastern slope of the Kuznetsk Alatau, in its northeastern part. It has the form of a stock elongated in a northeastern direction. The area of the massif exposed at the surface is 1.25 km². In the northeast and southwest the body of the stock is cut by disjunctive faults.

The alkaline rocks of the massif, in the form of nepheline syenites, were first discovered by workers of the Goryachegorsk party during geological mapping in 1959–1961. The massif was considered homogeneous in composition, composed of nepheline syenites. It occurs among Proterozoic–Cambrian rocks, which here form an anticline of northwestern strike, and is confined to its eastern limb.

As a result of field observations carried out by the author in 1966, a rather complex material composition of the massif was established, with the successive formation of:

1 phase — dolerites,
2 phase — melteigites,
3 phase — nepheline syenites.

In addition, metasomatic syenites are widely developed, having arisen as a result of the activity of deep-seated alkaline solutions.

Rocks of the first phase

The first phase comprises dolerites. They apparently completely made up the stock of the form now observed, but as a result of later magma injections they were almost completely replaced, being preserved only in the peripheral parts of the massif and as a xenolith in its central part. In the peripheral part of the body the dolerites are dark-gray, fine-grained, with an average crystal size of 0.6 mm. The texture of the rock is diabase, with a mineral composition of titanaugite — 45%, plagioclase — 50%, accessory minerals — 5%.

Titanaugite occurs as small prismatic crystals with irregular outlines; it is pleochroic from light pink in \(Nq\) to colorless in \(Np\). The mineral is almost completely replaced by fine-scaly biotite of yellow-brown color.

Plagioclase is represented by elongated tabular grains with rectilinear outlines and polysynthetic twinning. In composition the mineral corresponds to bytownite No. 80. Among secondary minerals, sericite develops after plagioclase.

Accessory minerals are represented by magnetite in the form of small isometric grains and, in smaller amounts, by apatite.

In the direction from the periphery toward the center of the stock, the rock gradually becomes coarser-grained, and in the xenolith of the central part of the body the average crystal size is 1–1.2 mm. Here the rock externally has a spotted character owing to the uneven distribution of the dark-colored mineral and feldspar. The grain-size structure is likewise heterogeneous—there are areas of pegmatitic character with pyroxene crystals up to 2 cm in size. The mineralogical composition of the dolerite is: pyroxene—50%, plagioclase—47%, accessories—3%.

Pyroxene, as in the peripheral part of the stock, is titanaugite in the form of elongate-prismatic crystals with irregular boundaries; the mineral has distinct pleochroism from pink along \(N_q\) to colorless with a faint pinkish tint along \(N_p\), with optical constants: \(C N_q = +51^\circ\), \(2V = +56^\circ\), \(N_g = 1.723 \pm 0.002\), \(N_p = 1.697 \pm 0.002\), \(N_g - N_p = 0.026\). Judging from the data of chemical analyses according to U. A. Diriu (1965), the described mineral contains about 20% calcium oxide. The pyroxene is partly replaced by chlorite.

Plagioclase, in the form of tabular crystals idiomorphic with respect to pyroxene, corresponds in composition to bytownite No. 87. The mineral is substantially replaced by sericite.

In the rock there is titanium-rich basaltic hornblende, the appearance of which is connected with later metasomatic processes. The amount of hornblende varies from zero to complete replacement of pyroxene. The mineral is represented by elongate-prismatic crystals with distinct pleochroism from red-brown along \(N_g\) to light brown along \(N_p\). Optical constants: \(C N_g = +13^\circ\), \(2V = -80^\circ\), \(N_g = 1.698 \pm 0.002\), \(N_p = 1.677 \pm 0.002\), \(N_g - N_p = 0.021\). Judging from the optical constants, the hornblende belongs to kaersutite. In samples where the most complete replacement of pyroxene by hornblende has occurred, the plagioclase is completely decomposed and replaced by sericite, partly by chlorite.

As a result of later low-temperature processes, the kaersutite is replaced to a considerable degree by light-yellow epidote and chlorite.

Among the accessory minerals, apatite is observed in the rock in the form of well-formed prisms with a hexagonal cross section, up to 1.5 mm in size along the long axis. Magnetite is present in smaller amounts.

2. Rocks of the second phase

In the second phase, melteigites are formed. They have been preserved in the central part of the stock as a xenolith together with dolerites.

Macroscopically, the rock is dark gray and of porphyritic structure; in the segregations, nepheline crystals 3–6 mm in size occur in an amount of 10% of the total rock volume, and pyroxene 4–5 mm in size in an amount of 8–10%.

Pyroxene is represented by prismatic crystals with a cross section in the form of an elongated octagon. Most of the crystals of the segregations are of zonal structure—their central part consists of green diopside with weak pleochroism, and the peripheral part of titanaugite. Diopside has optical constants \(C N_g = +45^\circ\), \(2V = +60^\circ\), \(N_g = 1.735 \pm 0.002\), \(N_p = 1.798 \pm 0.002\), \(N_g - N_p = 0.027\), which corresponds to a mineral containing 68% CaFe atoms (Treger, 1958).

Nepheline in phenocrysts, in the form of well-formed crystals of prismatic shape, is completely replaced by cancrinite. The principal mas-

of a rock of nepheline structure is composed of 55% nepheline and 40% pyroxene. The pyroxene of the groundmass is titanaugite, in the form of elongated-prismatic crystals 0.3 mm in size, with a rim of aegirine-augite along the periphery. The latter sometimes completely replaces small crystals of titanaugite.

The nepheline of the groundmass, in the form of well-formed crystals with square or hexagonal cross sections, as in the phenocrysts, is completely replaced by cancrinite. Among the accessory minerals there is up to 3% magnetite, in the form of small isometric grains with irregular outlines, and apatite in smaller amounts.

As shown by the relationships among the minerals, crystallization of the melt began with the separation of diopside pyroxene. This is evidenced by numerous small corroded grains of pyroxene in nepheline crystals. After the crystallization of pyroxene, the nepheline of the phenocrysts separated.

The residual melt crystallized under sharply changed external conditions, with the formation of a fine-grained groundmass; in this groundmass nepheline was the first to separate, i.e., the sequence of separation of the minerals of the groundmass was a continuation of the sequence of separation of the phenocryst minerals. In the process of crystallization the pyroxene underwent a change in its composition from diopside to titanaugite and aegirine-augite.

3. Rocks of the third phase

In the third phase the formation of nepheline syenites takes place. In appearance the rocks are pinkish-gray, massive, medium-grained. The average grain size is 2–2.5 mm. Under the microscope the rock reveals a hypidiomorphic-granular structure with the following mineralogical composition: nepheline—40%, K-feldspar—20%, plagioclase—8%, cancrinite—5%, pyroxene—8%, hornblende—14%, olivine—3%, accessories—2%.

Nepheline is represented by well-formed crystals, idiomorphic with respect to hornblende and K-feldspar, in cross section in the form of a square or hexagon; with the exception of individual grains, it is replaced by cancrinite. The optical constants of nepheline: \(N_o = 1.535 \pm 0.02\), \(N_e = 1.530 \pm 0.002\), indicate a content of 10% of the \(KAlSiO_4\) molecule.

Pyroxene-augite, pale pink in color, in the form of prismatic crystals with irregular boundaries, has optical constants: \(CNg = +48^\circ\), \(2V = +55^\circ\), \(Ng = 1.728 \pm 0.002\), \(Np = 1.702 \pm 0.002\), \(Ng - Np = 0.026\). Judging from the chemical analyses of augites according to U. A. Deer, the mineral described belongs to ferroaugite, with a content of about 15–16% iron oxides. The augitic composition of the pyroxene has been preserved only in the central part of individual grains. From the center of such grains toward the periphery the mineral acquires a green color and passes into aegirine-augite and aegirine, forming an outer rim around the pyroxene grains.

Hornblende, in the form of grains with irregular outlines, is sharply xenomorphic with respect to nepheline, with distinct pleochroism from dark greenish-brown along \(Np\) to light yellow-brown along \(Ng\), with optical constants \(CNp = -3\text{–}5^\circ\), \(2V = +83^\circ\), \(Ng = 1.702 \pm 0.002\), \(Np = 1.695 \pm 0.002\), \(Ng - Np = 0.007\), which corresponds to arfvedsonite with a content of about 7% magnesium atoms and 93% atoms of \(Fe' + Fe''' + Mn\) (U. A. Deer, 1965). The mineral has reactional relationships with pyroxene, small relict corroded grains of which are observed in the central parts of arfvedsonite crystals.

Olivine has been preserved as small relict grains of isometric form in crystals of pyroxene and hornblende. The mineral is colorless.

with an angle of the optical axes equal to \(-59^\circ\), which corresponds to a content of 14% of the forsterite molecule. The olivine has undergone opacitization—along the periphery of the crystals and along cleavage cracks it has been replaced by an ore component.

Plagioclase is represented by elongate tabular crystals, xenomorphic with respect to nepheline and hornblende, with polysynthetic twinning. In composition the mineral corresponds to andesine No. 36. Among secondary minerals replacing plagioclase, sericite is developed in small amounts in the form of fine flakes.

Potassium feldspar–microcline is most often without a visible lattice, in the form of tabular crystals with sinuous lateral boundaries; it is brownish in color owing to secondary products and has the optical constants: \(2V=-77^\circ\), \(Ng=1.530+0.002\), \(Np=1.523+0.002\), \(Ng-Np=0.007\), which corresponds to a mineral with a content of 45% of the albite molecule.

Cancrinite occurs as grains up to 2.5 mm in size, with irregular outlines, colorless. The mineral is usually associated with nepheline, as if cementing the crystals of the latter. The optical constants of cancrinite, \(No=1.515+0.002\), \(Ne=1.495+0.002\), correspond to a mineral with a content of 15% of the vishnevite molecule (Treger, 1958).

The interrelations of the minerals show that crystallization of the magmatic melt began with olivine. When the temperature of the onset of pyroxene crystallization was reached, the olivine underwent incongruent melting. Its small relict grains can be observed in pyroxene crystals.

After crystallization of pyroxene, the separation of nepheline began, represented by well-formed crystals idiomorphic with respect to plagioclase and hornblende. By the end of nepheline crystallization the residual melt still remained saturated with alkalies, which caused the crystallization of alkali hornblende. Its crystals are clearly xenomorphic with respect to nepheline. At the same time, the previously separated augite was transformed into aegirine-augite and aegirine. Replacement of pyroxene by hornblende also occurs, as is indicated by relicts of pyroxene grains preserved in the central parts of hornblende crystals. The melt remaining after crystallization of hornblende had an alumino-alkaline composition. Its crystallization proceeded with the separation of plagioclase, potassium feldspar, and cancrinite.

4. Metasomatic syenites

A significant part of the massif consists of metasomatic syenites, formed at the expense of nepheline syenites and other rocks as a result of potassium-feldspar metasomatism manifested here. Under the influence of alkaline deep-seated solutions in the nepheline syenites and other rocks, potassium feldspar develops in the form of large metacrysts up to 3–4 cm in size. At the same time, replacement of the primary rock-forming minerals—nepheline and dark-colored minerals—takes place, and the rock acquires a more leucocratic appearance.

Externally, the metasomatic syenites are flesh-red or pinkish-gray, with a coarse-grained or porphyroblastic texture. Under the microscope an intensely altered primary composition of the initial rocks is observed. Simultaneously with the development of large metacrysts of potassium feldspar, decomposition and replacement of the primary minerals occur: of the dark-colored minerals, at the initial degree of metasomatism, the original form of the grains is preserved, replaced by chlorite and iron hydroxides. The nepheline crystals are at the same time replaced by cancrinite. With a more intense manifestation of the metasomatic process, only small accumulations of secondary minerals—chlorite and iron hydroxides—remain from the dark-colored minerals. Metacrysts of potassium feldspar replace

dark-colored minerals, nepheline, as well as primary crystals of potash feldspar. Thus nepheline syenites and other rocks gradually change their composition in the direction of formation of alkaline monomineralic rocks composed of potash feldspar.

The potash feldspar of metasomatic syenites is an untwinned microcline of brownish color due to secondary products, occurring as elongated-tabular grains, often well formed. The optical constants of the mineral are \(2V = 76^\circ\), \(Ng = 1.531 + 0.002\), \(Np = 1.526 + 0.002\), which corresponds to a content of about 49% of the albite molecule.

The potash feldspar is subjected to intense albitization. Albite develops in separate areas with the formation of thin polysynthetic twins, up to an almost complete replacement of individual grains of potash feldspar. Chemical analyses of metasomatic syenites composed almost entirely of potash feldspar give a sodium oxide content of 7.76% and a potassium oxide content of 2.56%.