ON THE ZONALITY OF BERESHITES OF THE BATANAYUL MASSIF OF ALKALINE ROCKS

A. A. MITYAKIN

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

The Batanayul massif of alkaline rocks is located in the northern part of the Kuznetsk Alatau along the Batanayul spring, a left tributary of the middle course of the Uryup River. Administratively, the area of the massif is part of the Tisul District of Kemerovo Oblast. In its composition the massif is multiphase and is made up of a variegated assemblage of rocks varying in composition from labrador bereshites to alkaline potassium syenites containing free silica. Labrador bereshites are the principal component of the massif. They form a body in the shape of a sill with a dip azimuth of 340° and a dip angle of 35°. The area of the sill outcrop at the surface is about 3.5 km². Near-surface conditions of formation of the massif are expressed in the presence, in all rocks, including the labrador bereshites, of a distinct porphyritic texture. Rapid and uneven cooling under these conditions was also manifested in the formation, within the sill of bereshites, of a zonality of mineralogical composition, clearly traceable in the direction from the footwall to the central part of the body.

On the footwall side of the sill, at the contact with the effusives of the enclosing sequence, the bereshites underwent chilling. Here they have a clearly porphyritic texture. In the principal glassy groundmass, dark gray with a brownish tint, there are phenocrysts of nepheline 1–1.5 cm in size, making up about 35% of the volume of the rock. Numerous amygdules up to 7–8 mm in size, filled with carbonate and chlorite, are visible in the groundmass.

Under the microscope, fluidality is distinctly visible. The principal glassy groundmass, containing numerous streaks of carbonate, flows around porphyritic segregations of nepheline (Fig. 1). The streaks bend around the corners of nepheline grains, repeat the shape of their boundaries, and have a plane-parallel orientation. The presence of an unrecrystallized principal glassy groundmass bearing traces of fluidality indicates that the nepheline segregations were brought by the magma from deeper horizons. In the glassy groundmass, rare grains of pyroxene with grain sizes in tenths of a millimeter may be observed, and still more rarely plagioclase grains of the same size. In the direction from the footwall to the roof of the body the texture gradually begins to change, and at a distance of 25 m from the contact the rock assumes the following appearance:

The texture is porphyritic. Against a background of fine-grained dark-gray

from the groundmass are visible nepheline segregations, which by grain size are clearly divided into two groups. The first includes grains 1.5–1.8 cm in size, constituting no more than 4% of the rock volume. The second comprises grains 5–8 mm in size, constituting 50–51% of the total volume of the rock. In the segregations, besides nepheline, pyroxene is also present here in the form of prismatic grains 7–8 mm in size along the long axis, making up 15% of the total mass.

Fig. 1. Fluidal texture of the groundmass in bereshites from the sill sole (carbonate streams flow around porphyritic nepheline segregations). 1 — carbonate stream; 2 — glassy groundmass; 3 — porphyritic nepheline segregations.

The main mass of the rock is crystallized considerably more fully than at the sill sole. In the fine aggregate of chlorite, which constitutes the greater part of it, acicular grains of plagioclase and small grains of pyroxene are visible. The two groups of nepheline grains, distinguished by their size, are nothing other than two generations of nepheline. Grains of the first generation (1.5–1.8 cm in size), in their genesis, are similar to the grains described above from the bereshites of the lying wall, i.e., they were brought by the magma from deeper horizons. Their amount decreases from the sole toward the center of the body from 35 to 4%, in view of the fact that gravitational differentiation played a role: separating from the melt, they accumulated at the sole of the body. The crystals of nepheline of the first generation in the interval described have larger dimensions than at the sill sole (15–18 mm instead of 10–15 mm). This is explained by the fact that here, farther from the contact, under conditions in which a high temperature was preserved for a longer time, they had the possibility of further growth. Confirmation of this may be provided by the presence of small poikilitic inclusions of pyroxene in the peripheral parts of large nepheline grains. Evidently, overgrowth (capture) of pyroxene grains by nepheline took place here, since cases are observed of pyroxene grains not yet completely overgrown by nepheline. Grains of the second generation (5–8 mm in size) arose at the site of cooling of the melt, just as pyroxene arose at the same site. The latter, as indicated above, is practically absent at the sole of the body. In the direction from the sole toward the central part of the sill its amount gradually increases and reaches 20% (in the porphyritic segregations). The pyroxene, light green in color with weak pleochroism, according to its optical constants: \(CNg = 40^\circ\), \(2V = 54^\circ\), \(Ng = 1.723 \pm 0.003\), \(Np = 1.694 \pm 0.003\), \(Ng - Np = 0.029\), corresponds to diopside-salite with a content of about 7% \(FeO + Fe_2O_3\) (according to U. A. Deer, 1965). Nepheline, as at the sole of the body, is completely replaced by cancrinite with constants: \(No = 1.528\), \(Ne = 1.510\), \(No - Ne = 0.018\). Zeolite also develops in the nepheline in the form of thin branching veinlets.

Farther in the direction toward the central part of the body the rock again gradually acquires a different appearance. It has a porphyritic structure. In the segregations there is nepheline with grain size 5–8 mm in an amount of 55% of the total volume of the rock. Besides nepheline, the phenocrysts contain elongate-prismatic pyroxene grains 5–8 mm in size in an amount of 15% and tabular plagioclase grains also—

of the same size as the pyroxene grains, in an amount of 10% of the total volume of the rock. The pyroxene is analogous in composition to that described above; the composition of the plagioclase corresponds to labradorite No. 70. The main mass of the rock is fine-grained, composed of needle-like grains of plagioclase, small grains of pyroxene, with the participation of a fine aggregate of secondary minerals—chlorite and iron hydroxides. As is evident from the foregoing, in the third interval in bereshite there is no nepheline of the first generation. All the minerals—nepheline, pyroxene, and plagioclase—separated at the site of cooling of the magmatic melt.

In the roof of the sill the rock has an amygdaloidal structure, expressed much more distinctly than at the floor. The amygdules, up to 1 cm and more in size, usually have a rounded form and make up about 12% of the total volume of the rock. They are most often filled with carbonate or zeolite, more rarely with chlorite. The rock structure is porphyritic; in the segregations there is nepheline with grain sizes of 5–8, more rarely 10 mm, comprising 45–50% of the rock volume. Phenocrysts of pyroxene with grain sizes of 2–3 mm are rarely observed.

The main fine-grained mass of nephelinitic structure is composed of needle-like grains of plagioclase and small grains of nepheline, comprising about 30% of the groundmass. A fairly large quantity of iron hydroxides and chlorite is observed—possibly products of decomposition of the pyroxene of the groundmass.

As a result of the brief analysis of the zonality of the bereshite sill on the basis of petrographic study, the following conclusions may be drawn:

1. The presence of a chilled zone on the lying side of the bereshite sill, which here has a glassy groundmass with distinct fluidality and nepheline phenocrysts 10–15 mm in size, indicates that the magma which intruded at the site of formation of the sill from deeper horizons already carried with it segregated nepheline crystals. The small dip angle of the sill floor (35%) created favorable conditions for gravitational differentiation, as a result of which the nepheline crystals of the first generation, separated from the melt, were able to sink in the melt and accumulate at the floor of the sill; this is evidenced by the decrease in their content in the rock from 35% at the floor to zero in the center of the sill.

2. Uneven and rapid cooling of the magmatic melt under near-surface conditions was manifested in the formation of a series of zones, traceable in the direction from the floor of the sill to its center, with gradual transitions between them. With distance from the floor toward the center of the body, first pyroxene appears in the phenocrysts together with nepheline, and then plagioclase as well.

3. In the magmatic melt at the site of its crystallization, besides gravitational differentiation, other types of differentiation apparently also took place; their result was the enrichment of bereshite in the roof of the sill with nepheline, where it is present both as phenocrysts in an amount of 45–50% of the rock volume and in the groundmass, comprising 30–35% of the volume occupied by it.