ON THE PETROGRAPHY OF THE BATANAYUL ALKALINE ROCK MASSIF

A. A. MITYAKIN

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

The Batanayul alkaline rock massif 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 belongs to the Tisul District of Kemerovo Oblast.

The geological position of the region that includes the area under consideration is determined by its location within the Nazarovo marginal Devonian stage of the Kuznetsk Alatau, formed in the northern part of the East Martaiga median massif, composed of Upper Proterozoic and Lower Paleozoic deposits. The latter are overlain by a sedimentary–volcanogenic sequence of Lower and Middle Devonian age, intruded by a large number of intrusive bodies of alkaline and basic rocks.

The nepheline rocks of the massif, in the form of bereshites, were first discovered by M. M. Grunin and K. V. Ivanov during geological mapping work in 1940. Subsequently the massif was visited by M. V. Voroshilov (1957) and Yu. D. Skobelev (1959). Until recently, all investigators, including those named above, considered the bereshites to be a constituent part of the Devonian sedimentary–effusive sequence of the area. Within the bodies of bereshites, moreover, in addition to bereshites, nephelinites were distinguished. In recent years, geologists of the Goryanogorsk party (V. N. Koryagin and V. G. Mikhalev) expressed the opinion that the bereshites, occurring as sills, have an intrusive character. The author’s field observations, carried out in the summer of 1965, fully confirm this opinion. The following facts may be cited as evidence:

1. The bereshites have a distinct intrusive contact with the rocks of the enclosing effusive sequence. The latter bear clear traces of metamorphism, manifested in intensive carbonatization and the development of iron sulfides. Plagioclases present in porphyritic segregations are completely replaced by sericite, whereas at a distance of 10–15 m from the contact they appear entirely fresh.

2. At the contact with the bereshites, the greatest development in the effusives is shown by a system of fractures having the same elements of attitude as the bereshite body, and only weakly expressed away from the contact. This makes it possible to conclude that these fractures arose during the process of intrusion, the result of which was the formation of the named body.

3. The intrusive character of the bereshites is also indicated by the internal structure of the sill. At the boundary with the effusives there is observed a ma-

a thick chilled zone. Here the rock has a distinct porphyritic texture with nepheline phenocrysts. The groundmass is lilac-gray, vitreous. A large number of amygdales up to 6–7 mm in size, filled with carbonate and often chlorite, are observed. In the center of the body the rock is recrystallized much more completely: together with nepheline, pyroxene and plagioclase appear in the phenocrysts. The principal fine-grained mass is composed of plagioclase and pyroxene.

The total area of exposure of the massif is about 5.5 km², with maximum dimensions of 1.2 km across and 5 km along strike.

During field work, on the basis of the study of magmatic contacts between the rock varieties composing the massif, seven successive phases of intrusion were established, as a result of which the following were formed successively: labradorite bereshite—andesine bereshite—teschenite—porphyrite—nepheline monzonite—nepheline syenite—lime-alkaline and alkaline syenites—diabase dikes. The largest bodies, in the form of sills and stocks, are formed by labradorite bereshites, nepheline monzonites, and nepheline syenites. The other varieties are represented by small bodies in the form of dikes with a maximum thickness of 10–15 m. For all the rocks of the massif, formed under hypabyssal conditions, a distinct porphyritic texture is characteristic.

Labradorite bereshites constitute about 70% of the exposure area of the massif. They compose a sill-shaped body with a dip azimuth of 340° and a dip angle of 35°.

Macroscopically, rock specimens taken from the central part of the body possess a porphyritic texture. Against the background of a dark-gray groundmass, nepheline segregations with grain sizes of 5–8 mm are visible; in cross section they usually have a square form and make up 55% of the total volume of the rock. In addition to nepheline, the phenocrysts contain tabular grains of plagioclase up to 7–8 mm in size, amounting to 10%, and pyroxene in the form of prismatic grains of the same size, up to 16% of the rock mass.

Under the microscope it can be seen that the nepheline segregations are often represented by glomeroporphyritic aggregates consisting of several grains. The nepheline is completely replaced by cancrinite of light-yellow color, with constants \(N_o = 1.528\), \(N_e = 1.510\), \(N_o - N_e = 0.018\). Zeolite also develops along thin branching veinlets, forming a dendritic pattern. The pyroxene is light green, with weak pleochroism in green tones. It forms elongate-prismatic grains with an octagonal cross section and the optical constants: \(CNg = 40^\circ\); \(2V = 54^\circ\); \(Ng = 1.723 \pm 0.003\); \(Np = 1.694 \pm 0.003\); \(Ng - Np = 0.029\), which corresponds to diopside-salite with a content of about 7% \(FeO + Fe_2O_3\) (after U. A. Deer, 1965).

Plagioclase, represented by elongated tabular grains with broad polysynthetic twins, corresponds in composition to labradorite No. 70. It is partly replaced by sericite. The groundmass of the rock is composed mostly of a fine-grained aggregate of secondary minerals—chlorite and iron hydroxides, among which small acicular grains of plagioclase and pyroxene with an average size of 0.2 mm are visible.

Of the accessory minerals, magnetite is observed in the form of small isometric grains, usually associated with pyroxene, and apatite, represented by regular hexagonal prisms. Together the two minerals constitute about 4% of the rock volume. Judging from the relations among the phenocryst minerals, crystallization proceeded successively, beginning with nepheline, then pyroxene and plagioclase.

In the eastern part of the area, within the body of labradorite bereshites, a dike-like body of andesine bereshites 12 m thick is observed, having

...having the same elements of occurrence as the main body. The andesine bereshites arose as a result of the second phase of intrusion. With the first variety they have a sharp magmatic contact with the chilled zone. In the near-contact part, the bulk of the rock consists of partly devitrified glass of reddish-brown color under crossed nicols.

In the central part of the body these rocks have a clearly porphyritic texture; in the phenocrysts, nepheline, with grain sizes up to 2 cm, makes up 30% of the total mass, and plagioclase, in tabular grains of the same size, makes up 20% of the volume of the rock.

Microscopic study shows that the nepheline, as in the rocks of the first variety, is completely replaced by cancrinite and zeolite. Plagioclase, with polysynthetic twinning, is to a slight degree replaced by sericite. In composition it corresponds to andesine No. 48. The groundmass of the rock is finely recrystallized and is filled with chlorite, in which plagioclase laths 0.8–1.2 mm in size, pyroxene grains up to 3 mm in size in an amount of 10–15%, and small grains of nepheline replaced by cancrinite are visible. The pyroxene is pale green in color, with weak pleochroism, and has the optical constants: CNg = 40°; 2V = 58°; Ng = 1.717 ± 0.003; Np = 1.689 ± 0.003; Ng—Np = 0.028, which corresponds to diopside-salite with a content of about 6% FeO + Fe₂O₃ (after U. A. Deer, 1965). Among the accessories, magnetite and apatite are present. From the mutual relations of the minerals in the rock it may be concluded that its crystallization began approximately simultaneously with nepheline and plagioclase. The separation of pyroxene began later; it is present only in the groundmass.

The product of the next, third phase of intrusion is the teralite-porphyrites. They form fairly numerous small bodies in the form of dikes with thicknesses from 2–3 to 10 m. The strike of the dikes is sublatitudinal and submeridional. In outward appearance the rock is very similar to labradorite porphyrite—dark gray, of porphyritic texture; in the phenocrysts there are elongate-tabular grains of plagioclase, up to 2 cm along the long axis, making up about 35% of the volume of the rock. The plagioclase tablets have a linear and planar orientation concordant with the orientation of the dike contacts with the country rocks. Plagioclase, with polysynthetic twinning and uneven, sinuous outlines, corresponds in composition to andesine No. 48, and often has zonal structure. It is partly replaced by sericite and carbonate. The groundmass of the rock is fine-grained; its composition is as follows: nepheline 35%, pyroxene 30%, plagioclase 30%.

Nepheline is represented by well-formed elongate-prismatic grains with a regular hexagonal cross section, completely replaced by a mineral of the cancrinite group, forming scaly aggregates of honey-yellow color with the optical constants: No = 1.541 ± 0.002; Ne = 1.531 ± 0.002; No—Ne = 0.010. The increased refraction of the mineral and decreased birefringence in comparison with ordinary cancrinite are explained by an increased potassium content in its composition (Winchell, 1953).

The pyroxene is pale green, in the form of elongate prisms 1.5–2.0 mm in size, with weak pleochroism from light green along Ng to yellowish green along Np. CNg = 40°; 2V = 58°; Ng = 1.718 ± 0.003; Np = 1.690 ± 0.003; Ng—Np = 0.028, which corresponds to diopside-salite with a content of about 6% FeO + Fe₂O₃.

The plagioclase of the groundmass forms needle-like crystals with simple and polysynthetic twinning; the orientation of the grains is random. Among the secondary minerals, actinolite develops in small amounts along pyroxene grains; chlorite is also present, sometimes forming small accumulations in the form of amygdules, as is carbonate. Among the accessory minerals, magnetite and apatite occur in amounts of 2–3%.

In the fourth phase of intrusion, rocks are formed in which, among the feldspars, potassic feldspar predominates; it was absent in the three varieties listed above. These rocks compose a body that contacts on the west with labradorite porphyrites and has approximately the same elements of occurrence. The area of its outcrop at the surface is about \(1.5\ \mathrm{km}^2\). In mineralogical composition they occupy an approximately intermediate position between nepheline syenites and nepheline monzonites, but because of the rather high number of plagioclase (No. 45) they should evidently more properly be assigned to nepheline monzonites. The composition of the rock is as follows: potassic feldspar—45%, plagioclase—25%, pyroxene—13%, nepheline—10%, accessory minerals—2%.

Outwardly the rock is of porphyritic structure; among the phenocrysts there is potassic feldspar with grain sizes up to 6 cm, making up 30% of the volume of the rock, and nepheline in the form of prisms with square cross sections up to 1 cm in size, making up 8% of the volume of the rock.

The groundmass of the rock is dark gray and fine-grained. Under the microscope it is seen that the phenocrysts of potassic feldspar, which outwardly have the appearance of separate grains, are glomeroporphyritic segregations consisting of several grains with sinuous mutual boundary lines, together forming externally the shape of a single grain. In the grains of potassic feldspar there are observed regular, pegmatite-type intergrowths of nepheline up to 0.2 mm in size, making up about 10% of the area of the grain.

The optical constants of the potassic feldspar—\(Ng = 1.5285 \pm 0.001\); \(Nm = 1.523 \pm 0.001\); \(Np = 1.519 \pm 0.001\)—indicate a high content of the albite molecule, about 37%. The nepheline of the porphyritic phenocrysts, as also in the intergrowths, is completely replaced in the potassic feldspar by cancrinite and zeolite.

The groundmass of the rock has a hypidiomorphic-granular texture, with idiomorphism of pyroxene and plagioclase in relation to potassic feldspar.

Plagioclase, in the form of lathlike crystals with polysynthetic twinning and rectilinear lateral outlines, corresponds in composition to andesine No. 45. It is partly replaced by sericite. Pyroxene is present in the rock in two varieties. One of them, light green, forms short-prismatic crystals with a hexagonal cross section and weak pleochroism from light green along \(Ng\) to yellow-green along \(Np\), with optical constants: \(CNg = 42^\circ\); \(2V = 58^\circ\); \(Ng = 1.725 \pm 0.003\); \(Np = 1.697\); \(Ng - Np = 0.028\), which corresponds to salite with a content of 8% \(FeO + Fe_2O_3\) (after A. A. Deer, 1965). The pyroxene of the other variety, present in small quantity compared with the first, is colorless, in the form of elongate-prismatic grains with constants: \(CNg = 47^\circ\); \(2V = 57^\circ\); \(Ng = 1.703 \pm 0.002\); \(Np = 1.678 \pm 0.002\); \(Ng - Np = 0.025\), which corresponds to leucoaugite with a content of \(FeO + Fe_2O_3\) of 0.8%. The second variety of pyroxene is earlier; it is enriched in magnesium. Only separate relict grains remain from the leucoaugite, because during cooling it underwent incongruent melting, and from the melt there proceeded the growth of pyroxene more enriched in iron. This is indicated by reaction relationships between the two varieties. The potassic feldspar of the groundmass, similar in composition to that in the porphyritic phenocrysts, is represented by small grains of isometric or slightly elongated form. Among accessory minerals, magnetite and apatite are present.

In the fifth phase of intrusion there arise rocks in which, of the feldspars, only potassic feldspar is present. These are nepheline syenites, forming a stock-like body along the eastern side of Batanayul, about 100 m thick, of northeastern strike. Macroscopically—

... rock is pinkish-gray, of porphyritic structure; in the phenocrysts there is K-feldspar with grain size up to 1 cm, amounting to 30% of the rock volume, pyroxene with grain size up to 7–8 mm in the form of elongated prisms, amounting to 10%, and nepheline in the form of prismatic grains with a square cross section, amounting to 8% of the total volume of the rock.

Characteristic features are nests-schliere up to 3 cm in size, composed of pyroxene grains which appear to be cemented by potassium feldspar. The groundmass is fine-grained.

The pyroxene of the porphyritic phenocrysts is of two varieties—salite and aegirine-augite. The first is represented by prismatic grains with a hexagonal cross section, yellow-green in color, with pleochroism from yellow-green along Ng to colorless along Np, with optical constants: CNg = 44°; 2V = 59°; Ng = 1.721 ± 0.003; Np = 1.693 ± 0.003; Ng—Np = 0.028, which corresponds to the composition of a mineral containing about 6.5% FeO + Fe₂O₃. The salite has undergone intense opacitization. Replacement by magnetite begins at the periphery of the grains. Individual small grains are completely replaced by magnetite with preservation of the relict form of the mineral. Aegirine-augite forms flattened prisms with a cross section in the form of an elongated hexagon, with pleochroism from grass-green along Ng to yellow-green along Np. Its optical constants are: CNp = 30°; 2V = 82°; Ng = 1.743 ± 0.003; Np = 1.724 ± 0.003; Ng—Np = 0.039. It is clearly evident that the aegirine-augite is later than the salite, forming reaction rims around the latter. Confirmation of this is also the fact that in the principal fine-grained mass only aegirine-augite is present.

The potassium feldspar of the phenocrysts, as in the rock of the preceding variety, is characterized by a high content of the albite molecule. The main fine-grained mass of the rock contains 30% aegirine-augite; the remaining part consists of potassium feldspar and accessory minerals, among which there is much apatite and magnetite—2–3%.

In the next-to-last phase, nepheline-free syenites are formed, among which, by mineralogical composition, lime-alkaline hornblende-bearing and alkaline leucocratic varieties may be distinguished. Both varieties of rocks form, within the area, dike bodies up to 10 m thick, of sublatitudinal and submeridional strike. The lime-alkaline syenites are macroscopically pink, of porphyritic structure, with phenocrysts of plagioclase grains up to 8 mm in size, constituting about 8% of the rock volume, and single grains of K-feldspar. The groundmass is fine-grained.

The plagioclase of the phenocrysts is represented by well-formed tabular grains with polysynthetic twinning, corresponding in composition to andesine No. 38. It is partly replaced by sericite and carbonate. The single grains of potassium feldspar present in the porphyritic phenocrysts also form grains of tabular shape, with optical constants: Ng = 1.5285; Np = 1.519; Ng—Np = 0.0095, which corresponds to a mineral with a content of about 38% albite molecule.

The groundmass of the rock is hypidiomorphic-granular, with idiomorphism of hornblende and plagioclase relative to potassium feldspar. Of the hornblende, only the relict form of the grains has been preserved; it is completely replaced by carbonate, partly by chlorite, with segregation of iron hydroxides.

The plagioclase, in the form of columnar grains, has the same composition as in the porphyritic phenocrysts. The total mineralogical composition of the rock is: potassium feldspar—50%, plagioclase—40%, hornblende—10%. Among the secondary minerals, chlorite develops throughout the schliere in the form of a thin aggregate; carbonate occurs in smaller amount. The accessory minerals are represented by magnetite and apatite.

Alkaline leucocratic syenites—pink, with a fine-grained or porphyritic trachytic texture—are composed of potassium feldspar, analogous in composition to the varieties described above, with the participation of up to 3% quartz in the form of isolated small xenomorphic grains in the interstices between the grains of potassium feldspar.

In the final period of formation of the massif in the Batanayul area, diabase dikes were formed. This is a greenish-gray rock of porphyritic texture; in the phenocrysts there are isolated tabular plagioclase grains up to 2 cm along the long axis. In the rock, rounded amygdules up to 4 mm in size, filled with quartz, are observed.

Under the microscope the rock displays a typical ophitic structure of the groundmass, composed of 50% augite, 43% plagioclase, 5% magnetite, and 2–3% quartz. Plagioclase, in the form of columnar grains with polysynthetic twinning, corresponds in composition to labradorite No. 60. It is partly replaced by sericite and carbonate.

The pyroxene—violet-colored titanaugite—has the optical constants: \(CNg = 48^\circ\); \(2V = 50^\circ\); \(Ng = 1.743 \pm 0.003\); \(Np = 1.720 \pm 0.003\); \(Ng - Np = 0.043\), which corresponds to a mineral containing about 8% FeO + Fe₂O₃ and 4.8% TiO₂ (after U. A. Deer, 1965). Chlorite develops intensively after augite.

Magnetite, in the form of small irregular grains, is uniformly distributed over the entire area of the thin section, associating with pyroxene grains. Quartz is represented by isolated small xenomorphic grains.

As a result of this brief review of the petrography of the rocks of the Batanayul massif, the following conclusions may be drawn:

1. A gradual decrease in the role of Na alkalis and an increase in K alkalis from the early products of differentiation to the late ones is clearly traceable. The end members of differentiation are feldspar-free rocks of the first phase and purely potassium-feldspar rocks of the last. In the same direction there is a decrease in Ca content, as is evident from the basicity of the plagioclases.

2. There is a gradual increase in silica content from the rocks of the first phase to the last. Undersaturated rocks pass into saturated ones with the appearance of free silica.

3. Despite the considerable undersaturation in silica of the rocks of the first phases and the high iron content, olivine is absent from them. This indicates that the magma was oversaturated with Ca, as a result of which calcium–iron–magnesian pyroxenes formed. The presence in rocks of the first phase of highly basic plagioclase (labradorite No. 70), despite very high alkalinity, also testifies to oversaturation of the magma with Ca.

4. The high Ca content in the magma may also be indicated by the fact of complete replacement of nepheline by cancrinite. The last two conclusions provide grounds for supposing that assimilation by the magma of calcium-rich country rocks may possibly have taken place, which served as one of the causes of the formation of undersaturated nepheline rocks.