MINERALOGICAL-PETROGRAPHIC FEATURES OF THE GRANITOIDS OF THE KAMENSK MASSIF

(Northwestern part of Rudny Altai)

T. I. POLUEKTOVA

(Presented by Professor A. M. Kuzmin)

The Kamensk granitoid massif is located in the northwestern part of the Irtysh folded zone. The granitoids are exposed, chiefly, as separate rocky outcrops with characteristic weathering forms, described for the granites of Rudny Altai and Kalba by N. A. Eliseev. In plan, according to V. S. Kuzebny, the massif has an ellipsoidal shape and is elongated in a northwestern direction. Its area is 200 sq. km. The enclosing rocks are represented by greenstone schists of the Middle Devonian, which, in contact with the intrusive body, undergo metamorphism with the formation of spotted and banded hornfelses composed of pyroxene, garnet, quartz, epidote, and actinolite. The width of the exocontact aureole is 400–500 m, and in the northeastern part of the intrusion it increases to 1.5 km. The absolute age of the granites, according to determinations by F. S. Zakirova, is 270–300 million years.

The massif under study, over approximately 70–75% of its total area, is composed of coarse- and medium-grained porphyritic biotite granites, which are cut by rocks of later formations (in order of formation): medium-grained two-mica granites, aplite-like granites, aplites, and pegmatites. They occur in the form of flat sheet-like bodies, dikes, veins, or small lenses and are, as a rule, confined to tectonically weakened zones.

The present work gives the results of a study of the mineralogical-petrographic features of the biotite porphyritic granites.

Macroscopically the rocks are a light-gray, pinkish-gray granular aggregate of quartz, alkali feldspar, plagioclase, and biotite. According to the degree of grain size, coarse-grained granites of the central (main) part (facies) of the massif and medium- and fine-grained granites of the endocontact zone are distinguished.

The porphyritic segregations are formed by phenocrysts of alkali feldspar and plagioclase, twinned according to the Carlsbad and, more rarely, Manebach laws. The number of alkali feldspar phenocrysts averages 90–120 grains per 1 sq. m of rock; in the endocontact zone the same unit area contains 50–80 grains. Their dimensions range from 0.8×0.4×0.1 cm to 2.0×1.0×0.5 cm; individual crystals sometimes reach sizes of 5.0×3.5×0.5 cm. Well-formed crystals bearing the faces (010), (001), (201), (110) are rare.

The grains are usually cut by numerous cracks and are speckled with inclusions of plagioclase and quartz grains. Porphyritic segregations of plagioclase occur considerably more rarely (their number per 1 sq. m of rock is no more than 10–15) and are smaller in size ($1.0 \times 0.5$—$2.0 \times 0.8$ cm).

Microscopic study of the rocks showed the following composition: principal minerals: plagioclase, alkali feldspar, quartz, biotite; accessory minerals (zircon, apatite, monazite, tourmaline, xenotime, sphene, garnet, ilmenite, magnetite); secondary minerals (chlorite, sericite, muscovite).

Plagioclase is distributed unevenly in the rock: its amount ranges from 16.5 to 54, with an average of 23.3%; in the granites of the endocontact zone the content of this mineral decreases on average to 20.8%. On the basis of grain size, optical properties, and also the character of its relationships with other rock-forming minerals, the plagioclase has been assigned to two generations.

Plagioclase I, the earliest in time of crystallization, is represented by broad tabular crystals on (010) and prismatic crystals along [100], forming porphyritic inclusions in the rock. The former, as a rule, form twins according to the Carlsbad law; the latter occur both as individual crystals and as individuals twinned according to the Manebach law. The outlines of the mineral grains are usually uneven. Plagioclase crystals are characterized by zonal structure. The zoning is recurrent; the number of zones ranges from 6 to 12. The composition of individual zones, from the central to the peripheral, is presented in Table 1.

Table 1

Composition of zoned plagioclases

The transition from zone to zone is distinct. Within each zone the composition of plagioclase changes rather gradually from higher numbers to lower ones. The outer edge of each preceding zone often proves to be uneven, which is explained by some dissolution of its margin before deposition of the plagioclase substance of the next zone.

Along with typically zoned plagioclases, grains of patchy-zoned structure are not uncommon, emphasizing the blocky structure of the crystal (Fig. 1). The central part of the blocks is composed of oligoclase No. 27, the composition of which, toward the periphery of the block, smoothly descends through No. 22—25 to No. 20—18. The outer border of such a blocky crystal is composed of albite-oligoclase No. 13—10—8.

Plagioclase phenocrysts usually contain abundant poikilitic inclusions of quartz, which are concentrated along the periphery or in the central part of the grains and give it a skeletal, often case-like appearance.

Plagioclase II is part of the groundmass of the rock in the form of

The mineral contents are given in volume %.

prismatic crystals measuring 0.5×0.3—0.25×0.1 mm. Its grains are more often idiomorphic and form polysynthetic twins, less often Carlsbad and Manebach twins. The zoning is recurrent. The composition of their inner part is No. 25—18, of the outer part—No. 13—10—8.

The secondary alterations of plagioclase I and II are identical. In them, along the twin seam, pelite readily develops; along fine cracks, separate and clustered accumulations of sericite are formed.

Alkali feldspar is distributed extremely unevenly in the rock. Its content varies from 16.5 to 65%, averaging 40.5% in the granites of the main facies of the massif and 38% in the endocontact facies. This mineral is present, as indicated above, in two forms: as phenocrysts and as grains entering into the composition of its groundmass. The porphyritic segregations appear as more or less regular crystals, twinned according to the Carlsbad or Manebach laws. From the porphyritic segregations, as a rule, lappet-like offshoots extend, which penetrate between the grains of quartz and plagioclase (Fig. 2). Not infrequently, alkali feldspar, penetrating along

Fig. 1. Spotted-zonal structure of plagioclase.
Fig. 2, 3. Replacement of plagioclase by alkali feldspar.
Fig. 4. Relicts of plagioclase grains in microcline porphyroblasts.
Fig. 5. Aggregate of quartz grains with uneven, sinuous margins.
Fig. 6. Replacement of quartz by a “chain” of biotite.
Fig. 7. Replacement of alkali feldspar by biotite.
Fig. 8. Replacement of plagioclase by biotite.

fractures into plagioclase and quartz, replaces them, and the latter become relict inclusions in it (Figs. 3, 4).

Alkali feldspars are, to varying degrees, lattice-like or fibrous; on the basis of these features they have been assigned to microcline. In optical properties the microcline is not homogeneous. Thus, within one and the same grain the extinction angles on the plane (001) are equal to 0°, 4°, 6°, 9°, 13°, 17°, 19°. In a section perpendicular to Ng, Np : (001) = 2°, 4°, 5°, 8°, 10°.

For microcline the phenomenon of perthitization is very characteristic. Perthitic intergrowths form fractures in the mineral along the cleavage (001) and (010), follow fractures parallel to the plane of imperfect cleavage (110) and (100), and are also confined to the seam of the Carlsbad twin. Their composition corresponds to albite-oligoclase No. 13—10—8. The form of the perthitic formations is quite varied. In some grains they are observed as thin, linear or dotted and point-like veinlets; in other grains, usually noticeably deformed, the form of the perthites becomes complicated, up to irregularly lens-shaped or a complexly branching vein-like form.

Microcline grains are usually pelitized. Moreover, within one and the same grain the “pelite” is concentrated in patches and unevenly. In the pelitic material, abundant drop-liquid inclusions, magnetite grains, biotite flakes, and needles of rutile, apatite, and zircon are often visible.

The alkali feldspar in the main mass of the rock is also represented by microcline. Its grains, 2.0—10.0 mm in size, occupy the intergranular space or replace plagioclase and quartz. In optical properties and in the character of perthitization and pelitization, the described mineral is similar to the microcline of the porphyroblasts. This indicates their common origin, at the stage of potassium metasomatism.

The amount of quartz in the rocks studied ranges from 8.5 to 50%. The average in granites of the central part of the intrusive is 30.5%; in the endocontact facies it increases to 36.5%. Quartz is widespread both as individual grains from 1.5 to 5—7 mm across and as aggregates consisting of 2—10 differently oriented grains. Some quartz aggregates were essentially a single crystal that, as a result of deformation, split into several fragments. The latter then shifted relative to one another, as a result of which their disorientation arose. The form of the grains is irregular, rounded, angular. Among the angular varieties there are crystals with rectilinear outlines of faces intersecting at an angle close to a right angle. Grains with such forms of separation probably represent pseudomorphs of d-quartz after cristobalite. Along with rectilinear grain contours, individuals with uneven, serrated edges are often observed (Fig. 5). In some quartz grains a parquet-like structure is observed. The extinction of the mineral is wavy. In addition to the quartz grains described, myrmekitic quartz is sometimes encountered. Myrmekitic intergrowths become extinct either all at the same time or in groups.

Biotite is distributed unevenly in the granites: its content varies from 0.2 to 15.2%. The average in granites of the central part of the intrusive is 5%; in the granites of the endocontact it decreases to 3.5%. On the basis of the form of occurrence, as well as optical properties, two generations of biotite have been distinguished. Biotite of the early generation is observed as separate flakes included in plagioclase 1, quartz, and porphyroblasts of alkali feldspar. Pleochroism scheme: along Ng—reddish-brown, along Np—light pink. The refractive index of biotite 1 is rather constant and equal to 1.653—1.658, while the total iron content, op-

determined from the diagram of V. S. Sobolev (1950), is 48–50%.

Late biotite is represented by xenomorphic grains with uneven, “ragged” margins. Distributed in the rock in chains, the biotite penetrates along cracks into quartz (Fig. 6), plagioclase, and alkali feldspars and replaces them (Figs. 7, 8). The pleochroism of biotite II is sharp: in Ng it is dark brown to black, in Np—brownish yellow. The refractive index is 1.670–1.673; the total iron content averages 72–74%. Biotite is subject, to varying degrees, to muscovitization, more rarely to chloritization.

Accessory minerals occur as inclusions in biotite, plagioclase 1, in pelitized areas of feldspars, and in cracks of quartz grains. The main mass of accessories is associated with late biotite, individual flakes of which are often entirely “clogged” with zircon.

Secondary minerals are represented by sericite, developing after oligoclase; pelite—after alkali feldspar; and also by muscovite and chlorite replacing biotite.

Thus, as the investigations have shown, three stages can be discerned in the history of formation of the granites described. In the first, plagioclase I, quartz, biotite, and part of the accessory minerals crystallized. Alkali feldspar at this stage, if it did form, did so in very small amounts. The rocks formed corresponded to normal biotite granites. The next stage is characterized by a process of potassium–sodium metasomatism, which was preceded by tectonic crushing of the granites. Potassic metasomatism proper manifested itself in the formation of large porphyroblasts of alkali feldspars, with grains of various sizes and irregular outlines. In the course of their formation, potash feldspars replace the earlier segregated plagioclase 1, quartz, and biotite, and contain, in the form of inclusions, relics of these minerals. Potassic metasomatism is followed by sodic metasomatism, manifested in the formation of abundant albite. Microcline is replaced to varying degrees by albite; plagioclase 1 is overgrown by two or three zones of plagioclase No. 13–8. In the third stage, potassium–magnesium metasomatism develops, leading to the formation in the rock of biotite II and the main mass of accessory minerals associated with it. Biotite, distributed in the rock in chains, as “streams,” replaces all previously segregated minerals. The formation of biotite and accessory minerals completes the process of granite formation. In later postmagmatic stages secondary minerals are formed—sericite, muscovite, chlorite.