ON THE PETROGRAPHY OF THE MUSTAG PLUTON

(GORNAYA SHORIA)

E. I. LISENKOV

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

The Mustag pluton, of essentially granitoid composition, is situated somewhat west of the Kondoma iron-ore district and as a whole represents a concordant body extending in a northeastern direction almost from the Kondoma River in the south to the headwaters of the Bolshoi Taz and Viktor’evka rivers in the north. The body is about 40 km long and 7–10 km wide. From the east the pluton is bordered by a series of stock-like granitoid satellite bodies, of which the largest are the massifs of Mount Sarlyk and Mount Ien. Situated in the core of a large anticline of northeastern strike, the intrusive body is in contact with rocks of an undivided, essentially carbonate–siliceous sequence of Upper Proterozoic age.

The material composition and structure of the pluton have been poorly covered in the geological literature. Some information on the petrography of the intrusive body is available in the works of Yu. A. Kuznetsov [5], A. M. Kuz’min [6], N. A. Batov [1], M. A. Kashkai [4], V. I. Kaminskaya [3], and A. G. Volodin [2].

The author’s investigations have shown that the Mustag pluton is distinguished by the complex composition and facies variability of the rocks composing it; among these, apart from vein formations, three different-age groups are clearly distinguished: 1) an essentially gabbro–diorite group; 2) coarse-grained biotite granites; 3) fine-grained alaskite granites. A brief characterization of the named groups of rocks is given below.

1. Rocks of the gabbro–diorite series are exposed by the present erosion surface in the form of small bodies (from the first tens of meters to 2.5 km across) of irregular, sinuous shape, spatially tending toward the periphery of the pluton. The total area of exposures of gabbro–diorite bodies amounts to about 10% of the area of the pluton. In mineralogical composition, among the rocks of this group there are distinguished pyroxenites, hornblende and olivine gabbros and gabbro-norites, gabbro-diorites, diorites, and quartz-bearing diorites, connected by gradual transitions. Gabbro-diorites and diorites are encountered most frequently. Macroscopically, relatively fresh varieties of the rocks of the gabbro–diorite series are usually medium-even-grained, less commonly porphyritic, dark-gray and gray in color. The quantitative composition of the minerals, especially the mafic minerals, varies over wide limits.

Gabbro and gabbro-norites possess a gabbroic microstructure, with transitions in the endocontact parts into a fine-grained ophitic one.

[[beginning of page clipped]] are composed of plagioclase No. 66–68, monoclinic pyroxene—salite (\(cNg = 43^\circ\), \(+2V = 58\text{–}60^\circ\), \(Ng = 1.712 \pm 0.003\), \(Np = 1.69 \pm 0.002\)), hypersthene (\(-2V = 60^\circ\), \(Ng - Np = 0.014\)), containing 40% ferrosilite [7, p. 98], magmatic hornblende (\(-2V = 80^\circ\), \(cNg = 14\text{–}17^\circ\); pleochroism: along \(Ng\)—brown, dark greenish-brown; along \(Np\)—straw-yellow). The pyroxenes are often completely replaced by hornblende, as a result of which all gradual transitions are observed to hornblende gabbros, in which the amphibole content reaches 55–60%. Olivine and olivine-bearing gabbros and gabbro-norites are most characteristic of the southern part of the pluton. The olivine content rarely exceeds 10% of the rock volume. In optical properties (\(2V\) varies from \(-82^\circ\) to \(+87^\circ\)) the composition of the olivine corresponds to chrysolite [7, p. 68]. The mineral is usually replaced by serpentine or tremolite. The accessory minerals of the gabbro-norites and gabbros (sphene, apatite, titanomagnetite) make up 2–5% of the rock volume. Secondary alteration of the rocks is expressed in saussuritization of plagioclase and replacement of pyroxenes and brown-green hornblende by uralite.

Gabbro-diorites display a gabbroic or prismatic-granular microstructure and consist of plagioclase No. 55–50, ordinary magmatic hornblende (\(cNg = 14\text{–}16^\circ\), \(-2V = 70\text{–}75^\circ\); pleochroism: along \(Ng\)—green, brownish-green; along \(Np\)—light yellow), augite-salite (\(cNg = 45^\circ\), \(+2V = 51^\circ\)), hypersthene (\(-2V = 50\text{–}61^\circ\)), a small admixture of brown biotite replacing hornblende, quartz, and orthoclase. The accessory minerals are magnetite, apatite, and sphene. The average quantitative-mineralogical composition of the gabbro-diorites (from 10 thin sections) is as follows: plagioclase—42%, salite—4%, hypersthene—11–12%, hornblende—30–32%, biotite—5%, quartz and orthoclase—up to 4%, accessory minerals—2–3%. Secondary alterations are the same as in the gabbros.

Quartz-bearing diorites have a prismatic-granular structure and differ from the gabbro-diorites in a lower content of mafic minerals, increased content of quartz, K-feldspar and biotite, and in the invariably distinct zonal structure of plagioclase, whose composition in zones from the cores of grains to the periphery changes from No. 68–70 to No. 25. The average composition (from 15 thin sections) of the diorites is as follows: plagioclase—45%, hypersthene—3%, amphibole—33%, biotite—6%, quartz and K-feldspar—8%, accessory minerals—2%. Hypersthene and hornblende are analogous in their properties to the corresponding minerals of the gabbro-diorites. Quartz and K-feldspar occur in micropegmatitic intergrowth and fill the interstices between plagioclase grains. Accessory minerals are sphene, apatite, magnetite, and, rarely, zircon. Secondary alteration of the diorites is expressed in the formation of fibrous uralite-actinolite (\(cNg = 17\text{–}21^\circ\), \(-2V = 74^\circ\)), saussurite, sericite, and chlorite.

In the contacts of the gabbroids with the enclosing rocks, pyroxenites are sometimes encountered, composed of 90–95% salite (\(cNg = 45^\circ\), \(+2V = 61^\circ\), \(Ng = 1.737 \pm 0.002\)) and basic plagioclase No. 76–80.

All the gabbroids described above are characterized by a number of common features indicating their hybrid origin. Evidently, the genesis of the rocks of the gabbro-diorite series must be linked with intensive deep-seated assimilation by granitic magma of limestones and other rocks rich in iron and magnesium. The genetic connection of the gabbro-diorites with the granites is also indirectly confirmed by the absence of the former beyond the limits of the pluton.

The rocks of the gabbro-diorite series are subjected to intense contact influence from the later biotite and alaskite granites. In direct contact the gabbroids have been transformed [[text continues on next page]]

into fine-grained hypersthene–biotite hornfelses with a typical granoblastic structure, or into coarse-grained hornblendites. The age relations of the gabbroids and granites are determined, moreover, by the presence in the former of small cutting bodies and veins of biotite granites containing xenoliths of basic rocks.

1. Coarse-grained biotite granites are the most widespread among the rocks of the pluton. Visually these are weakly porphyritic, coarse-grained, light-gray and brownish-gray rocks. At contacts with the enclosing rocks, a thin (up to 0.5 m) zone of thermal hardening of the granites is locally observed. The quantitative mineralogical composition of the biotite granites (average of 27 thin sections) is expressed in the following volume percentages: alkali feldspar—34, plagioclase—28, quartz—30, biotite—7, accessory minerals—1.

Alkali feldspar is represented by microcline-perthite and forms tabular crystals or irregularly shaped grains from 0.8 to 15 mm across. The amount of perthitic albite intergrowths No. 8–10 ranges from 10 to 60–70%, averaging 20–25% of the volume of the microcline-perthite grains. Both exsolution perthites and replacement perthites are observed. Study of the alkali feldspars in 40 thin sections showed that the potassium phase is ungridded or weakly gridded microcline (−2V ranges from 80–86°; the angle Nm: ⊥ (001) varies from 7 to 14°), forming simple Manebach twins.

Quartz occurs in the granites as xenomorphic grains from 0.2 to 5 mm in size. The mineral is characterized by undulatory extinction, which becomes especially sharp in cataclased granites.

Plagioclase forms idiomorphic crystals from 0.3 to 12 mm in size and corresponds in composition to oligoclase No. 21. Polysynthetic twins according to the albite and albite–pericline laws are common. Very characteristic is the presence of myrmekitic intergrowths of quartz in plagioclase at the boundary of the latter with microcline-perthite. In the biotite granites, a second generation of plagioclase is noted—albite No. 8–10—which is observed in replacement perthites and also as rims around first-generation plagioclase and microcline.

Biotite forms platy crystals from 0.5 to 3 mm across. The mineral is distinguished by sharp pleochroism from dark brown along Ng to straw-yellow along Np and belongs to a strongly ferruginous variety—annite (Nm varies from 1.667 ± 0.003 to 1.678 ± 0.005; Ng–Np = 0.050–0.055).

Hornblende occurs as small (from tenths of a millimeter to 2 mm), well-formed crystals and is represented by the common variety (cNg = 17–19°; −2V = 80°; pleochroism: along Ng—dark green, along Np—light, yellowish-green). The amount of hornblende does not exceed 2–3% of the rock volume.

Accessory minerals are zircon, monazite, orthite, apatite, sphene, and magnetite, most of which are confined to biotite.

Secondary alteration of the biotite granites is expressed in chloritization of biotite, sericitization of plagioclase, and locally albitization and greisenization. The greisenized granites compose the massif of Mount Kizemes (the southern part of the pluton). They are characterized by the presence of muscovite (3–4%), replacing biotite and feldspars, and a slight admixture of tourmaline (up to 2%). In zones of crushing, the granites are transformed into cataclasites and mylonites. In less altered granites, cataclasis is manifested in sharp undulatory extinction of quartz and in bending of plag—

biotite walls. The cataclastic granites are usually intensely epidotized and chloritized.

3) Alaskite granites compose the northwestern part of the pluton. These are usually light-colored, fine-even-grained rocks, more rarely porphyritic and medium-grained, everywhere exhibiting a miarolitic texture. Cavities measuring from 2 to 10–15 mm across are often filled with druses of small quartz crystals. Among the alaskite granites one may distinguish granophyric and aplite-like granites and granite porphyries, which form gradual mutual transitions.

The aplite-like granites possess an aplitic, more rarely granitic, microstructure. The quantitative composition of the minerals varies within broad limits and is expressed as follows (average for 4 thin sections): potassium-sodium feldspar—39%, plagioclase—26%, quartz—33%, biotite—2%. Accessory minerals—zircon, magnetite, more rarely orthite and tourmaline—make up less than 1% in the rocks. The potassic phase of the potassium-sodium feldspars, in its optical properties, corresponds to intermediate microcline (usually \(-2V = 76^\circ\) and does not fall below \(68^\circ\)). The amount of perthitic albite intergrowths in the alkali feldspars is 10–15%. Quartz forms irregularly rounded grains and has approximately equal idiomorphism with microcline-perthite, or is somewhat more xenomorphic than the latter. Plagioclase is observed in the form of irregular grains corroded by microcline and, in composition, corresponds to oligoclase No. 26–30. In quantitative terms, plagioclase is always considerably subordinate to microcline. The mineral is intensely sericitized. Biotite forms small flakes replaced by chlorite, epidote, and rarely muscovite.

Granophyric granites, in mineralogical composition, are generally similar to aplite-like granites and differ from the latter in their micrographic structure, their lower content and more acidic composition of plagioclase (No. 14–18), and, in many cases, the presence of riebeckite (up to 2%).

Granite porphyries occur in the endocontact zone. The rocks exhibit a porphyritic structure. Phenocrysts of rounded quartz grains, tabular crystals of microcline-perthite and plagioclase, making up 25–35% of the rock volume, are situated in a fine-grained micropegmatitic groundmass consisting of quartz, potash feldspar, a small admixture of biotite, and more rarely riebeckite. The composition of the zonal plagioclases of the phenocrysts changes from No. 30–33 in the inner zones to No. 15 in the outer zones.

The alaskite granites are later than the biotite granites, as is demonstrated by the intrusive relationships of the rocks and by the presence in the alaskite granites of xenoliths of coarse-grained biotite granites up to 0.4 m in size (headwaters of the Kara-Su River).

Dike rocks have a limited distribution and are represented by aplites, microgranites, granite porphyries, and felsites. A nearly complete absence of pegmatites and quartz veins in the pluton is characteristic. Dikes of the second stage are not expressed.

Thus, among the diverse rocks composing the Mustag pluton, three heterochronous groups are clearly distinguished by petrographic features, spatial localization, and relationships; these groups probably correspond to three successive phases in the formation of the pluton. In the earliest phase, rocks basic and intermediate in composition were formed, bearing features of a hybrid origin and spatially closely associated with the granites of the later phases.

The next in time of formation are the coarse-grained biotite granites most widely distributed in the pluton (the main phase). The formation of the pluton ends with the intrusion of alaskite granites (third phase) and then of vein rocks of acidic composition.

REFERENCES

1. N. A. Batov. Geology and mineralogy of the iron-ore deposits of the Kondoma group. Materials on the Geology of the West Siberian Territory, issue 18, 1935.

2. A. G. Volodin. On the age of the granites of the Mustag and Sarlyk plutons in Gornaya Shoria. Transactions of the Mining-Geological Institute, West Siberian Branch of the Academy of Sciences of the USSR, issue 17, 1956.

3. V. I. Kaminskaya. Geological features of the Kondoma iron-ore district. Proceedings of the Siberian Branch of the Academy of Sciences of the USSR, Geology and Geophysics, No. 3, 1965.

4. M. A. Kashkay. On the petrography of the intrusive rocks of the iron-ore deposits of the Kondoma district. Transactions of the Petrographic Institute of the Academy of Sciences of the USSR, issue 5, 1935.

5. Yu. A. Kuznetsov. On the intrusions of the Kuznetsk Alatau and their ore potential. Bulletin of the West Siberian Geological Prospecting Trust, issue 3–4, 1932.

6. A. M. Kuzmin. Geological survey of Gornaya Shoria and the district of the Stalin Plant. In: Mineral Raw-Material Base of the KMK, 1933.

7. V. E. Tröger. Tables for the Optical Determination of Rock-Forming Minerals. Gosgeoltekhizdat, Moscow, 1958.