ACCESSORY MINERALS OF XENOLITHS FROM THE KAMENSKY GRANITOID MASSIF

(Rudny Altai)

T. I. Poluektova

(Presented by Professor A. M. Kuzmin)

This paper describes the features of the distribution of accessory minerals in xenoliths of shales and in the granitoids enclosing them.

The Kamensky granitoid massif is situated in the northwestern part of the Irtysh zone of folding. The length of the intrusive body is 20 km; its width varies from 6 to 11 km. From the southeast and northeast the granitoids are in contact with greenstone shales of Devonian age. The southwestern and northwestern contacts of the massif are overlain by a thick sequence of Quaternary deposits. According to geological and geophysical data, the body is a complex discordant laccolith gently plunging to the southeast. The age of the pluton is probably Permian.

Rocks of five intrusive phases take part in the structure of the massif (in order of formation): porphyritic biotite granites; medium-grained two-mica leucocratic granites; aplite-like granites; aplites and pegmatites. The granites of the first, main intrusive phase are the most widely developed; they occupy approximately 70–75% of the area of the massif.

Porphyritic biotite granites are dark-gray, pinkish-gray, or light-gray rocks. Porphyritic segregations are represented by tabular crystals of microcline ranging in size from 3 × 2 cm to 1 × 0.5 cm; individual crystals are broken by numerous fractures. The number of phenocrysts is 50–75 per 1 m² of rock. The groundmass consists of orthoclase—35%, plagioclase—27%, quartz—35%, and biotite—3%.

Orthoclase is developed as xenomorphic grains 2.5–2.0 mm in size. \(2V = 75–85^\circ\). Secondary alteration of the mineral is expressed in weak pelitization.

Plagioclase occurs in short-prismatic crystals 0.3–5 mm in size, with characteristic polysynthetic twins, sometimes weakly sericitized. Its composition corresponds to oligoclase No. 22–26.

Quartz is represented by sharply xenomorphic and isometric grains ranging in size from 0.01 to 2.5 mm. The bulk of the grains is confined to the interstices between plagioclase crystals. The optical properties of quartz are ordinary. Extinction is wavy, sometimes mosaic. Its grains, as a rule, are filled with gas–liquid inclusions.

The granites of the endocontact facies of the intrusion are similar in their mineral composition to the granites of the intrusive facies proper. They differ from the latter chiefly in the smaller grain size of the minerals, the increased content of biotite (10%), and the lower content of quartz (25%). The optical properties of the minerals are, on the whole, close to those of the porphyritic biotite granites of the main facies. Biotite shows a certain difference: it forms schlieren distributed in the rock, as a rule, unevenly. The size of the biotite flakes is up to 3–3.5 mm; pleochroism is strong, from dark brown with a reddish tint along Ng to pale yellow along Np. The total ferruginosity of the mineral is 88–85% [3].

The granites of the endocontact facies are characterized by the presence of a large number of xenoliths of the enclosing rocks, with sizes from 0.5 to 40 m in cross section.

The xenolith under study has a rounded or nearly rounded form; its size is 35 m in cross section. In composition the xenolith corresponds to strongly metamorphosed schists, in which injections of granitic melt are observed along the schistosity plane. The composition of the injected schists is: quartz—75%, biotite—16%, muscovite—5%, plagioclase—2.5%, pyroxene—1%, hornblende—1%; epidote—0.5%.

Quartz is present in isometric, rounded aggregates of grains, the edges of which are often corroded; extinction of the mineral is wavy. Grain size is 0.05 mm. Biotite is represented by small flakes elongated in the direction of schistosity; it sometimes forms inclusions in quartz; it is pleochroic from dark brown along Ng to light yellow along Np; its total ferruginosity is 40%, increasing to 73% in contact with the granite. Muscovite is relatively coarse-flaked; its optical properties are ordinary; as a rule it develops on biotite. Plagioclase forms isometric, tabular crystals, rarely of xenomorphic form; its composition corresponds to oligoclase No. 20–25. Pyroxene occurs as xenomorphic, rarely idiomorphic grains replacing biotite. The color of the mineral is pale bluish green with Ng = 42°, corresponding to the composition of salite. Hornblende is dark green with Ng = 15°. Optical constants: Ng = 1.677; Np = 1.640; Ng − Np = 0.037.

Epidote develops as irregular grains with anomalous interference colors characteristic of it; Np = 1.71, Ng = 1.73. In its optical properties it is close to clinozoisite.

Accessory minerals were obtained from samples weighing 2.5 kg, which were taken along a definite section (Fig. 1). In the samples of xenolith, injected schists, and the granites enclosing them, zircon, apatite, monazite, magnetite, ilmenite, rutile, hematite, tourmaline, garnet, orthite, pyrite, epidote, and iron hydroxides were established; their quantitative contents are summarized in Table 1.

Fig. 1. Geological structure diagram of the area: 1—coarse-grained biotite granites; 2—xenolith of injected schists; 3—sample numbers and sampling points.

Zircon is most widely distributed in the granites, where its content is 754.25 g/t; in contact with the xenolith it increases to 899.40 g/t (Fig. 2). The mineral described is represented by crystals of types 8, 10, 11, and 12 [1] (Fig. 3).

Zircons of the 8th crystallomorphological type are charac—

and frequently occurring (68%). In contact with injected schists their number decreases almost by half—to 32%. The length of the crystals varies from 0.29 to 0.07 mm; the width, from 0.09 to 0.03 mm. The elongation value, i.e., the ratio of the crystal length to its width, varies from 6.4 to 1.5, with an average of 3; in contact with the xenolith, 3.8. The coloration of the zircon individuals described is colorless, pink, and reddish-brown. In the granite sample from the contact with the xenolith, the number of colorless individuals increases, while pink and reddish-brown ones occur more rarely. All zircon crystals of this type are filled with gas and gas–liquid inclusions. Gas bubbles are concentrated, as a rule, in the center, sometimes elongated in the form of several chains parallel to the crystal faces. In individual zircons, mainly from the contact with the xenolith, inclusions of biotite are established. The surface of the crystals is even and lustrous, the edges rectilinear. A certain exception is represented by reddish-brown zircons, the faces of which are rough, cut by fine cracks, along which iron hydroxides develop.

Fig. 2. Variation diagram of the content of accessory minerals in the granite and xenolith of the Kamensky massif: 1—zircon; 2—apatite; 3—monazite.
The numbers 10, 11, 12, and 13 are sampling sites indicated in Fig. 1.

Fig. 3. Types of accessory zircon crystals in the granites and xenoliths of the Kamensky massif:
$m\{110\}$, $a\{100\}$, $s\{111\}$, $\lambda\ 311$.
The numbers correspond to the crystallomorphological zircon types of V. A. Ermolaev.

The content of zircon crystals of the 10th type in the granite sample from the contact with the xenolith is higher than away from it and amounts, respectively, to 8 and 16%. The length of the polyhedra of this type is within the range from 0.32 to 0.12 mm; the width, 0.068–0.043 mm. The average elongation value is 3.3; in contact with the xenolith it increases to 4.6. All crystals, as a rule, are reddish-brown in color; in contact with the xenolith, colorless and pink ones appear. The surface of the crystals is rough, pitted, the edges wavy. On individual specimens a reddish-brown “shirt” of iron hydroxides is observed.

The 11th crystallomorphological type is rare and occurs as single colorless crystals. Their length is 0.28 mm, width 0.07 mm, elongation 3.4.

Individuals of the 12th type are most often encountered in granites from the contact with the xenolith, where their number amounts to 44%. The length ranges from 0.35 to 0.13 mm; the width, 0.06–0.4 mm. The elongation value varies from 2 to 10.7, with an average of 2.6. All representatives of zircons of this type are colorless, with even, lustrous faces, filled with gas and gas–liquid inclusions.

Xenoliths of injected schists are poor in zircon; its content is 2.25 g/t, and in contact with granite it increases to 16.5 g/t (Fig. 2). In their morphological features the crystals are characterized by pronounced asymmetry of faces; additional forms {311} are present, sometimes in incomplete number; the apices of individual individuals often belong to different crystallomorphological types. Conventionally, the zircons are assigned to crystals of types 8, 9, and 11; among them, in the sample of injected schists, crystals of type 8 are widespread, while in contact with granite types 9 and 11 appear, and type 8 occurs in subordinate quantity.

Crystals of type 8 are colorless; their length varies from 0.42 to 0.23 mm; width—from 0.07 to 0.06 mm; elongation—3.8, and in contact with granites it increases to 6.2. Abundant inclusions of gas are characteristic.

The 9th morphological type was encountered as single crystals in samples from contact with granites. Its length is 0.19 mm, width 0.10 mm. The elongation value is 1.8. The color is reddish-brown. The surface of the faces is uneven and rough; numerous overgrowths of a dark mineral are observed.

Crystals of the 11th type are widespread in the sample from contact with granites. The length of these individuals ranges from 0.20 to 0.14 mm; width—from 0.05 to 0.04 mm. The elongation value varies within 2.6–5. The color is reddish-brown owing to a “jacket” of iron hydroxides.

Apatite in the granites is noted in the sample from contact with the xenolith, where its content is 24 g/t (the highest value). It occurs in prismatic, more rarely dipyramidal crystals; in contact with injected schists, rounded or nearly rounded grains are encountered. The size of the crystals (in the direction of the long axis) is 0.38–0.40 mm; in contact with the xenolith it decreases to 0.18 mm. The elongation value varies from 1.4 to 2.5. Apatites are colorless, grayish-white, and contain inclusions of small gas bubbles and finely dispersed carbonaceous-graphitic matter, which imparts a dark to black coloration to the mineral. The surface of the faces is rough, the edges are smoothed, and the apices of the dipyramid are rounded.

In injected schists, apatite is a rather rare mineral; its content is 0.5 g/t, and in contact with granite it increases to 1.5 g/t (Fig. 2). It occurs in prismatic crystals 0.20–0.25 mm in size; elongation ranges from 1.3 to 2.6. The edges of the faces are smoothed and curved; the interfacial angles are blunted. The color of the mineral is yellow; brown spots of iron oxide hydrates are often observed on the faces. In the sample from contact with granites, colorless crystals predominate, filled with inclusions of gas bubbles.

Monazite is the most widespread mineral of the granites and xenoliths of injected schists. Its amount in the granite is 235.7 g/t, and in contact with the xenolith it increases to 965 g/t (Fig. 2). It is observed as tabular crystals of brownish-brown, copper-yellow color, 0.40 to 0.50 mm in size. In injected schists the monazite content is 74 g/t; in contact with granites it decreases to 52 g/t. Its crystals have a platy habit, 0.025–0.050 mm in size; the edges of the faces are straight, the surface even and smooth. The color is pale yellow; brown bands of iron oxide hydrates are sometimes observed on the grains; these are especially characteristic of monazites from contact with granites.

Magnetite occurs constantly in the samples. Its amount in granite is 49,628 g/t; in contact with the xenolith it sharply decreases to 1,445 g/t (Fig. 4). The crystals are octahedral in habit; by size, two varieties are clearly distinguished: small ones, 0.10–0.13 mm in size

with even, shiny faces, rectilinear edges, and large, 0.23–0.30 mm in size; the faces of such crystals are, as a rule, sculptured, the edges serrated, and the surface rough and pitted. In the sample from the contact with the injected schists, the first variety of magnetite crystals predominates.

In the injected schists the magnetite content is 375 g/t; at the contact with the granite the concentration of the mineral increases to 1393 g/t; its octahedral crystals, 0.025–0.075 mm in size, have smooth, shiny faces.

Fig. 4. Variation diagram of the contents of accessory minerals and total iron content of biotite in granites and xenoliths of the Kamensky massif: 1—magnetite; 2—ilmenite; 3—total iron content of biotite. The numbers from 10 to 13 are the sampling sites indicated in Fig. 1.

The amount of ilmenite in the granites is 33.85 g/t; at the contact with the xenolith it increases to 575.30 g/t. The crystals are thin-tabular, representing combinations of the pinacoid forms {0001} and rhombohedra {10$\overline{1}$1}, {22$\overline{4}$3}. The size of the individuals is 0.3–0.68 mm. The surface of the faces is uneven and rough, the edges are corroded, and individual crystals are leucoxenized.

In the injected schists ilmenite occurs more rarely; its content is 10.50 g/t, and at the contact with the granite it increases to 41.35 g/t. It is distributed in the form of tabular crystals 0.025–0.075 mm in size. Even, smooth, shiny faces are characteristic; leucoxenization is not manifested.

Rutile was found only in a sample of granites from the contact with the xenolith, where its content is 12 g/t. The crystals are prismatic, 0.30–0.40 mm in size. Typical features of this mineral were noted

knee-shaped twins, similar to those described in reference manuals. The color is bright red.

Hematite is distributed only in the granites, where its amount is 105.25 g/t; at the contact with the xenolith it decreases to 7.35 g/t. It is observed in fragments of tabular crystals 0.25–0.30 mm in size.

Tourmaline was found in the xenolith of injected schists, where its content is 203 g/t, whereas at the contact with granite it is recorded as single grains. In order to study in detail the crystallographic features of the mineral, goniometric measurements were carried out on the best-formed tourmaline crystals. The absence of striation on the crystal faces made it possible to obtain sharp, clear signals. Crystals 0.3–0.4 mm in size were selected for measurement.

According to the goniometric study, four simple forms [4] were established: \(a\)—\(\{11\bar{2}0\}\), \(m\)—\(\{10\bar{1}0\}\), \(\sigma\)—\(\{21\bar{3}0\}\), \(r\)—\(\{10\bar{1}1\}\) (Fig. 5). The results of the crystal measurements are summarized in Table 2.

Table 2

The tourmaline crystals are reddish brown to brown; the surface of the faces is smooth and lustrous. Small overgrowths of biotite are observed on individual specimens. The size of the crystals along the long axis is 0.14–0.28 mm, and across it 0.11–0.20 mm. The ratio of crystal length to width is 2. Some contain inclusions, according to electron-diffraction data, of biotite flakes. By its optical properties (\(Np = 1.632\), \(Nm = 1.652\)), the tourmaline belongs to the calcic-sodic ferruginous varieties [2].

Garnet in the granites was found in a sample from the contact with injected schists—40.8 g/t—and in the xenolith from the contact with granites—20.5 g/t. The crystals have a rhombododecahedral habit, ranging in size from 0.075 to 1.5 mm. The color is pale pink.

Orthite, in a small amount—1.9 g/t—is recorded in a sample of injected schists from the contact with granite, in the form of resin-black grains with a brownish translucence along the margins; \(Ng = 1.67\); \(Np = 1.64\).

Pyrite—a substantially hydrothermal mineral—occurs in samples of granites; moreover, at the contact with the injected schists it occurs more rarely than away from it. The crystals are cubic, weakly striated, 0.13–0.25 mm in size. The surface of the faces is covered with a brown film of iron hydroxides. In the xenolith, pyrite was noted in the sample from the contact with granite, where its content is 0.25 g/t; it occurs exclusively as rounded, rough yellow grains, whose size varies from 0.025 to 0.25 mm.

Principal Conclusions

1. The suite of accessory minerals in both the xenoliths and the granites enclosing them is the same, with the exception of tourmaline and orthite, which are characteristic of the injected schists.

Fig. 5. Tourmaline crystal: a \(\{11\overline{2}0\}\), r \(\{10\overline{1}1\}\), m \(\{10\overline{1}0\}\).

1. In morphological features, the accessory minerals of the granites and xenoliths are generally analogous.

2. The quantitative ratios of the accessory minerals of the granites and xenoliths differ sharply.

3. In the granites, the amount of zircon, apatite, monazite, and ilmenite increases at the contact with the xenolith; the content of magnetite and hematite decreases, while the overall ferruginosity of biotite \(\left(\dfrac{\mathrm{Fe}}{\mathrm{Mg}+\mathrm{Fe}}\right)\) increases.

4. In the xenolith of injected schists, elevated contents of accessory minerals have been established at the contact with the granite, which probably indicates the introduction into it of the elements of accessory minerals from the granitoid intrusion.

REFERENCES

1. V. A. Ermolaev. On a method of mass crystallomorphological analysis of accessory zircon. Izv. Tomsk Polytechnic Institute, vol. 121, 1963.

2. E. Larsen, G. Berman. Determination of Transparent Minerals under the Microscope. Nedra, 1965.

3. V. Sobolev. The significance of the ferruginosity of femic minerals and auxiliary diagrams for determining the compositions of biotites. Mineralogical Collection, No. 4, Lviv Geological Society, 1950.

4. Dana. The System of Mineralogy. 1900.