FEATURES OF THE DISTRIBUTION OF ACCESSORY ZIRCON IN THE GRANITES OF THE MOKHNATUKHINSKY MASSIF

(RUDNY ALTAI)

T. I. POLUEKTOVA

(Presented by Prof. A. M. Kuzmin)

As is known, zircon is among the most widespread accessory minerals of intrusive rocks, for example, granites [2, 4, 6, 7], and is widely used for solving a number of special problems in petrology and geochemistry [5, 8]. The results of work by many researchers show that the morphological and chemical features of this mineral reflect the physicochemical conditions under which the rock was formed [7]. The present article gives data on the character of the distribution of accessory zircon in the granites of the Mokhnatukhinsky massif. Special attention is paid to the size and morphological features of the crystals, in particular, to the ratio of their length to width.

The Mokhnatukhinsky massif is located in the northwestern part of Rudny Altai on the Uba River, a right tributary of the Irtysh River, and represents an asymmetric laccolith. The age of the pluton is Permian. The main field of the intrusive body is composed of coarse-grained porphyritic granites.

The porphyritic segregations are represented by microcline and plagioclase. Microcline phenocrysts are distributed unevenly in the granites. Their number varies from 50 to 200 per 1 m² of rock; their dimensions range from \(3 \times 1.5\) to \(10 \times 4\) cm². Large microcline crystals in the endocontact zone of the intrusion bear traces of marked corrosion: the edges of the faces are serrated; the grains, as a rule, are cut by cracks along which biotite develops. Individuals twinned according to the Carlsbad law are frequently encountered. Porphyritic segregations of plagioclase occur as tabular crystals measuring \(2 \times 5\)–\(0.8 \times 2.5\) cm. Its composition corresponds to oligoclase and andesine-oligoclase.

The main mass of the granites consists of plagioclase, potassium-sodium feldspars, quartz, biotite, accessory and secondary minerals. Plagioclase is distributed unevenly in the rock; its amount in the intrusion ranges from 4 to 28% (Fig. 1, a), and the average content is 16%. In the endocontact zone of the massif, the amount of plagioclase increases and varies within 4–45% (Fig. 1, b); its average content in the endocontact granites is 20%. The mineral occurs in the form of short-prismatic, tabular individuals measuring from 2 to 5 mm. Polysynthetic twins are common. The composition of plagioclase varies from No. 18 to No. 38. Potash feldspars are present in the form of grid, weakly grid, and non-grid modifications of microcline. Its amount in the massif varies from 5 to 32% (Fig. 1, a), and in the endocontact, 5–38%

Fig. 1. Variation diagram of mineral contents in granites of the Mokhnatukhinsky massif: a — in the central part, b — in the endocontact part. 1 — quartz, 2 — plagioclase, 3 — potassium-sodium feldspars, 4 — biotite, 5 — zircon.

Axes in the diagrams: content of rock-forming minerals, vol. %; zircon content, g/t; sample numbers.

(Fig. 1, b). The average content of the mineral in the granites of the intrusion is 34%; in the endocontact it decreases to 28%. The grains of potassic feldspar are allotriomorphic in form; their size is 1.5–5.5 mm. The quartz content in the rock varies from 20 to 70% (Fig. 1, a); toward the periphery of the intrusion the limit of variation in the amount of silica in the granite decreases and amounts to 35–60% (Fig. 1, b). Quartz is present as rounded, xenomorphic grains, and also as idiomorphic poikilitic inclusions in microcline and plagioclase. The amount of biotite over the entire area of the intrusive body is uniform and amounts to 5–10%, only at one point (23) reaching 20% (Fig. 1, a, b). The mineral appears as platy and scaly grains of dark-brown color. Pleochroism is strong, from straw-yellow along \(N_p\) to red-brown tones along \(N_g\). The total ferruginosity of biotite varies from 48 to 80%, reaching a minimum value in granites from the contact with the enclosing rocks. Secondary minerals are represented by pilite and sericite, developed on plagioclase, and by muscovite replacing biotite.

Material for the study of accessory minerals was obtained from samples weighing 2.5 kg, which were taken along specified geochemical profiles with a distance between points of 500 m, and in the endocontact part of the intrusion—at 100 m intervals in the direction away from the contact with the enclosing rocks toward the center of the intrusion. The samples were processed according to the generally accepted scheme.

Zircon, one of the most widespread accessory minerals of the granites studied, is characterized by a very great irregularity of distribution, which is especially clearly expressed in the central part of the intrusion (Fig. 1, a, b). Thus, the difference between the maximum and minimum amount of the mineral described in the massif is 827 g/t, and in the endocontact zone, 556 g/t. The arithmetic mean content of zircon in the center of the intrusion is higher than at the periphery and amounts, respectively, to 311.6 g/t and 190 g/t. The coefficient of variation, which determines the deviation of the mineral content from the arithmetic mean, is lower in the granites of the central zone of the massif than in the endocontact zone and is equal, respectively, to 0.18 and 0.3%.

In the granites 15 morphological types of zircon crystals have been established; among them polyhedra of the 8th, 9th, 10th, 11th, and 12th types* are encountered most often, but the 8th crystallographic type is predominantly widespread (Fig. 2), and a detailed characterization is given of the features of its distribution.

Crystals of zircon of the 8th morphological type are characterized by well-developed faces \(\{100\}\) and \(\{110\}\). The faces of the form \(\{311\}\) are sharply asymmetric, unequal in size, and are observed, as a rule, in incomplete number. By color, colorless, pink, yellow, and red-brown zircons are distinguished.

Colorless individuals are distributed irregularly in the massif; their frequency of occurrence in the samples varies from 42 to 78% (Fig. 3, a); in the endocontact zone of the intrusion the amount of colorless zircons increases with distance from the contact with the enclosing rocks toward the center of the body (Fig. 3, b). The surface of the crystal faces is smooth, lustrous; the edges are rectilinear. Individual individuals are filled with inclusions, among which are deter—

Fig. 2. Eighth crystallomorphological type of zircon. \(a\ \{100\}\), \(m\ \{110\}\), \(s\ \{111\}\), \(\lambda\ \{311\}\).

* The type number is given in accordance with the table of typomorphy of zircon crystals by V. A. Ermolaev [3].

Fig. 3. Variation diagram of the distribution of colorless and colored zircon crystals of the 8th morphological type in the granites of the Mokhnatukha massif: a—in the central part of the intrusion; b—in the endocontact zone.
1—colorless; 2—pink; 3—yellow; 4—red-brown; 5—arithmetic mean of the colored zircons.

rounded gas inclusions, gas-liquid inclusions of rounded-elongate form, and also grains of zircon, biotite, and ilmenite. Most colorless zircon crystals are zoned, and the number of zones sometimes reaches 12. The absolute dimensions of zircon polyhedra in both the central and the endocontact parts of the massif vary from 0.09×0.003 mm to 0.4×0.1 mm. The average dimensions range from 0.15×0.05 mm to 0.26×0.08 mm. The magnitude of maximum elongation, i.e., the ratio of the crystal length to its width, is not constant and varies from 3.2 to 5.4; in the endocontact part of the intrusive body the limits of variation increase and amount to 3.5–7. The minimum elongation of zircon crystals in the massif is constant and equal to 2. The average ratio of the length of a polyhedron to its width in the granites of the central part of the intrusive body and along the periphery varies from 2.5 to 3.9, with fluctuations in the ratio caused chiefly by variation in the maximum elongation (Fig. 4, a, b).

Pink zircon crystals were not encountered in all samples. Their calculated frequency of occurrence in the intrusive body fluctuates within the range 3–17%; in the endocontact part of the massif the upper limit of the frequency of occurrence increases to 29% (Fig. 3, a, b). The absolute dimensions of pink individuals vary from 0.2×0.06 mm to 0.4×0.1 mm, the average from 0.1×0.04 to 0.32×0.10 mm. The ratios of crystal length to width are not constant and vary within broad limits. Thus, the magnitude of the greatest elongation of crystals in the intrusive body varies from 2.5 to 8.3; of the smallest—from 1.8 to 8.3; of the average—2.2–8.3; in the marginal zone of the granite body the maximum elongation fluctuates from 2 to 5.4; the minimum—from 2 to 4. The magnitude of the average ratio of crystal length to width varies from 2 to 4.2 (Fig. 4, c, d).

The number of yellow zircon crystals in the massif varies from 5 to 16%; in the endocontact zone their content decreases from 28 to 5% with distance from the contact toward the center of the body (Fig. 3, a, b). The polyhedra are characterized by poorly developed faces {311}; the edges are smoothed or slightly wavy. Numerous overgrowths of ilmenite are observed on faces {110}. The absolute dimensions of the polyhedra vary from 0.162×0.04 mm to 0.216×0.05 mm. The maximum magnitude of elongation varies within the range from 2.2 to 6, the minimum from 2 to 2.5; at one of the points of the granite pluton it decreases to 1.2. The average elongation fluctuates from 2 to 3 (Fig. 4, e); in the marginal zone of the intrusive body the ratio of the greatest crystal length to width decreases, with distance from the contact with the enclosing rocks and from the central part of the massif, from 6 to 4.2; the magnitude of the smallest elongation of the crystals is rather constant and amounts to 2–2.5; only in one case does it increase to 4.2. The average elongation varies within the range 2–4.2 (Fig. 4, f).

Red-brown zircons are distributed unevenly in the massif and were not encountered in all samples. In the marginal part of the intrusive body, red-brown individuals were noted in increased content in a sample from the contact with the granites (8%); in subsequent samples, with distance from the margin of the intrusive body toward its center, their frequency of occurrence decreases, with the exception of one sample located at a considerable distance from the contact with the enclosing rocks, in which the quantity of this dark-colored variety of zircon rises sharply to 50% (Fig. 3, a, b). A distinctive feature of the red-brown crystals is strong fracturing, pitting of the faces, and serration of the edges. The absolute dimensions of the crystals vary from 0.80×0.08 mm to 0.29×0.07 mm. The average size of the polyhedra fluctuates from 0.15×0.06 mm to 0.25×0.07. The ratio of crystal length to width in the intrusive body varies from 1.9 to 3.9, and in its endocontact part—from 2.8 to 4.3.

To clarify the relationship between the frequencies of occurrence of colorless and colored zircon crystals of the 8th morphological type, the arithmetic mean of the frequencies of all colored varieties was calculated (Fig. 3, a, b). As can be seen from the graphs, the frequencies of occurrence of colorless and colored individuals are inversely related, i.e., with an incre—

Fig. 4. Variation diagram of the elongation of zircon crystals of the 8th morphological type in granites of the Mokhnatukhinsky massif. The left diagrams are for the central part of the intrusion, the right diagrams for the endocontact zone: a, b — colorless; c, d — pink; e, f — yellow zircons. 1 — maximum elongation, 2 — minimum, 3 — average; g, h — mean elongation value for colorless and colored zircons. (For diagrams g–h, the symbols are given in Fig. 3.)

with an increase in the content of colorless zircons, the number of colored ones decreases, and vice versa.

The arithmetic mean value of elongation of all colored varieties of zircon and the length-to-width ratio of colorless individuals are also inversely related: with a decrease in the elongation of the colorless zircons, the value for the colored zircons increases (Fig. 4, ж, з).

Conclusions

The material presented makes it possible to draw the following conclusions.

1. The Mokhnatukhinsky granite intrusion has a complex history of formation, as indicated by the uneven distribution both of the rock-forming and accessory minerals, in particular zircon of the δ-th morphological type. Variations in the elongation values of zircon crystals also indicate instability of the physicochemical conditions of the environment.

2. Colorless and colored individuals of zircon are formations of different times. This is indicated by the established inverse relationship between the frequency of occurrence of colorless and colored crystals, and by the same relationship between their elongation values.

LITERATURE

1. A. I. Bazhenov, V. A. Ermolaev. Distribution and typomorphy of crystals of accessory zircon in the rocks of the Elekmonar multiphase intrusive massif (Gorny Altai). Mat. po miner., petrograf. i pol. iskop. Zap. Sib. i Krasnoyarskogo kraya, issue 2, Tomsk State University Press, 1964.

2. K. N. Braun, N. A. Ashikhmina, T. S. Magidovich. Features of the distribution of accessory minerals in Lower Paleozoic granitoids of the lower course of the Dzhida River (Buryat ASSR). In: Akts. min. i el-ty kak kriterii komagmatich. i metallogenich. spetsial. magmatich. kompleksov. USSR Academy of Sciences Press, 1965.

3. V. A. Ermolaev. Morphology of zircon crystals from Paleogene deposits of the margins of the Tomsk swell. Zap. Vsesoyuzn. min. ob-va, issue 2, 1961.

4. V. V. Lyakhovich, N. I. Noneshnikova. Accessory minerals of granitic intrusions of Western Tuva and associated vein rocks. Tr. IMGRE, issue 7, 1961.

5. V. V. Lyakhovich. Study of microelements and accessory minerals in modern petrography. Tr. IMGRE, issue 1, 1957.

6. B. K. Lvov, N. I. Petrova. Accessory zircon from granitoids of the eastern slope of the Southern Urals. In: Min. i geokh. LGU, 1964.

7. S. D. Turovskii. On the morphological features of zircon from igneous rocks of Northern Kirghizia. Tr. in-ta geol. AN Kirg. SSR, issue 6, 1955.

8. Hoppe Hünter. Die Verwendbarkeit morphologischer Erscheinungen an akzessorischen Zirkonen für petrogenetische Auswertungen. Abhandl. Deutsch. AK. Wiss. Berlin, No. 1, 1963.