Distribution Features of Accessory Zircon in Granites of the Kamensky Massif

(northwestern part of Rudny Altai)

T. I. Poluektova

(Presented by Professor A. M. Kuzmin)

In recent years an ever increasing number of researchers have been studying accessory minerals. The heightened interest in these minerals is explained by the great possibilities for using them to solve various geological problems [1, 2, 3, 4, 5]. In many works, of all the minerals the greatest attention is given to accessory zircon, which is being used mainly for the purposes of correlating granitic rocks that differ in age and conditions of formation. As correlation criteria in subdividing granitoids, researchers use the morphology of zircon crystals, its physical properties—chiefly color—and also the content in it of trace elements.

The present work gives the results of a study of the regularities in the distribution of accessory zircon in a granite massif in a horizontal direction from the contact with the enclosing rocks toward the center of the body, and also in its vein derivatives. At the same time, the most detailed analysis is given to the character of changes in the elongation of crystals of both various morphological types and colorless and colored crystals of one and the same type.

The Kamensky granitoid massif is located in the northwestern part of the Irtysh folded zone among greenstone schists of Middle Devonian age and occupies an area of about 200 sq. km. The absolute age of the granites, according to determinations by F. S. Zakirova, is 230–250 million years. The massif is composed of rocks of five intrusive phases (in order of formation): coarse- and medium-grained porphyritic biotite granites; medium- and fine-grained two-mica granites; alaskite-like granites; aplites; pegmatites.

Granites of the first phase are the most widespread and make up 75–80% of the area of the intrusive body. Among them one can

It is possible to distinguish rocks of the central part of the massif and of the endocontact zone that differ in degree of grain size: in the center of the body they are coarse-grained, and in the marginal zone—medium- and fine-grained. Their mineralogical composition is fairly uniform: microcline, plagioclase, quartz, biotite, accessory minerals (zircon, apatite, monazite, xenotime, garnet, sphene, ilmenite, magnetite, tourmaline, rutile, orthite, pyrite), and secondary minerals (chlorite, epidote, zoisite). The quantitative ratios of minerals in the granites are not constant and often vary within wide limits. The amount of plagioclase in the central part of the intrusion varies from 16.6 to 54%, averaging 23.5%; in the granites of the endocontact the content of this mineral ranges from 10.5 to 35% (average 20.8%). Two generations of plagioclase have been distinguished. Plagioclase I, zonal and earlier, forms porphyritic segregations in the rock as tabular grains with uneven, sinuous outlines. The composition of the zones changes from No. 27 in the center of the crystal to No. 18 in the outer zone. Plagioclase II is present in the groundmass of the rock as prismatic zonal grains. The composition of the inner zone corresponds to No. 25—18, and that of the outer to No. 13—8. Microcline numerically predominates over plagioclase. Its content varies from 16.5 to 65% (average 38%). In the rock it forms comparatively large porphyroblasts, and also enters the groundmass as irregular grains. In optical properties the microcline is heterogeneous. Extinction angles in the section (001) range from 0° to 15—17°, and the optic axial angle is \(2V = (-)78—84^\circ\). The quartz content in the granites varies from 50 to 85%, averaging 30.5% in the central zone of the intrusion and 36.5% in the endocontact zone. In the rock it forms separate grains or intergrowths of several grains, and also occurs as poikilitic formations in porphyritic segregations of plagioclase and microcline. Its optical properties are ordinary. Biotite is the latest in time of formation; it is distributed extremely unevenly in the rock, its amount lying within the range 0.2—15.2%. It is represented by xenomorphic grains with uneven, “ragged” edges. The grains form clustered accumulations or are distributed in chains, “cutting across” all the rock-forming minerals. Pleochroism is sharp: for \(Ng\)—dark red, dark brown to black; for \(Np\)—light pink. The refractive index is fairly constant and equals 1.670—1.673 ± 0.01; total iron content is 72—74%. Accessory minerals occur as inclusions in biotite or fill intergranular spaces in its immediate vicinity. Zircon, moreover, is sometimes found as inclusions in plagioclase I.

Intrusive formations of the subsequent phases occur in the massif as vein bodies with thicknesses from 10 cm to 1.5—3 m. Their mineralogical composition is analogous to that of the biotite granites of phase I. The quantitative ratios of minerals in the rocks are fairly constant. In optical properties the rock-forming minerals are close to those from the granites of phase I. The biotite content in rocks of the later phases decreases sharply, and in the aplites it is 0.2—0.1%. The assemblage of accessory minerals is the same as in the granites of phase I, but their quantity is considerably smaller. Among the accessory minerals in all rocks of the massif, zircon is one of the constant and most abundant minerals.

For the purpose of studying the distributional features of accessory minerals, in particular zircon, samples weighing 2.5 kg were taken from rocks of each intrusive phase. Sampling of the biotite coarse-grained granites, characterized by uniformity, was carried out along horizontal profiles every 1 km; of medium-grained granites from the endocontact zone—every 100 m.

In the rocks of the massif, zircon is distributed extremely unevenly.

In the biotite granites of the central part of the massif its content ranges from 13.5 to 89.4 g/t. A statistical analysis of the distribution of zircon showed that samples with its content in the interval 73.5–164.5 g/t occur most often (Table 1, Fig. 1). The arithmetic mean zircon content in the granites of the central part of the massif is 340 g/t. The coefficient of variation is 19.5%. In the marginal facies of the intrusion the amount of zircon decreases and averages 167.5 g/t. The coefficient of variation is 14.4%.

Table 1. Variation diagrams of zircon content and of the distribution of its crystal types in the rocks of the Kamensky massif. Figs. 1–2. Variation diagrams of zircon content in biotite granites of the first phase (Fig. 1) and in rocks of different intrusive phases (Fig. 2). Figs. 3–5. Variation diagrams of the distribution of zircon crystal types in biotite granites of the central part of the massif (Fig. 3), in biotite granites along the direction from the contact with the enclosing rocks toward the center of the body (Fig. 4), and in rocks of different intrusive phases (Fig. 5). Fig. 6. Variation diagram of the distribution of colorless and colored zircon crystals in the rocks of the massif.

In the rocks of the late differentiates the average zircon content decreases successively. Thus, in medium-grained two-mica granites it is 58.4 g/t; in aplite-like granites, aplites, and pegmatites, respectively, 14.5, 15.9, and 3.8 g/t (Table 1, Fig. 2).

Zircon in the rocks of the massif is represented by crystals of six morphological types: 8, 9, 10, 12, 15, 19* (Fig. 1). In addition to the named types, crystals with a more complex development of simple forms are present, as well as transitional varieties from one type to another. Rounded forms of zircon grains are sometimes observed, formed as a result of dissolution by postmagmatic solutions [1]. Individuals of types 9, 15, and 19 are exceptionally rare and were encountered in biotite coarse-grained granites. The crystals are distributed unevenly within the massif. The greatest fluctuations in content are characteristic of multifaceted crystals of types 8 and 10. Thus, the number of type 8 crystals varies from 25 to 90%, and that of type 10—from 5 to 65%. Type 12 crystals are distributed more or less uniformly; their content fluctuates from 6 to 12% (Table 1, Fig. 3). With distance from the contact of the granite body with the enclosing rocks toward its central part, approximately over a distance of 500 m, the content of type 8 crystals regularly decreases, whereas the number of individuals of types 10 and 12, on the contrary, increases (Table 1, Fig. 4). In two-mica granites, aplite-like granites, and aplites, zircons of type 8 predominate, while individuals of types 10 and 12 occur in subordinate amounts; in aplites the latter are not established at all (Table 1, Fig. 5).

Fig. 1. Types of zircon crystals from rocks of the Kamensky intrusive massif
a {100}; m {110}; S {111}; e {101}; r {311}; u {331};

Zircons in the rocks of the massif are colorless, light yellow, light brown, brown, and dark brown. In addition, zonal crystals are noted, in which the color changes from zone to zone in the following sequence: the central zone is brown to dark brown, the next is light yellow, and the outer zone is colorless. Such a change in the coloration of zones indicates later crystallization of the colorless zircons. The distribution of zircons of different colors throughout the massif is uneven.

Among type 8 crystals, colorless ones predominate. Their content in biotite granites reaches 65%; toward the periphery of the massif their number increases to 95%. In the rocks of the late differentiates, colorless zircons of this type occur less often, and the number of colored ones, on—

* In the literature there is still no unified summary of the crystallomorphological types of zircon; therefore each researcher creates “his own” crystal types. In our opinion, the morphological series of E. A. Ermolaeva should be taken as the basis for the typification of zircon crystals [2].

on the contrary, increases. A similar pattern in the change of zircon coloration in the direction from rocks of the early phases to the late phases has also been established for crystals of type 12 (Table 1, Fig. 6). Polyhedra of type 10 are mainly brown and dark brown. Colorless varieties were recorded in isolated samples from aplitoid granites.

The morphology of colorless and colored crystals of each of the distinguished types, both in rocks of the same petrographic composition and in those of different composition, is characterized by its own features. To reveal these features more clearly, the author applied a statistical analysis of the degree of elongation of the crystals. The results of the statistical processing are presented in the form of diagrams (Table 2, Figs. 1–8). As can be seen from the diagrams, the elongation of colorless and colored crystals is different. Thus, the coefficient \(l^{*}\) of colorless crystals of type 8 in the granites of the central part of the intrusive body ranges from 1.2 to 8.2. Crystals with elongation in the interval from 2.6 to 2.9 occur most often

Table 2. Variation diagrams of the elongation values of zircon crystals in rocks of the Kamensky massif.

* To avoid frequent repetitions, the value of crystal elongation in the text is called the coefficient \(l\) \(\left(l=\frac{\text{length}}{\text{width}}\right)\).

(table 2, Fig. 1) (average 2.7). The coefficient \(l\) of colored individuals of this same type varies from 1.7 to 8.6. The most widespread are crystals with elongation from 2.1 to 2.9 (Table 2, Fig. 2); the average value is 3.0. In the granites of the endocontact zone, the ratio of crystal length to width fluctuates within comparatively narrow limits. For example, the elongation of colorless crystals of type 8 in these rocks varies from 1.6 to 3.7, with an average value of 2.5. The coefficient \(l\) of colored zircons fluctuates within the range 1.6–4.2 (average 2.8). A similar regularity in the variation of elongation has also been established for crystals of types 10 and 12. The variation in elongation of these crystals in biotite granites is shown in the diagrams (Table 2, Figs. 3, 4).

In the transition from rocks of early phases to rocks of later phases, the elongation of colorless and colored crystals of the types considered decreases regularly. Moreover, the sharpest decreases in the coefficient are characteristic of its maximum value. The average and minimum values undergo insignificant fluctuations (Table 2, Figs. 5, 6, 7, 8).

The average sizes of the crystals are given in Table 3. Analysis of the table shows that colorless crystals are larger than colored ones. In the granites of the marginal zone of the massif, as well as in two-mica, aplitoid granites and aplites, the sizes of zircon crystals decrease. Colored individuals of type 10 are somewhat not subject to this regularity; their size increases in the biotite granites of the endocontact and in later formations.

Table 3

Average sizes (in mm) of zircon crystals in the rocks of the Kamensky massif

Concluding the description of the morphology of the crystals, it is necessary to dwell on certain features of the surface of their faces and on the nature of inclusions in the zircons described.

On the surface of the crystal faces in all varieties of rocks, stepped relief, roughness, and other defects of the faces are clearly distinguishable. The edges are smoothed or serrated. Dark-colored varieties of zircon are cracked. On the faces \(\{110\}\), and very rarely \(\{100\}\), intergrowths of biotite are observed, and in the form of fine dust—ilmenite and magnetite. It is characteristic that the intergrowths are observed mainly on brown and dark-brown individuals. A similar phenomenon was noted by A. P. Berzina and V. I. Sotnikov (1963). They noted the wide distribution of zircons with intergrowths and their confinement to granites that had undergone metasomatic alteration in the form of potassium feldspathization and greisenization.

Unlike the colored ones, the colorless crystals have even, rectilinear edges; their faces are smooth and lustrous. Colorless individuals are often observed in parallel intergrowths and overgrowths with colored ones. It is characteristic that such intergrowths and overgrowths are typical of zircons from biotite granites; they have not been encountered in rocks of the later differentiates.

Inclusions in zircon are rare. In colorless crystals, gas bubbles, gas–liquid inclusions, and also small crystals of light-colored zircon, flakes of biotite, and plates of ilmenite are sometimes observed. Gas bubbles are, as a rule, confined to the central part of the crystal, and in zonal individuals—to the boundaries of individual zones. Along with the inclusions noted, inclusions of tubular form with sharp, rectilinear boundaries are often encountered in colorless crystals. Usually they are oriented at some angle to the direction of the fourth-order symmetry axis. The nature of such tubular formations remains unclear for the time being. It can only be noted that they occur more often in zircons from aplite-like granites and aplites.

Thus, as a result of the study of accessory zircon in the Kamensky granitoid massif, it appears possible to draw the following general conclusions.

1. Zircon is distributed extremely unevenly in the granites. The greatest fluctuations in the content of this mineral are characteristic of coarse-grained biotite granites. In rocks of the endocontact zone of the massif, the amount of zircon varies within narrower limits, and its content is lower than in the granites of the central belt. In rocks of later formations—two-mica granites, aplite-like granites, aplites, and pegmatites—the amount of zircon decreases regularly.

2. In terms of the features of the combination of simple forms, zircon crystals belong to types 8, 9, 10, 12, 15, and 19. These types are distributed unevenly in the rocks of the massif. Sharp fluctuations in content are characteristic of types 8 and 10; the amount of type-12 zircons changes within insignificant limits.

3. A definite regularity has been revealed in the nature of the distribution in the rocks of the massif of each of the morphological types. Thus, type-8 zircons predominate in the biotite granites of the endocontact and in rocks of later formations; types 10 and 12 are more often encountered in the biotite granites of the central zone of the intrusive. In the aplites, the latter crystals have not been established.

4. In the granites of the massif, zircons are colorless, light yellow, light brown, brown, and dark brown. The coloration of crystals of one and the same type is not the same in the rocks of the massif. In biotite granites, zircons of types 8 and 12 are predominantly colorless or light yellow. In rocks of subsequent differentiates they acquire brown and dark-brown coloration. Individuals of type 10 in all rocks of the massif are brown and dark brown.

5. The presence of zonal crystals with a dark-colored inner zone and a colorless outer zone indicates a later separation of the colorless individuals. Taking into account the distribution of colorless and colored crystals of different types, it may be assumed that in the biotite granites the first to crystallize were brown and dark-brown zircons of type 10, then crystals of type 12 were separated, and the last were colorless zircons of type 8.

6. The physicochemical conditions of crystallization of colorless and colored zircons of one and the same type are different, as indicated by the different values of their elongation. Especially sharply different are the conditions

crystallization in the biotite granites of the central and endocontact zones and in the two-mica granites. In aplitoid granites and aplites the physicochemical conditions of crystallization are similar, since the deviations in zircon elongation in these rocks are insignificant.

1. The absolute sizes of colorless zircons are greater than those of colored ones. The average sizes of crystals decrease in the granites of the marginal zone of the massif and in the rocks of late formations. The decrease in zircon size is fully consistent with the general decrease in the grain size of these rocks. Colored zircons of type 10 are somewhat not subject to this regularity; their sizes, on the contrary, increase in the biotite granites of the endocontact part of the intrusive body and in the rocks of late differentiates.

REFERENCES

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2. Ermolaev V. A. Features of the typomineralogy and typomorphism of accessory zircon in magmatic complexes of different ages in Eastern Transbaikalia. In: Materials on Geology and Useful Minerals. Notes of Siberia. Tomsk University Press, 1961.

3. Lyakhovich V. V. Accessory minerals as indicators of geological processes. In: New Methods in Mineralogy and Petrography and the Results of Their Application. Gosgeoltekhizdat, 1963.

4. Lvov B. K. Petrology, Mineralogy, and Geochemistry of the Kochkar District (Southern Urals). Leningrad State University Press, 1965.

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