Variation of Accessory Mineral Assemblages at the Contact Between Granites of the Turochak Intrusion and Dioritoids of the Ulmen Intrusions (Gorny Altai)

A. I. Bazhenov, T. I. Poluektova

(Presented by Professor A. M. Kuzmin)

Accessory minerals of individual granitoid massifs of Gorny Altai have in recent years been studied intensively [1, 2, 3, 5].

The purpose of the present work is to clarify the mineralogical transformations in dioritoids under the influence of the younger intrusion of the Turochak granites. Sampling of the granites of the Turochak massif was carried out at intervals of 1 km from the contact toward the center of the body; the dioritoids of the Ulmen intrusive body were studied in the direction away from the contact with the granites (Fig. 1). The weight of all samples was 2.5 kg. The quantity of accessory minerals in the crushed material was counted by the “track” method.

The area described is situated in the middle course of the Ulmen River, a right tributary of the Biya River. Its geological structure includes Lower and Middle Cambrian formations, combined into the Manzherok \((Cm_1^1mn)\), Kayanchin \((Cm_1^2kn)\), and Kaim \((Cm_2^1km)\) suites. Among intrusive formations, the granites of the Turochak pluton—its northeastern part—and the dioritoids of the Ulmen massif are widely developed; their ages are determined respectively as pre-Devonian and Salairian.

The Turochak granite massif is a large batholith. Its northeastern contact dips gently to the northeast, as indicated by the comparatively great width of the contact aureole. In chemical composition the granites occupy an intermediate position between alkaline and calc-alkaline granites according to Daly [4], differing from alkaline granites by oversaturation in \(\mathrm{Al_2O_3}\), and from calc-alkaline granites by somewhat lower numerical values of the characteristics \(a\) and \(b\). These granites consist of plagioclase—31.7%, microcline—33.1%, quartz—21.2%, biotite—2.9%, and accessories—1.1%. The amount of plagioclase in the endocontact zone of the intrusion decreases as the contact with the hornfelses is approached (Fig. 2); its composition changes from oligoclase No. 20–25 in the central part of the massif to andesine No. 40–32 in the marginal zone. Microcline occurs in xenomorphic, more rarely idiomorphic grains in two generations. Microcline 1 is the earliest and is widespread in the central part of the batholith in the form of microcline-perthite, in which albite segregations have a fine-veinlike form. The mineral is usually pelitized. Microcline 2 is developed in the endocontact belt of the intrusion, where it composes fresh grains with clearly ex-

Fig. 1. Schematic geological structure of the area: 1 — modern alluvial deposits; 2 — Kaim Formation: conglomerates, tuffs of intermediate and basic composition, porphyrites; 3 — Kayanchinskaya Formation: limestones; 4 — Manzherok Formation: siltstones, shales, tuffs, porphyrites, limestones; 5 — granites of the Turochak massif; 6 — dioritoids of the Ulmen massif; 7 — tectonic faults; 8 — sampling points and sample numbers.

lattice. The biotite content regularly increases from the periphery toward the center (Fig. 2). Its total ferruginosity decreases in the same direction (Fig. 2). Biotite is present in two generations. Biotite 1 forms flakes, has the pleochroism usual for it, and is distributed predominantly in the center of the massif. Biotite 2 is later and occurs mainly in the endocontact zone. It forms a rim around magnetite, penetrates along cleavage cracks into plagioclase, occurs as veinlets in microcline, and fills intergranular spaces in quartz. Quartz always has cloudy extinction.

Fig. 2. Variation diagram of the content of biotite and plagioclase and of the total ferruginosity of biotite in the endocontact zone of the Turochak pluton: 1 — plagioclase, 2 — total ferruginosity of biotite

The Ulmen diorite massif is a stock elongated in a sublatitudinal direction and is cut off in the west by the Turochak massif. In their chemistry, the dioritoids are close to the average composition of olivine-free gabbros according to Daly [4], differing from the latter in lower contents of Al₂O₃ and CaO. They are composed of plagioclase — 59.5%, hornblende — 23.1%, orthoclase — 3.2%, diopside — 11%, and accessories — 3.2%. The composition of plagioclase changes from No. 27 at the contact with the granites to No. 80–85 with increasing distance from them. The amount of hornblende, with increasing distance from the contact with the Turochak granites, decreases from 49% to 13% at the most distant point. Two varieties of amphibole are distinguished in the rock. Hornblende 1 is brown, apparently earlier, and occurs in hypidiomorphic, more rarely allotriomorphic, grains; \(N_g = 1.65\text{–}1.70\). Hornblende 2 is green, forms allotriomorphic grains, and replaces brown hornblende and diopside, forming a peculiar rim around the latter. The amount of diopside decreases with increasing distance from the contact with the granites. Microcline appears near the contact with the Turochak granites.

The list of accessory minerals in the granites and dioritoids is given in Table 1.

Zircon is one of the most characteristic and widespread accessories of the granites. Its amount decreases with increasing distance from the con-

Content of accessory minerals in the intrusive rocks of the area

Table 1

* Sphene.

Continuation of Table 1

*) Numerator—sphene content; denominator—leucoxene content.

contact with the hornfelses toward the center of the intrusive body (Fig. 3). The dioritoids of the Ulmen massif are poorer in zircon, and it is not found in all pan samples. An increase in its content toward the contact with the granites is observed—21 g/t—and a decrease to tenths of a g/t with distance from it, with the exception of two samples (P-18a, P-20), in which the amount of zircon increases, respectively, from 4.2 to 4.4 g/t.

Fig. 3. Variation diagram of the content of accessory zircon, apatite, and sphene in the endocontact zone of the Turochak intrusion:
1—zircon, 2—apatite, 3—sphene

The typomorphism of zircon crystals was determined according to the method of V. A. Ermolaev [5]. Of the 25 crystallomorphological types described by this author, four have been established in the Turochak granites: the eighth, ninth, tenth, and twelfth types (Fig. 4). Crystals of the 8th type are weakly flattened along the faces (110); the faces {311} are weakly expressed. The surface of the crystals is pitted. With distance from the contact of the intrusive body, the crystals become more perfect. The sizes of individuals of this type, even in one and the same pan sample, vary within wide limits, but their average size in different samples is consistent. In the endocontact zone of the massif, zircon crystals are larger than in the central zone. With distance from the contact, the elongation*) of the crystals gradually decreases—

*) By elongation \((P)\) of a crystal is meant the ratio of its length \((l)\) to its width \((m)\):

\[ P = \frac{l}{m}. \]

decreases from 2.7 to 2.1 (Fig. 5). Zircons of the 10th crystallographic type are strongly fractured; the faces are corroded along (100), with numerous black encrustations confined to the periphery of the faces. With distance from the endocontact toward the center of the intrusion, they are characterized by a greater isometry of individuals. Crystals of the 9th type are slightly flattened along (100); the {311} faces are developed incompletely; on the faces there are numerous black encrustations; the surface of the faces is uneven. Elongations of the crystals vary from 2.2 to 2.7. The twelfth crystallographic type is distinguished by an abundance of faces that bear clear traces of inhibited growth. The surface of the faces is smooth and lustrous. Elongations of individuals vary from 0.8 to 6.5.

Fig. 4. Types of crystals of accessory zircon in the granites of the Turochak and dioritoids of the Ulmen intrusions. The numbers correspond to the crystallomorphological types of zircon of V. A. Ermolaev.

In the dioritoids, the 9th and 8th types are most widespread; the 10th is encountered much more rarely. Strong fracturing and “fusion” of the crystals are a distinctive feature of types analogous to those from the granites. Numerous black encrustations are present on the faces of the crystals; individuals from the endocontact parts of the massif are especially rich in them. Parallel intergrowths along the (010) faces of two crystals are not uncommon. The size of the intergrown individuals varies. The larger one is crushed; the second is smaller, with even, smooth faces. The elongation of crystals of the 9th type throughout the massif is approximately consistent and varies within

Fig. 5. Variation diagram of the elongation of accessory zircon crystals of the 8th type and the β-radioactivity of zircon: 1—elongation of zircon crystals of the 8th type; 2—β-radioactivity of zircon.

from 1.7 to 2.5; at the point most distant from the contact with the granites it is 1.1. Crystals of types 10 and 8 are comparatively rare. Their morphology is analogous to individuals of the corresponding types from the granites.

The coloration of crystals of the mineral described changes in the granites from colorless to dark brown. Each type has its own specific color (Table 2). In the marginal part of the intrusive body, type-8 zircons have a brown coloration; with increasing distance from the contact, colorless ones appear, with light-brown predominating; finally, in the most distant part, colorless and light-yellow ones predominate, with brown ones entirely absent. In two samples taken from the contact with hornfelses, zoned zircons with a more darkly colored center were encountered. Crystals of type 10 are for the most part brown; only in two samples were 10% colorless and 10% gray recorded. Type 9 is characterized by a light-brown coloration, with a subordinate number of brown individuals. All encountered crystals of type 12 are colorless.

In the dioritoids, gray and brown individuals are most widespread (Table 2). At the contact with the granites, 80% of type-9 crystals are dark brown, 20% are light-colored; with increasing distance from the contact, the content of lightly colored ones increases. Farther along the section, brown individuals appear again, and at the points farthest from the contact the chief role belongs to gray and light-brown crystals. Only at points located at some distance from the contact with the granites (P-16, P-6, P-7, P-17) do brown, gray, and light-brown crystals predominate.

In ultraviolet rays, the mineral described from the contact with hornfelses luminesces with a brownish light; in samples from the central part of the Turochak massif it glows with yellow and light-yellow light. Zircons from the dioritoids luminesce in brown and orange tones, more rarely in light-orange and light-yellow tones.

Zircon crystals contain diverse inclusions; individuals of type 8 are richest in them. In crystals of types 9 and 10, because of their dark color, inclusions are not visible. Among the inclusions, dark drop-shaped ones are distinguished, evidently gas-liquid, concentrated in the center of the crystal. Inclusions of tubular form, brown and green in color, are oriented parallel and subparallel to the fourth-order axis of symmetry. Often, in two parallel intergrowths of crystals, one individual contains both gas-liquid and tubular inclusions, while in the other they are absent. In the dioritoids, crystals of type 9 are rich in inclusions predominantly of tubular form, parallel to one another and oriented at a small angle to \(F_1\).

At different points of the Turochak massif the \(\beta\)-radioactivity of zircon is not the same. In rocks of the endocontact it has elevated activity (0.88 conventional units) in comparison with samples distant from the contact (0.003 conv. units). These data indicate a general tendency toward a decrease in the radioactivity of the mineral with increasing distance from the contact (Fig. 5). Zircons from the dioritoids have very low radioactivity; moreover, at the contact with the granites it is less than at points distant from the latter.

In thin sections of the garnites, colorless zircons are observed as inclusions in plagioclase and orthoclase. Dark-colored ones occur in biotite and are surrounded by intense pleochroic halos. In the dioritoids it is concentrated in brown hornblende, forming a light-yellow, weakly pleochroic aureole.

Apatite is present in all crushed samples of granites and dioritoids. Its amount varies from 0.32 g/t in the central part of the grani-

Coloration of accessory zircon crystals

Table 2

A — colorless; B — light yellow; V — light ocher; G — gray; D — brown; E — reddish-brown; Zh — dark brown.

of the Turochak intrusion to 125 g/t in its marginal zone (Fig. 3).

In the endocontact zone the crystals are prismatic, short-columnar, and represent a combination of the simple forms $\{10\bar{1}0\}$, $\{10\bar{1}1\}$, $\{0001\}$ (Fig. 6). Elongation is usually 2–1.7. The color is gray or brown owing to a film of iron hydroxides coating it. The surfaces of the individuals are pitted; abundant overgrowths of small magnetite grains are often noted on the faces; the presence of dark-brown gas–liquid inclusions is characteristic, as a rule concentrated in the center of the crystal. In ultraviolet rays apatite luminesces with a bright-blue light.

Fig. 6. Crystals of accessory apatite. Upper row—from granites of the Turochak intrusion; lower row—from dioritoids of the Ulmen massif

In the dioritoids, apatite is the most widespread accessory mineral; its amount varies over a wide range, reaching a minimum (95.2 g/t) at the contact with the Turochak granites and a maximum (206 g/t) in the central part of the intrusion. Quantitative mineralogical counts in thin sections show that its content at individual points of the massif rises to 2–2.5%, which allows it to be assigned to the secondary rock-forming minerals. It occurs in the rock in two varieties. The first is widely distributed at the contact with the granites and is represented by milky-white, large, short-columnar crystals with a combination of the simple forms $\{10\bar{1}0\}$, $\{11\bar{2}1\}$, $\{0001\}$ (Fig. 6); rounded grains with poorly distinguishable crystallographic forms are not uncommon. The surface of the faces is strongly corroded, with coarse vertical striation; the crystals bear traces of dissolution, which gives them a rounded form. In the contact zone, zoned crystals with a darker central zone are not uncommon. Among the inclusions, rodlike brown and yellow-brown ones and dark, almost black gas–liquid inclusions are distinguished. The second variety of the mineral described is widely developed in areas remote from the contact with the granites and is represented by water-clear, long-prismatic crystals with rare gas–liquid inclusions.

In thin sections of the granites, apatite occurs as numerous inclusions in biotite, and sometimes in orthoclase. In the dioritoids, both of its varieties are observed in all rock-forming minerals, but it is especially abundant in hornblende, where they are arranged as chains of crystals oriented along cleavage cracks.

Sphene and leucoxene are distributed as follows: the amount of sphene decreases from the endocontact zone of the Turochak pluton toward its center. The predominant mass of sphene in the granite is represented by light-yellow, pink, translucent crystals with lustrous faces and sharp interfacial angles. In the contact zone, brown flattened individuals occur. Fine black inclusions, apparently of ilmenite, are quite common. Titanite is most widespread in the dioritoids, where it is recorded everywhere in amounts from 7 to 196 g/t. Its minimum contents are observed at the contact with the Turochak massif, and the maximum in the center of the Ulmen intrusive. The habit of the crystals is fairly uniform—envelope-shaped flattened individuals; in the contact zone they occur as irregular grains. Sometimes it forms rims around ilmenite; as inclusions it is observed in orthoclase.

Leucoxene is a product of sphene decomposition and is present in all heavy-mineral concentrates of the dioritoids. Its content increases toward the contact with the granites, and the amount of sphene correspondingly decreases. This mineral is represented by rounded light-yellow grains. Sometimes relicts of brown sphene can be observed inside.

Tourmaline—schorl—is recorded in increased amounts in granite samples from the contact with hornfelses. In the dioritoids it was encountered in one heavy-mineral concentrate, spatially distant from the contact with the granites.

Fluorite is present as isolated irregular, rounded, water-clear grains in granite samples. In the dioritoids, varieties colored in greenish-violet tones of varying intensity occur. The emerald-green fluorites from the contact of the granite with the hornfelses are somewhat distinctive. On individual crystal fragments, octahedral faces with coarse striation parallel to the edges of the octahedron are preserved.

Magnetite is one of the most widespread accessory minerals. Its amount in the granites of the marginal part of the massif is considerably less than in the central part: it ranges from 23 to 480 g/t, as against 228–1172 g/t in the center of the intrusive. The sizes of its octahedral crystals vary from 0.14 to 0.27 mm, with the larger ones tending toward the center of the pluton. In the dioritoids, the magnetite content varies over wide limits, reaching a maximum (3792 g/t) at the contact with the Turochak granites and a minimum (20 g/t) at a considerable distance from them. A distinctive feature of the mineral described in these rocks is the rounded shape of the grains. In samples from the contact with the granites, about 20% of the crystals are represented exclusively by spherical formations. With distance from the contact, the mineral acquires its characteristic octahedral faceting. In thin sections of the granites, two varieties of magnetite are clearly distinguished. The first, earlier one forms idiomorphic inclusions in plagioclase and xenomorphic ones in K-feldspar. The second variety forms a fine dusting in biotite. In the dioritoids it is likewise recorded in two varieties. The early one fills intergranular spaces, while the late one forms a dusting in green hornblende.

Ilmenite is a relatively rare mineral, present as irregular grains, rarely as tabular crystals included in K-feldspar and plagioclase.

Pyrite, in cubic, more rarely octahedral and pentagondodecahedral crystals, was encountered in granite samples from the near-contact belt. In the dioritoids it is recorded almost constantly. In one sample

in granites from the contact with hornfelses, a flattened crystal of wolframite of reddish-brown color was found. Among the secondary accessory minerals, epidote is widespread everywhere; its amount in the dioritoids increases toward the contact with the granites.

The material presented makes it possible to draw the following conclusions regarding the transformation of the material composition of the dioritoids under the influence of the Turochak granite intrusion.

1. An extremely uneven distribution of rock-forming and accessory minerals in the dioritoids. 2. An increased content of dark-colored minerals with approach to the contact with the granites. 3. The presence, near the contact with the granites, of two generations of hornblendes. 4. Accessory minerals, as a rule, bear traces of dissolution, especially increasing at the contact with the granites. 5. The presence of two generations of apatite and magnetite, one of which is syngenetic with the parent rock, the other epigenetic. 6. The material composition of the granites of the endocontact zone changes quite regularly with distance from the contact toward the center: the amount of plagioclase in the granites increases, while the content of biotite and its total iron content decrease. 7. The amount of accessory minerals (zircon, apatite, sphene) in the Turochak granites decreases from the periphery of the intrusion toward the center. Magnetite is in an inverse relationship. 8. In the endocontact belt of the granite massif, the accessory minerals are strongly corroded and fractured; with distance from the contact they are distinguished by good preservation and distinct crystallographic faceting. 9. In the dioritoids at the contact with the granites, all accessory minerals are strongly corroded; with distance from the contact they acquire more or less distinct crystallographic faceting.

REFERENCES

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2. A. I. Bazhenov. Change in the radioactivity of accessory zircon in vertical and horizontal sections of the Elekmonar multiphase granitoid massif (Gorny Altai). In: Collected Materials on Mineralogy, Petrography, and Useful Minerals of Western Siberia. Tomsk State University Press, 1962.

3. A. I. Bazhenov. On the question of changes in the radioactivity of accessory sphene, using the rocks of the Elekmonar multiphase granitoid massif (Gorny Altai) as an example. Izv. TPI, vol. 121, 1963.

4. R. O. Daly. Igneous Rocks and the Depths of the Earth. ONTI, 1936.

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6. V. V. Lyakhovich, B. P. Zolotarev et al. Accessory minerals in the granitoids of Gorny Altai. In: Questions of Mineralogy, Geochemistry, and Genesis of Deposits of Rare Elements, Academy of Sciences of the USSR, issue 2, 1959.

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