Izykh Gabbroid Massif

(Kuznetsk Alatau)

G. A. Ivankin, D. I. Tsarev

(Presented by the Department of General Geology)

The gabbro massif described below is located on the left side of the valley of the Bely Iyus River opposite the settlement of Mendolya. Its rocks form the upper part of the mountain of the same name, the summit of which rises above the surrounding surface in the form of a monadnock.

The peculiar structure of the pluton, the features of the petrographic composition of the rocks, and the degree of their alteration have not yet been covered in print. Taking this into account, in the present article the authors report the principal results of their investigations.

The pluton can be traced from the valley of the Bely Iyus River through Mount Izykh in a SW direction beyond the limits of the area studied. It intrudes the rocks of the carbonate-amphibolite sequence and, together with them, forms a comparatively large roof pendant of the Ulen-Tuim (Beloiussk) granitoid massif (Fig. 1). The Izykh pluton has a lopolith-like form elongated in a NE direction and is layered. The clearly expressed banding of the rocks composing it makes it possible to refine the structure of the massif, the main features of which are shown in Fig. 1. In addition, we emphasize that the banding of the rocks has a very steep dip (at angles of 75–80°) to the NE along the SE boundary of the body and an opposite dip in its NW part. Southwest of the summit of Mount Izykh, the banding dips to the NE at angles of 60–65°.

Among the rocks composing the pluton, the most widespread are the following varieties: 1) olivine gabbro, saussuritized and amphibolized; 2) amphibolized and cataclased gabbro; 3) amphibolized melanocratic gabbro; 4) amphibolized quartz gabbro; and 5) anorthosites. In addition, this same complex probably includes the garburgites that form dikes cutting the gabbroids. Table 1 gives a brief characterization of the noted rock varieties.

The common characteristic features of the Izykh gabbros are that they are all metagabbros; the hornblende occurring in them is always secondary, replacing pyroxene, has a bluish-green color, and its grains often have splintery outlines; \(2V\) ranges from \(-76\) to \(-86^\circ\), \(CNg\)—from 15 to 23°; the primary dark-colored minerals are represented by augite (\(CNg = 44^\circ\), \(Ng - Np = 0.015\)), and in the olivine variety by augite, hypersthene, and magnesian olivine (\(Ng - Np = 0.035\)); the plagioclases are labrador-bytownite from No. 70 to No. 76.

Amphibolitized and cataclastic, and melanocratic amphibolitized gabbro are the most widespread. Quartz amphibolitized gabbro and anorthosites are more limited in distribution and occur as separate irregularly shaped areas and bands.

Fig. 1. Geological map of Mount Izykh. 1 — Quaternary deposits; 2 — marbles of the Karateg sequence; 3 — tonalite-diorites of the first phase of the Ulen-Tuim pluton; 4 — granodiorites of the first phase of the Ulen-Tuim pluton; 5 — granites of the second phase of the Ulen-Tuim pluton; 6 — gabbroids of Mount Izykh with elements of prototectonic banding; 7 — melanocratic varieties of the gabbroids of Mount Izykh; 8 — contact amphibolite-gneisses; 9 — dikes of harzburgites; 10 — hybrid gabbrodiorites; 11 — disjunctive faults.

In addition to the rocks described, which have preserved an intrusive appearance, the massif includes varieties that acquire all the features of metamorphic formations. For the petrographic characterization of these rocks, which we call contact amphibolite-gneisses, we give a generalized description of them.

Contact amphibolite-gneisses are observed in the near-contact part of the gabbroids with the rocks of the Ulen-Tuim intrusive complex and are products of the contact effect of the latter on the Izykh gabbro and on the rocks of the Karateg sequence enclosing the gabbro. In appearance the amphibolite-gneisses resemble fine-grained gneiss-like diorites. Their color is usually gray. Previous investigators called them hornfels. However, besides typical hornfels, which contain about 5–10% femic minerals, amphibolite-gneisses predominate in this group of rocks. The principal mine-

Brief petrographic characteristics of the Izyk gabbroids
Table 1

minerals composing the amphibolite-gneisses are: 1) plagioclase—30–60% (No. 40 to No. 60, in relicts of primary grains—up to 71); 2) hornblende—30–50% ($2V$ from $-72$ to $-84^\circ$, $CNg = 17$–$19^\circ$); 3) diopside—up to 40% ($Ng - Np = 0.028$, $CNg = 39^\circ$, $2V = 57^\circ$); 4) quartz—10%, accessories (magnetite, sphene)—5%; 5) rarely biotite. The rock structure is gneissoid; under the microscope it is nematoblastic (Fig. 2a). The amphibolite-gneisses border the gabbroids of Mount Izykh from the northwest and southeast in belts up to 700 m wide. Elements of their gneissosity are arranged parallel to the contacts of the gabbroids with the Ulen-Tuim granitoids and parallel to the structures of remnants of marbles of the Kharates sequence; they dip predominantly beneath the gabbroid massif at angles of 70–85°, and rarely show reverse dips. Individual linearly elongated bodies of amphibolite-gneisses occur within the gabbroid massif, envelop gabbro boudins, and penetrate into fractures. In places, processes of metasomatic granitization of the amphibolite-gneisses are observed, expressed in banded replacement of the gneiss substrate by granite in accordance with its texture. These data indicate that the amphibolite-gneisses possessed plastic flow during their formation and, being plastic, were readily subjected to processes of granitization. It is interesting to note the fact that amphibolite-gneisses are observed only in the near-contact zone of the gabbroids with the first syenite-diorite phase of the Ulen-Tuim intrusion; they are absent at the contact with the second, granite phase.

Of the dikes cutting the metagabbroids of Mount Izykh, dikes of ultrabasic rocks and olivine gabbros are of greatest interest. We shall attempt to focus attention on them, and shall not deal with the other dike formations, as less important from our point of view.

Dikes of ultrabasic rocks are composed of serpentinized harzburgites; they have a thickness of 2–3 m and, with respect to the structures of the metagabbroid massif, are discordant. The color of the rock is black; the structure is fine- to medium-grained; the texture is massive or weakly banded; traces of coarse jointing are observed. Brown crusts form on weathered surfaces. Under the microscope a mesh and cross-fibrous structure is observed. Mineralogical composition: 1) olivine—5% ($Ng - Np = 0.036$); 2) relict grains of rhombic pyroxenes replaced by serpentine and tremolite (enstatite—$Ng - Np = 0.010$, hypersthene—$Ng - Np = 0.012$, $2V = -72^\circ$); 3) serpentine—45%; 4) tremolite—up to 45% ($Ng - Np = 0.023$); 5) ore minerals—up to 5%; 6) green spinel—0–1%.

Dikes of olivine gabbro have been encountered in the northeastern part of the Izykh massif and beyond its limits. In the first case they cut the metagabbroids; in the second, the granitoids of the Ulen-Tuim pluton, and consequently are comparatively young formations among the intrusive rocks of the district. The thickness of the dikes reaches 6 m. The minerals composing these gabbros are characterized by freshness. The rock has a dark-gray to black color, a medium-grained structure, a massive texture, and consists of 45% plagioclase No. 70, 50% mafic minerals (pyroxene, hornblende, olivine), and 5% magnetite. The pyroxenes are represented by augite and, to a lesser degree, hypersthene. The olivine content is 3–4 grains per thin section. Almost all pyroxene grains have a rim of green hornblende. In all probability, amphibolization of the pyroxenes occurred during crystallization of the magmatic melt. The microstructure of the gabbro rock is ophitic, and in individual areas—poikilo-ophitic.

For the geochemical characterization of the gabbroids and contact amphibolite-gneisses, Table 2 is presented, in which the results of spectral analyses are summarized. By comparing the most frequently occurring-

the content of chemical elements confirms the genetic affinity of the gabbroids and amphibolite-gneisses. The observed higher content of titanium and zinc is probably explained by the influence of superimposed processes in the contact zone.

Table 2

Content of chemical elements in the rocks of the Mount Izykh massif according to the results of spectral analysis of hand specimens

Comparing the quantitative composition of the oxides of the contact amphibolite-gneisses with the average composition of the most typical gabbro samples (Table 3), we see that the former contain more SiO₂, total iron, TiO₂, and alkalis, and are depleted in calcium and magnesium. It follows from this that, as a result of the contact action of the Ulen’-Tuim intrusion on the gabbroids of Mount Izykh, silica, titanium, iron, and alkalis were introduced into the latter, while calcium and magnesium were removed from them.

In tectonic terms, the Mount Izykh massif is situated in the axial part of the Izykh syncline, which forms part of a larger structure—the horst-anticline; it occupies a subconcordant position with the structures of the enclosing rocks: its southwestern boundary is only slightly cross-cutting relative to the latter, while the northwestern and southeastern boundaries extend along the strike of the marbles of the Karater suite. Among the elements of prototectonics, banding, linear flow structures, and jointing cracks are manifested. At the same time, it is not always possible to distinguish primary banding from secondary banding that arose as a result of metamorphic reworking. Both generally have one and the same strike, parallel to the contacts, usually dip toward the massif at angles of 65–80°, and are expressed by the alternation of melanocratic metagabbro with ordinary metagabbro and anorthosites. Among the prototectonic jointing cracks, the longitudinal ones are most clearly distinguishable; the others are obscured by secondary fracturing. The intrusion of various dikes and normal-fault–strike-slip displacement are associated with secondary fracturing. Linear flow structures are caused by the mutually parallel orientation of elongated grains of dark-colored minerals.

The age of the metagabbroids of Mount Izykh is very difficult to determine. The massif cuts through the Karatag sequence \((Pt_3)^1\) and is cut by the Ulen-Tuim intrusion. We do not have other factual materials that would make it possible to narrow the age limits of the pluton. The strong degree of metamorphism practically excludes the possibility of using methods for determining absolute age.

Table 3

Chemical composition of rocks of the Izykh intrusive complex

Numerical characteristics according to A. N. Zavaritsky

1, 4, 6 — amphibolitized and cataclased gabbro.
2 — olivine gabbro, saussuritized and amphibolitized.
3, 5 — melanocratic gabbro, amphibolitized.
8 — harzburgite.
7 — contact amphibolite-gneiss.

Thus, the gabbroid rock complex of Mount Izykh is characterized by a number of specific features distinguishing it from other intrusive formations of the region of the middle course of the Bely Iyus River. Judging from the chemical composition (see Table 3 and the petrochemical diagram, Fig. 3), the rocks of the complex show the greatest similarity to the gabbro of Mount Kogtakh, located 20 km northeast of Mount Izykh, but they differ in a number of essential features, of which the following should be emphasized: 1) the rocks of Mount Izykh are metagabbro and serpentinized harzburgites; 2) according to the results of areal metallometric sampling carried out by geophysical parties, an increased concentration of Cr, Ni, Co is established in the rocks of Mount Izykh; these elements are not characteristic of the Kogtakh massif; and 3) titanium minerals (ilmenite and titanomagnetite), present in increased quantities in the rocks of the Kogtakh massif, are practically absent in the rocks of Mount Izykh.

In studying the granitoids of the Ulen-Tuim pluton we found that the hybrid rocks, which usually form elevated areas in the relief, contain an increased amount of pyroxenes (up to 35%) and abound in xenoliths of country rocks of the Izykh gabbro type and of effusives of the Karatag sequence. This suggests that the pyroxenes are xenogenic minerals captured by the granitoid magma during assimilation of the magmatic rocks of the roof. In support of this idea, microscopic study revealed that at times the pyroxenes in the thin section occur as separate isolated groups of grains among plagioclases (Fig. 2, e). In some cases the grains in these groups are “cemented” by an ore mineral, forming a sideronitic texture. Such areas of pyroxene rock resemble relics of gabbro xenoliths. Everywhere the pyroxene is replaced by hornblende or by chlorite-actinolite aggregates. The pyroxenes are represented by monoclinic (augite, diopside) and orthorhombic (bronzite, hypersthene) varieties. Quantitatively, the monoclinic varieties predominate over the orthorhombic ones.

The results of metallometric sampling carried out by geophysical parties of the Western Integrated Geological Exploration Expedition of the Krasnoyarsk Geological Administration show that elevated Cr and Ni contents occur mainly in samples taken from Mount Izykh and from areas where hybrid pyroxene-bearing rocks are distributed (Fig. 4). Areas with development of effusives of the Karatag sequence and rocks of the Bui intrusion do not reveal any appreciable content of these elements.

Analyzing these data, one may conclude that the gabbroids of Mount Izykh once had a comparatively wide distribution in the area of the middle course of the Bely Iyus River. They most probably formed sheet-like and funnel-shaped bodies elongated in a northeasterly direction along the strike of the folded structures of the volcanogenic-sedimentary geosynclinal sequence (Karatag). This volcanogenic-sedimentary sequence is exposed south of Mount Izykh, is intensely dislocated, and is saturated with sills and dikes of gabbro-diorites, gabbro-diabases, and epidiabases of the gabbro-diorite-diabase formation, known in neighboring areas as the Bui intrusion. The conformity of the Izykh gabbroids to the folded structures of the country rocks and their rather intense tectonism suggest that their formation took place in the pre-ore period and was connected with deep faults that arose during extension of the Earth’s crust in the marginal trough of the geosyncline at the initial stage of tectogenesis [8]. With the emplacement of the gabbroids, the cycle of geosynclinal volcanism was completed. As for the dikes of garburgite composition, they should be regarded as the final product of differentiation of the basaltic magma chamber that produced the Izykh gabbros. This is indicated by the spatial confinement of the garburgites to the gabbroid massif, by their identical degree of metamorphism, pointing to a small gap in the time of formation, by the presence of elevated contents of the same trace elements (Ni, Co) in both rock types, and by identical metamorphogenic minerals (green spinel). Consequently, it becomes necessary to distinguish an independent Izykh gabbro-pyroxenite intrusive complex and to assign this complex to some definite magmatic formation. The petrographic composition of the gabbroids, the form of the body, its position in geological structures, association with an effusive-sedimentary geosynclinal complex, and the presence of quartz gabbros—all this is characteristic of the gabbro-pyroxenite-dunite formation distinguished by Yu. A. Kuznetsov in the Urals [5]. A distinctive feature of the Izykh—

Fig. 2. Photomicrographs of petrographic thin sections.
a — contact amphibolite-gneiss; b — mesh structure in cataclased gabbro; c — relic pyroxene in a grain of uralitic hornblende in amphibolitized gabbro; d — hornfelsed gabbro; e — amphibolitization of a pyroxene grain in amphibolitized gabbro; f — grouped distribution of pyroxene in hybrid rocks of the Uspek-Tuim pluton.

... rocks is the absence of dunites in their composition, and the ultrabasic rocks are later formations than the gabbroids. A case somewhat similar to ours was described by A. P. Lebedev (1962) for the Svarants massif in southern Armenia [7]. There the Upper Eocene (?) gabbroids are cut by dikes of ultrabasic rocks (magnetite olivinites and kazankites). In works on the Kuznetsk Alatau, however, descriptions of intrusive complexes similar to the Izykh complex are not encountered. Thus, a difficulty arises in determining the formational affiliation of the rocks of the Mount Izykh massif. The authors are nevertheless inclined, with considerable reservations, to assign them to the gabbro–pyroxenite–dunite formation.

Fig. 3. Petrochemical diagram. 1 — Izykh gabbro; 2 — Kogtakh gabbro; 3 — contact amphibolite-gneiss of Mount Izykh; 4 — harzburgite of Mount Izykh

The younger dikes of olivine gabbro that cut the Izykh metagabbros and the Ulen-Tuim granitoids also deserve attention. Similar formations occur in several districts of the Kuznetsk Alatau. A. F. Korobeinikov, in his work on the Kommunar ore field [6], describes dikes of olivine gabbro cutting the Solgonskii granitoid massif, which is correlated with the Ulen-Tu...

Iim pluton ($\mathrm{Cm}_3$–O). He provisionally assigns these dikes to the Devonian gabbro–syenite intrusive complex distinguished by M. P. Kortusov in the Mariinsk taiga (Kuznetsk Alatau) [3].

Fig. 4. Schematic geological map of the area of Mount Izykh. 1 — gabbroids of Mount Izykh; 2 — granitoids of the Ulen’-Tuim pluton; 3 — hybrid rocks of the Ulen’-Tuim pluton; 4 — carbonate rocks of the Karatag Formation; 5 — amphibolized diabases of the Karatag Formation; 6 — bodies of gabbro-diorite and gabbro-diabase of the gabbro-diorite–diabase formation; 7 — aureoles of elevated Ni and Cr contents according to the results of metallometric sampling.

In agreement with these data, we also provisionally assign the dikes of olivine gabbro in our area to this intrusive complex.

REFERENCES

1. A. N. Zavaritskii. Igneous Rocks. Publishing House of the Academy of Sciences of the USSR, 1956.

2. G. A. Ivankin, Yu. A. Kuznetsov. Magmatism and metamorphism of the Kuznetsk Alatau. Geology of the USSR, vol. XV, part 1, Krasnoyarsk Territory, Gosgeoltekhizdat, 1961.

3. M. P. Kortusov. On the gabbro–syenite intrusive complex of the Mariinsk taiga (Kuznetsk Alatau). Materials on Mineralogy, Petrography, and Useful Minerals of Western Siberia. Tomsk University Press, Tomsk, 1962.

4. Yu. A. Kuznetsov. On the principles of distinguishing and classifying magmatic rocks. In: “The Basic Ideas of M. A. Usov in Geology.” Publishing House of the Academy of Sciences of the Kazakh SSR, 1960.

5. Yu. A. Kuznetsov. Main Types of Magmatic Formations. Nedra Publishing House, Moscow, 1964.

6. A. F. Korobeinikov. Principal features of the fracture tectonics of the Kommunar gold-ore field. Geology and Geophysics. Publishing House of the Siberian Branch of the Academy of Sciences of the USSR, No. 11, 1964.

7. A. P. Lebedev. Ore-bearing olivinites of the Svarants deposit in southern Armenia as a special genetic type of magmatic iron ores. In: Geology of Ore Deposits. Publishing House of the Academy of Sciences of the USSR, No. 6, 1962.

8. A. Rittmann. Volcanoes and Their Activity. Translated from German. Mir Publishing House, Moscow, 1964.

9. V. L. Khomichev. Intrusive magmatism of the Tuim district of Khakassia. In: “New Data on the Geology of the South of Krasnoyarsk Territory.” Krasnoyarsk Book Publishing House, 1963.