PETROLOGY OF THE GABBRO–SYENITE INTRUSIVE COMPLEX OF THE SAKSYR RIDGE (KUZNETSK ALATAU)

E. S. Edintsev, S. S. Ilyenok

(Presented by the Seminar of the Department of Petrography)

The rocks of the gabbro–syenite complex make up a massif measuring \(6 \times 7\) km (Fig. 1) in the northwestern part of the Saksyr Ridge. The enclosing rocks are limestones and dolomites of the Martokha Formation of the Upper Proterozoic and diorites of the Tygyrtysh intrusive complex of Ordovician age. Near the contact with the intrusion these rocks underwent contact metamorphism, which in carbonate rocks is expressed in marmarization and serpentinization, and in diorites in hornfelsing. The width of the zone of marmarization is 100–200 m; that of hornfelsing is the first tens of meters; and the serpentinites form small bodies (\(10 \times 20\) m, \(20 \times 50\) m) immediately at the contact or at a short distance from it. The formation of the complex proceeded in three phases: 1—gabbro-diorites, gabbro, olivine gabbro, anorthosites, gabbro with columnar hornblende, quartz-bearing gabbro; 2—syenite-diorites, syenites; 3—quartz and quartz-bearing syenites, alkaline syenites.

The rocks of the first phase compose two large (\(1.5 \times 5\) km, \(2 \times 7\) km) and a number of small bodies in the area of Mount Kara-Tag and 4 km north of it (the periphery of the massif). The northern body has a linearly elongated form with contacts dipping to the southeast (rare measurements of the orientation of dark-colored minerals) and is composed of gabbro and quartz-bearing gabbro. The southern body on the right bank of the Kamyshta River has a form close to funnel-shaped, and farther to the northeast (on the right bank of the Kamyshta River) acquires linear outlines. The main part of this body is composed of gabbro, quartz-bearing gabbro, and gabbro-diorites, among which anorthosites and gabbro with columnar hornblende are distinguished as bands up to 600 m wide, and olivine gabbro as smaller (1–2 m) bands. The boundaries between individual petrographic varieties, with the exception of gabbro with columnar hornblende, are indistinct. The contact between gabbro with columnar hornblende and normal gabbro is sharp; the columnar crystals of hornblende are oriented strictly parallel to the contact.

The rocks of the second phase form two bodies measuring \(1.5 \times 3\) km and \(0.5 \times 1.0\) km, situated south of Mount Kara-Tag. Both bodies are composed of syenite-diorites, which near the contact with the limestones of the Martokha Formation pass into syenites. The transition is indistinct; the width of the syenite band is about 250 m.

The syenites of the third phase of this complex occupy the central part of the massif and compose one large (\(3 \times 6\) km) body and a number of small ones.

Fig. 1. Geological plan of the northwestern part of the Saksyr Ridge: 1 — alluvial deposits; 2 — diorites of the Tygertysh intrusive complex. Rocks of the gabbro-syenite complex: 3 — gabbro, quartz-bearing gabbro, gabbro-diorites; 4 — anorthosite; 5 — gabbro with acicular hornblende; 6 — syenite-diorites; 7 — quartz-bearing syenites, quartz syenites; 8 — alkaline syenites; 9 — carbonate rocks of the Upper Proterozoic; 10 — measurements of bedding elements; 11 — measurements of the orientation of mafic minerals in gabbroids; 12 — tectonic disturbances.

The large body has a zonal structure: its central part is composed of quartz and quartz-bearing syenites, while along the contact with the carbonate deposits of the Martukha Formation there is a belt of alkaline syenites, the width of which along the western contact is 100 m, and on the left bank of the Kamyshta River increases to 700 m. The transition from quartz syenites to alkaline syenites is gradual: quartz disappears from the syenite composition, and plagioclase is completely replaced by sodic feldspar. A brief characterization of the rocks of the complex is given below.

Gabbro, quartz-bearing gabbro, and gabbro-diorites are the most widespread rocks. Macroscopically, these are coarse-crystalline rocks of dark-gray color with a faint violet tint (due to plagioclase), with a massive, more rarely trachytoid texture. The structure of the rocks is gabbroic in combination with hypidiomorphic-granular. Quantitative mineral composition: plagioclase — 53–69%, pyroxene 20–23%, amphibole — 3–15%, biotite 3–4%, quartz — 0–4%, ore and accessory minerals 1–2%. Plagioclase is represented by sharply idiomorphic tabular-prismatic crystals 0.1–7.0 mm in size. In composition it corresponds to andesine No. 40–43 and labrador No. 52. Monoclinic pyroxene \((2V=+54^\circ,\ c:Ng=38^\circ,\ Ng=1.716\pm0.003,\ Np=1.690\pm0.003,\ Ng-Np=0.026)\) forms isometric crystals 0.1–3.0 mm in size and corresponds to diopside with 62% diopside and 38% hedenbergite component. Amphibole is observed as irregularly shaped grains 0.1–3.2 mm in size. Pleochroism is clearly expressed: \(Ng\) — yellowish-brown with a reddish tint, \(Nm\) — light brown, \(Np\) — light yellow. Judging by the optical constants \((c:Ng=15^\circ,\ 2V=-81^\circ,\ Ng=\)

= 1.700±0.003, Np = 1.678±0.003, Ng−Np = 0.022), the amphibole belongs to titanian hornblende. Spectral analysis established in the mineral: Mg > 1%, Ti > 1%, Al > 1%, Fe > 1%. Biotite (Ng about 1.622) is represented by flakes 0.1–2.5 mm in size. In transmitted light the mineral has a brown color with sharp pleochroism in lighter tones. Quartz is present as xenomorphic grains 0.1–2.0 mm in size. Ore and accessory minerals are represented by magnetite, apatite, and sphene. In addition, in the heavy-mineral concentrates tens of grains of ilmenite, single grains of zircon, pyrite, and chalcopyrite were also identified.

Olivine gabbro has a limited distribution and is recorded only in several places as bands 1–2 m wide among normal gabbro. Macroscopically this is a medium- to coarse-crystalline black rock with brown crusts of iron hydroxides on the weathered surface. The texture of the rock is gabbroic. The quantitative mineralogical composition is as follows: plagioclase — 26.91%, pyroxene — 53.08%, olivine — 18.73%, biotite — 0.62%, ore mineral — 0.66%. Plagioclase (labrador No. 60–62) is observed as tabular crystals 2–3 mm in size. Pyroxene is represented by prismatic crystals 1–3 mm in size. The mineral is partly replaced by brown amphibole and chlorite. By optical properties (c:Ng = 46°, 2V = ±59°, Ng = 1.730±0.003, Np = 1.702±0.003, Ng−Np = 0.028) it belongs to augite. Olivine forms isometric crystals up to 3 mm in size. Along the periphery of the crystals and along cracks, replacement of olivine by an aggregate of serpentine and magnetite is observed. Biotite occurs as sharp-edged flakes 0.2–0.5 mm in size. In transmitted light the mineral is brownish-brown with sharp pleochroism. The ore mineral is represented by rare grains of titanomagnetite 0.05–0.2 mm in size. In heavy-mineral concentrates from olivine gabbro, tens of grains of pyrite, chalcopyrite, and apatite, and single grains of ilmenite and rutile were additionally identified.

Anorthosites form, in the area of Mount Kara-Tag, a band about 100 m wide among gabbro and gabbro-diorites. Along strike the band has been traced for 600 m. Macroscopically these are coarse-crystalline rocks of light-gray color with a weak greenish tint. The texture of the rock is close to prismatic-granular. The quantitative mineralogical composition is as follows: plagioclase — 88.1%, hornblende 11.5%, ore and accessory minerals 0.4%. Plagioclase (labrador No. 50–55) forms elongate-prismatic crystals 0.5–2.0 mm in size. Hornblende is represented by prismatic crystals 0.5–1.0 mm in size; pleochroism from grass-green to yellow-green tones is well expressed. Among ore minerals titanomagnetite, magnetite, pyrite, chalcopyrite, and ilmenite are present. The accessory mineral is represented by sphene.

Gabbro with acicular hornblende is developed only in the area of Mount Kara-Tag. Macroscopically this is a medium- to coarse-crystalline black rock with a trachytoid texture. The structure of the rock is hypidiomorphic-granular with elements of gabbroic structure. The quantitative-mineralogical composition is as follows: plagioclase — 52.2%, amphibole with relicts of pyroxene — 43.4%, ore and accessory minerals 4.4%. Plagioclase (labrador No. 55) is represented by prismatic grains 0.1–5.0 mm in size. Amphibole forms acicular crystals 0.1–6.0 mm in size. Their outlines are usually uneven, with frequent projections. Pleochroism of the mineral: Ng — brown, Nm — light brown, Np — light yellow; absorption scheme Ng > Nm > Np. By optical constants (2V = 78–79°, c:Ng ± 12°) the amphibole belongs to brown hornblende. According to spectral-analysis data the mineral contains 0.6% titanium. In the cores of amphibole crystals in some

in some cases, grains of unaltered monoclinic pyroxene have been preserved. Ore and accessory minerals are represented by magnetite, ilmenite, pyrite, apatite, sphene, and zircon.

Syenite-diorites are coarse-crystalline rocks of pinkish-gray color with a massive texture. Under the microscope a hypidiomorphic texture is observed. The quantitative mineralogical composition of the rock is as follows: plagioclase—55.7%, sodium-potassium feldspar—25.7%, amphibole—7.2%, biotite—4.9%, quartz—4.7%, ore and accessory minerals—1.8%. Plagioclase (oligoclase—No. 20–23) is represented by prismatic crystals 0.1–4.5 mm in size. Sodium-potassium feldspar forms irregular, more rarely prismatic, crystals 0.1–0.3 mm in size. Biotite is observed as deformed flakes with patchy striations. Judging by the refractive index \((N_g = 1.670 \pm 0.003)\), the mineral belongs to lepidomelane with an iron content of 70%. Quartz is represented by irregular grains 0.1–1.5 mm in size. Amphibole \((C:N_g = 12^\circ)\) forms prismatic crystals 0.1–3.0 mm in size. In transmitted light the color of the mineral is green; strong pleochroism in light-green tones is observed. In the cores of amphibole crystals, small grains of monoclinic pyroxene are sometimes observed. Of the accessory minerals, apatite, sphene, and zircon are present in the rock. Ore minerals are represented by titanomagnetite, ilmenite, and pyrite.

Quartz and quartz-bearing syenites are medium- to coarse-crystalline rocks of gray-pink color with a massive texture. The texture of the rocks is hypidiomorphic-granular. The quantitative mineralogical composition is as follows: microperthite—42–56%, plagioclase—28–39%, amphibole—0–6%, biotite—3–8%, quartz—2–13%, ore and accessory minerals—1.0–1.5%. Microperthite is represented by prismatic and irregularly shaped crystals 0.1–7.0 mm in size. Sodium-potassium feldspar \((2V = 72^\circ,\ Nm \perp 001 = 2^\circ)\) is cut by numerous veinlet-spotted intergrowths of albite. Plagioclase (albite No. 5–8) forms prismatic crystals 0.1–3.5 mm in size. Quartz is observed as xenomorphic grains 0.1–3.5 mm in size. In the rock it is distributed unevenly. Biotite forms flakes of irregular shape 0.1–3.5 mm in size. The mineral is often decomposed and replaced by chlorite and iron hydroxides. Amphibole was encountered only in one thin section and is represented by irregularly shaped crystals 0.1–1.5 mm in size. The mineral is strongly replaced by chlorite, epidote, and iron hydroxides. Accessory and ore minerals are represented by apatite, zircon, and magnetite.

Alkaline syenites are coarse-crystalline rocks of red color. The texture of the rock is hypidiomorphic-granular. The quantitative mineralogical composition is as follows: microperthite—31.5%, biotite—9.6%, pseudomorphs after amphibole—7.8%, ore and accessory minerals—1.1%. Microperthite is represented by prismatic crystals 0.5–7.0 mm in size. Microperthitic intergrowths of albite have a spotted-veinlet form. Biotite is observed as irregularly shaped grains 0.3–0.6 mm in size. The mineral is strongly replaced by an aggregate of biotite, quartz, and iron hydroxides. Pseudomorphs after amphibole are represented by rhomboidal segregations 0.2–1.0 mm in size, composed of chlorite and iron hydroxides. Of the accessory and ore minerals, apatite and magnetite are present in the rock.

As is seen from the diagram (Fig. 2), the rocks of the first phase of the gabbro-syenite complex are predominantly undersaturated with silica \((Q < -6)\) and poor in alkalis \((a:c < 2)\); the rocks of the second and third phases, on the contrary,

saturated, weakly oversaturated with silica, and rich in alkalis ($Q = -3.5$–$12.2$, $a:c > 3$). The comparatively gentle orientation of the vectors corresponding to the rocks of the second and third phases in the $csb$ plane indicates a significant potassium content in the alkalis. In addition, the syenites are characterized by high ferruginosity ($f':m' = 1.1$–$11.0$), which, according to Yu. A. Kuznetsov, testifies to their connection with magma of basaltic composition. In chemical composition, the rocks of the first phase are comparable with the average types of diallagite (analysis 7), olivine gabbro (analysis 8), gabbro (analyses 2, 3, 4, 5, 9), melaphyre (analyses 1, 6, 10), quartz gabbro (analyses 11, 12, 13), and anorthosite (analysis 14) according to R. Daly, differing from the average type of olivine gabbro by a higher, and from the average type of melaphyre by a lower, content of femic components ($b = 46.2$ versus $29.9$ and $b = 21.8$–$22.1$ versus $22.6$),

Fig. 2. Diagram of the chemical compositions of rocks of the gabbro-syenite complex of the Saksyr Ridge (after A. N. Zavaritsky): 1 — rocks of the first phase; 2 — rocks of the second phase; 3 — rocks of the third phase.

from the average type of quartz gabbro by a higher content of alkaline aluminosilicates ($a = 10.9$–$12.7$ versus $9.6$) and a lower content of femic components ($b = 16.7$–$18.4$ versus $18.5$), and from the average type of anorthosite by a greater amount of femic components ($b = 11.8$ versus $5.8$).

The syenite-diorites are comparable with the average type of trachyandesite according to R. Daly, having a smaller amount and a larger amount of ferrous aluminosilicate ($a = 14.5$–$15.6$ versus $18$, $c = 4.6$–$5.2$ versus $2.8$). The figurative points corresponding to the syenites (analyses 19–26, 29–31) on the diagram (after A. N. Zavaritsky) are located inside a triangle whose vertices are the figurative points of the average types of syenites, alkaline syenites, and alkaline granites according to R. Daly. The syenites represented by analyses 27 and 28 are, in composition, close to the average type of nordmarkite according to R. Daly, differing from it by a higher content of ferrous aluminosilicate ($c = 2.0$–$2.3$ versus $0.8$).

In the chemical composition of its rocks, this complex is close to the gabbro-syenite complex of Mount Kalbuk in Gornaya Shoria (Il’enok, 1964), and according to the existing classification (Kuznetsov, 1964) it belongs to the gabbro-syenite subtype of the gabbro-monzonite-syenite formation.

In the area described, the intrusive formations of the gabbro-syenite complex cut through the deposits of the Martyukha Formation of the Upper Proterozoic and the diorites of the Tygertysh intrusive complex of the Ordovician. In addition, a dike-like body of olivine gabbro in the valley of the Nenya River (20 km northwest of the Saksyr Ridge) cuts the effusives of the lower part of the Byskar Series of the Lower–Middle Devonian (Bogdanov, 1962). The absolute age of biotite from the syenite-diorites (samples taken 2 km south of Mount Kara-Tag), determined in the Central Laboratory of the WSGU by V. M. Kisenko using the potassium–argon method, was 428 and 429 million years, and that of amphibole was 434 and 437 million years. On the basis of these data it may be said that the formation of the gabbro-syenite complex took place during the period from the Upper Silurian to the Lower Devonian.

REFERENCES

1. V. M. Bogdanov. Sequence and age of intrusive complexes of the Temir region and the vicinity of Lake Bulan-Kul in the southeastern part of the Kuznetsk Alatau. Materials on the Geology and Mineral Resources of Krasnoyarsk Krai, issue 3, 1962.

2. S. S. Il’enok. Petrology of the Gabbro-Syenite Complex of Gornaya Shoria. Tomsk University Press, 1964.

3. Yu. A. Kuznetsov. Principal Types of Magmatic Formations. Moscow, Nedra, 1964.

4. S. M. Chetverikov. Handbook of Petrochemical Recalculations. Moscow, Gosgeoltekhizdat, 1948.