NEPHELINE SYENITES IN ROCKS OF OGNITS IGNEOUS COMPLEX (SOUTH-EASTERN PART OF EAST SAYAN): COMPOSITION, AGE, TECTONIC SETTING

Khromova E.A.

Geological Institute SB RAS, Ulan-Ude, Russia

The Devonian granite, granosyenite and syenite massifs of the Ognits igneous complex are widespread in the south-eastern part of the East Sayan (Gordienko, 1969, 1987). They are mostly composed by subalkaline and alkaline granitoids, including nepheline-containing varieties as well. On the one hand, the area of the Ognits complex intrusive body development is characterized by presence of belts and massif chains along the faults. On the other hand, it is specific of pluton areal development in the regions of big fault conjugation. The belts are up to 250km long and 25-40km wide. The largest knot is located in zone of the Oka and Main Sayan deep fault contiguous development. The formation of granitoids in this area is related to three stages of various ages: Early Paleozoic (Tannuolsky, Sarkhoi complexes); MidPaleozoic (Ognits and Boto-Gol complexes) and Late Paleozoic (Dzhugoyarsky and Samsalsky complexes). Among the MidPaleozoic granitoids, nepheline syenites are of the greatest interest. About ten massifs composed by similar rocks are known in the south-eastern part of the East Sayan. They are ubiquitously associated with bodies of alkaline granitoids. The Nyurgan massif alkaline rocks together with rocks of Ekhe-Khaigass, Sentsa, Soroksky, Zhakhna, Udachny, Boto-Gol, Khusha-Gol and other MidPaleozoic (Devonian) massifs are combined into the Ognits igneous complex. Nepheline-containing massifs are characterized by small size and multiphase structure. In the present paper, nepheline syenites are described. They make up large volume among rocks of the Nyurgan granitoid massif within the Oka zone of caledonides in the south-eastern part of the East Sayan. In addition to nepheline syenites, syenites, syenite-granites, granosyenites, diorites and granodiorites of the Ognits igneous complex are widely spread in the region. They compose a group of intrusions in the Sentsa, Tissa and Zhom-bolok river basins. The granitoids of the complex cut all the known sedimentary-metamorphic stratas of the Proterozoic and Low Paleozoic in the area, and the former are its youngest intrusive formations.

The Nyurgan massif is one of the big alkaline-granitoid ones in the studied area. It occupies axial part of the Nyurgan ridge and localizes in zone of the north-west striking large fault. The massif lies in the Sentsa and Buryat-Gol interfluve. It occupies the area of 20 km² and has rather complex morphology. The massif extends for 13 km in the north-western direction, and it is 0.5-2 km wide. Contours of the pluton are irregular and form narrow elongated crests in the enclosing rock. The massif is bounded by the Sentsa fault in the north-west. It contacts with sandstones and aleurolites of the Dibin suite in the north-east and limestones and dolomites of the Mongoshin suite of the Late Proterozoic in the south-west. The contact influence on the Dibin suite rocks resulted in formation of wide (200-300m) band of hornfelsed rocks. Contact-metasomatic changes in rocks of the Mongoshin suite are poorly observed. They include insignificant scarning of limestones and migmatization of shales.

By quantitative predominance of rocks, it has been revealed that granosyenites, quartz and alkaline syenites are mostly distributed in the lower part of the massif. Nepheline syenites occupy the central part of the massif. The dikes that are represented by fine-grained granites occur rather rarely. Nepheline syenites are macroscopically represented by light grey, grey rocks with greenish hue. They are commonly coarse-grained, more rarely middle-grained with specific greasy luster of nepheline. “Variolitic” sculpture is observed on the weathered surface. The rock has hypidiomorphic-grained structure and trachydoidal texture. Alkaline feldspars are represented by microcline-perthite that is characterized by ribbon type of perthite replacement with subsequent albitization. Albite either forms small inclusions in microcline or rims its grains. Nepheline is of grayish interference colour, and it forms irregular grains within 1-1.5 mm in size. Along microfractures, the mineral is replaced by cancrinite that is also developed as fringe on carbonate. It is particularly clearly observed at their contact with nepheline. Femic minerals are mainly represented by biotite being strongly pleochroic in thin section. Biotite lamellas range from 0.5 to 3 mm in size. Sometimes glomero-like growths of amphibole small grains (0.5-1 mm) occur.

By Na₂O+K₂O content, nepheline syenites belong to alkaline sequence. They are characterized by positive Eu anomaly, whereas negative one is specific of granites and granosyenites.

Feldspars are primary minerals in rocks of the Nyurgan massif. Microcline is in fact the only salic mineral in alkaline syenites. As a rule, potassium feldspar, its plates and grains are oversaturated with microperthite ingrowths of albite by 15-20 and sometimes 50%. Secondary albite is often represented by well limited plates being small in size. Feldspar composition from rocks that compose the Nyurgan massif is similar to theoretical one. Calcium content in albite being a bit higher is an exception. Nepheline content is irregular in nepheline syenites of the Nyurgan massif. The mineral grains commonly make up 15-25% of the rock total volume. Nepheline crystals have rounded form in thin sections. The mineral is practically not subjected to postmagmatic transformations. It is water-transparent. Sometimes the mineral slightly grows turbid in cases of nepheline replacement by cancrinite. Cancrinite forms fringes round nepheline grains. Calcite is observed at contacts during cancrinitization. Clinopyroxene occurs as irregular grains in rocks. Crystals widely vary in size, i.e. from 1 mm to 4 mm. The content of the mineral in rock does not exceed 5%. Spotted colour is typical of clinopyroxene in thin sections (alternation of green and violet colours). The component composition of clinopyroxene (in mol. %): 19.5 aegirine, 12.4 diopside, 64.6 hedenbergite. Biotite is subordinate in the Nyurgan massif rocks. In thin sections, it is represented by flakes and lamellas that are various in form and pleochroic in brown colour. Amphibole occurs rarely in rocks of the Nyurgan massif. In addition, the mineral grain size does not exceed 1 mm. Amphibole is bright green in thin sections. Due to the amphibole nomenclature (1997), the studied amphiboles of the Ognits igneous rock complex belong to hastingsites and ferroedenites as well by their composition. Calcite is specific of the Nyurgan massif alkaline rocks. As a rule, it occurs as poikilitic inclusions in nephelines. They are round, drop-like extracts in thin sections with specific pearly colour. Calcite large grains commonly contain small crystals of ore mineral. Paragenesis of calcite with biotite is likely to be associated with it. The accessory minerals are mainly zircons. Sphene, fluorite and ore minerals such as pyrite, ilmenite, titanomagnetite and magnetite also occur.

The data of rock absolute age in the Ognits igneous complex are contradictory. Some authors (Klyarovsky et al., 1965) present figures of alkaline rock absolute age (K-Ar technique) within 400-420Ma in the Nyurgan and Boto-Gol massifs. However, the data by Gordienko exist (Gordienko, 1969) who presents absolute age values (K-Ar technique) for similar rocks of the Oka (Zhom-bolok) fault zone 210 ± 31 Ma for nepheline syenites and 360 ± 5 Ma for biotite microcline granites. The studied granitoids are comagmatic with volcanites. Their age corresponds to the Early Devonian and equals 402 ± 14.9 Ma (Vorontsov, 2006). Rb-Sr isotope analysis of the Nyurgan massif granosyenites has been carried out during our studies at the Geological Institute SB RAS. The measurements have been made with Finnegan MAT-262 mass-spectrometer. Strontium isotope standard value equaled 0.7058 ± 0.00097. Uncertainties of ⁸⁷Sr/ ⁸⁶Sr ratio determinations equaled not more than 1 and 0.05% respectively. By the obtained Rb-Sr data, isochrone on six bulk samples and two feldspar monofractions from granosyenites has been calculated. The age 454 ± 28 Ma has been obtained. Taking into consideration that nepheline syenites postdated granosyenites, the obtained values of isotope age indicate the MidPaleozoic age of Ognits complex massif formations. In addition, the age of the East Sayan Ognits complex is determined by the fact that granitoids cut deposits of the Upper Nerkhin suite (S₂-D₁) and are also present in pebbles of the MidDevonian Khaltin suite conglomerates. Mantle matter participation in formation of granitoids should not be ruled out due to low initial ⁸⁷Sr/ ⁸⁶Sr ratio 0.703 (average from 6 samples).

The described nepheline syenites are likely to be associated with formation of microcline granite and syenite intrusions within the zone of the Oka deep fault higher magmatic activity. Host carbonate rocks and gabbroids also played a great role in formation of alkaline rocks.

References:

Gordienko I.V. Paleozoic magmatism and geodynamics of Central Asian Fold Belt. Moscow: Nauka. 1987. 236p. (in Russian)

Gordienko I.V. Devonian volcano-plutonic formation in south-eastern part of East Sayan. Ulan-Ude. Buryat izd-vo. 1969. 115p. (in Russian)

Vorontsov A.A., Dril’ S.I., Dudov E.N., Andryushchenko S.V. Rb-Sr isotope systematics of riftogenic trachybasalt-trachyte-alkaline rhyolite associations of Siberian Platform southern rim: geochronological data on Kropotkinsky (Khoito-Oka interfluve, East Sayan) and Khambinsky (West Transbaikalia) paleograbens. In: III^rdRussian Conference on isotope geochronology: isotope dating the processes of ore formation, magmatism, sedimentation and metamorphism. Materials of the Conference. Moscow. V1. 2006 p. 164-168. (in Russian)

Klyarkovsky V.M., Kostyuk V.P. About age of alkaline rocks in eastern part of East Sayan. In: Doklady of USSR Acad. of Sci. v.162. N2. 1965 p.405-407. (in Russian)