EVOLUTION OF THE GRANITOID MAGMATISM OF WESTERN TIEN-SHAN

Dalimov T.N.*, Akhundjanov R.A.**, Ganiev I.N.*, Ishbaev Kh.D.*

*National University of Uzbekistan named after Mirzo Ulugbek, Tashkent, Uzbekistan, ingn@rambler.ru

**Institute of Geology and Geophysics named after H.M.Abdullaev Academy of Sciences of

Uzbekistan Republic, Tashkent, rkh.akhundjanov@mail.ru

Western Tien-Shan, which is stretched in the sublatitudinal direction to the distance more than 1500 km, is one of the most interesting magmatic provinces of the intracontinental mobile belts of Eurasia. It is characterized by duration of magmatic processes from the Precambrian to the Mesozoic, the variety of their products and development in the volcanic, the plutonic and volcano- plutonic forms. Its formation is caused by the evolution of Turkestan, Zaravshan and Southern Gissar of the paleo-oceanic ponds of paleozoic age. Are car-mark geodynamic situations of origin, development of paleo-oceanic structures, formation of the active outskirts different types, to collision and intra-plate stage (Dalimov, 1993, 2003). Western Tien Shan is characterized by the magmatism, development in the wide time interval and relates to the petrographical provinces with the predominant granitoid magmatism. On the prevalence its products compose about 40% of area of region. By the important feature of the spatial arrangement of granitoid rocks is the concentration them plutons in the arch of the anticlinal structures of sublatitudinal direction and to the areas of maximum development ultrabasic-basic of the magmatism, which is located by the mantle products (dunites, olivinite, peridotites, serpentinites, picrite, wehrlites, gabbroids), in the zones of deep faults. It is seemed that they are formed into two stages with different geodynamic regimes: rift and subduction. The age of these ultramafites it is not reliably established and varies from the Precambrian to the Late Silurian. Further evolution of magmatism is following. In the early carbon after carbonate accumulation in D₂- C₁ as a result of mantle diapirism in the frontal part of the pressure dames appear the chambers of the accumulation of the magmas of picrite composition, enriched by iron and titanium. The consequence of this differentiation is the formation of the melts of basic composition and the formation of the mesoabyssal bodies of gabbroids (Rb- Sr; 358-328 mln. years, I^Sr = 0,705-0,706). In further late-carbonic evolution are observed the apogee of magmatism, sharp predominance of the role of the mixed mantle-crustal sources of melts, appear lime- alkaline volcanic associations. The sequence of the formation of magmatic series, “batholithic” nature of the development of plutonism are represented as the proof of prolonged existence of the magma pools and their vertical migration from the lower parts of the crust into the upper. Granitoid plutons are examined as the derivatives of mantle- crustal diapir, phenomena of the mixing of basic melts with the material of granite-gneiss substratum.

Late Carbon − Early Permian magmatic associations are characterized by the fact that they are formed the intra-plate conditions. In the peripheral magmatic cameras the formation of hypabyssal small intrusions and dikes occurs, while in the near-surface parts of the crust explosive volcanism predominates. Subalkaline, especially increased potassium alkalinity and caused by this “monzonit” incline of the rocks of small intrusions is one of the characteristic features of the intra-plate stage of magmatism. This phenomenon is represented as the consequence of the influence of mantle fluids on the residual chambers of the magmas of different composition, placed at different levels of the earth's crust. Latest is formed the association of the rare-metal rocks, been stocks, with necks, by the dikes of trachybasalts, syenite-porphyries, ongonites, ongoriolites, alkaline ongoriolites and small sizes by the intrusions of leucogranites. The source of their melts is defined as metamagmatical mantle-crustal genetic type.

In the structure of batholiths concentric zonality is outlined - this is the change towards to center of plutons of basic and intermediate rocks of initial phases by the granitoid rocks of acid and ultraacidic composition. In the framing, in the smaller measure, in the internal part of plutons are placed the small intrusions of monzonitoid and subalkaline granitoid rocks. Their position is observed not in all areas because of with overlapping of large areas by Mesocenozoic cover. There with is not excluded the role of transverse “anti-Tien Shan” faults.

The lithosphere of the areas of the granitoid magmatism of Western Tien Shan is characterized by the presence of the clearly expressed high-speed anomalies in the low-speed crust, which are interpreted as basic magmatic rock (gabbroids) and amphibolite (Dalimov and other, 2003; Khamrabaev and other, 1998). In the aforecited features of granitoid magmatism out above are appeared the features of similarity to generation conditions for the batholiths of Central-Asian foldbelt (Yarmolyuk, Kovalenko, 2003), where the granite formation is represented as the product of mantle-crustal interaction, which caused anatexis and appearance of the magma chambers of granitoid plutons.

References

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Dalimov T.N., Ganiev I.N., Shpotova L.V., Kadyrov M.Kh. Geodynamics of Tien Shan. // Tashkent: University, 1993. 200p.

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Yarmolyuk V.V., Kovalenko V.I. Geodynamic situations of the formation of batholiths in the Central-Asian foldbelt // Geology and geophysics, 2003, T. 44, № 12. P.1305-1320.