POSTCOLLISIONAL GRANITES IN THE SOUTH TIEN SHAN VARISCAN COLLISIONAL BELT, KYRGYZSTAN

Abstract

Two major types of the potassium-rich postcollisional granites of Permian age were studied in the South Tien Shan Variscan Collisional Belt. The first type, metaluminous granites of the Jangart complex, are located along the southeastern boundary of this belt with the Precambrian Tarim Block. The plagioclase rims on K-feldspar megacrysts (rapakivi texture), extremely high FeOt/(FeOt+MgO) ratio, indications of the low H2O and O2 fugacities, high concentrations of the incompatible elements especially light rare earth elements (LREE), Ba, Nb along with heightened contents of the compatible elements especially Ni, Cr and moderately initial 87Sr/86Sr ratio (Sr0=0.7075) make those granites similar to other rapakivi granites. The second type, peraluminous granites of the Inylchek complex, is located along the northwestern boundary of the belt with the Caledonian Kazakh Microcontinent. Those granites are rich in F, B, Li, Rb, Cs, Sn, Ta and heavy rare earth elements (HREE), but are poor in Ba, Sr, Ni and Cr and are characterized by the relatively high initial 87Sr/86Sr ratio (Sr0=0.7098). Li–mica granites are distributed widely among them. The granites of a transitional type (Uchkoshkon complex) occupy an intermediate geographic position between major granite types. The initial Sr isotopic composition of the transitional granites varies widely (Sr0 from 0.7080 to 0.7256). The variation in chemical composition of the coeval postcollisional granites across the Collisional belt, together with the variability of initial Sr isotopic ratios of the transitional granites, are thought to indicate basement heterogeneity. This heterogeneity, in turn, is probably related to thrusting of the Tarim Block under South Tien Shan during the collision.