Chapter 2. Postcollisional intrusions of the Kokshaal Segment of South Tien Shan

Postcollisional granites of the Kokshaal Segment of South Tien Shan comprise about 20 postkinematic intrusions composed of biotite-amphibole granites, biotite granites and Li-F topaz-bearing leucogranites. The granites were emplaced coevally with tholeiitic mafic rocks and alkaline syenites. Geochemically the granites are classified as A-type and characterized by elevated Fe/(Fe+Mg) and K2O/Na2O values and high concentrations of Na2O+K2O, Rb, HFSE. On the discrimination diagrams Y-Nb and Rb-(Y+Nb) compositions of the granites plot into the field of intra-plate granites. On a regional scale, the compositional variations of the Kokshaal granites can be explained by fractionation of potassium feldspar and amphibole. The granites were probably derived from the crustal protoliths represented by Precambrian metamorphic rocks of the Tarim microcontinent. Crystallization ages of the Kokshaal granites, established utilizing U-Pb zircon dating, fall in the relatively narrow range between 280 and 295 Ma corresponding to the early Permian, which is in agreement with ages of postcollisional granites elsewhere in Tien Shan. Genesis of granites was related to trans-crustal shear zones.