The petrochemical characteristics of Hercynian granitoids in Tianshan and the geodynamic implications

Abstract

Four genetic types of granitoids, i.e. mantle-derived type (M-type), syntexis type (SY-type), trasformation type [T-type, including migmatic(MT), peraluminous (PT) and K-rich (KT) subtypes], and A-type were classified in the Hercynian granitoids in Tianshan area. Their geological and petrochemical characteristics were studied. With the evolution of granitoids type to A-type and with the ages of granitoids becoming young, the content of SiO2 and alkali increased and the size of magmatic bodies decreased. The basicity of syntexis type and the acidity of K-rich transformation granites are of the highest value in the central Tianshan, followed by northern Tianshan and southern Tianshan successively. The peraluminous transformation granitoids are of same petrochemical characteristics in the entire region of Tianshan. These petrochemical characteristics and their spatiotmeporal variations imply that:1) The developing sequence of the granitoids in the Tianshan collisional orogen is generally M-type-SY-type-MT-type-PT-subtype -KT-subtype-A-type. 2) The sequence shows the directionally compositional evolution of granitoids in a collisional orogen, which illustrates the vertical and lateral crust growth and the enhancement of crustal maturity. 3) The concept of "orogenic rock" should include granitoids of the syntexis and transformation type, and the petrochemical and distributing heterogenity of granitoids were resulted from the nature and setting of various tectonic units. 4) The PT-subtype granitoids mark the collision of continental blocks. 5) The KT-subtype granitoids could be used as the marker of the climax of decompression-geothermal increase. 7) A-type granitoids are the indicator of the end of orogenesis.