Geochemical characteristics and genesis of adakite-like granites at Yema Nanshan in the western segment of the Central Qilian Mountains

Abstract：Calidonian granites are widely developed in the Yema Nanshan area in the western segment of the Qilian orogenic belt. The geochemical characteristics of most of the granites are similar to those of adakites： SiO2≥56%, Al2O3≥15.0%, low MgO (average 2.04%), high Sr (257-889μg/g, average 546.42μg/g), low Y (7.1-25.3μg/g，average 16.67μg/g), average Sr/Y=5.89, no pronounced Eu anomaly and Sr positive anomaly. Adakites in this area have five rock types, quartz diorite, quartz monzodiorite, quartz monzonite, monzogrannite and hornblende granite. These rocks commonly contain hornblende, belonging to I-type granites. The geochemical diagram indicates that the granites formed in a pre-collision island-arc environment, with a zircon U-Pb age of 444±33 Ma, in a plate suduction convergence stage of the Qilian orogenic belt. Therefore, it may be inferred that adakite-like granites of Yema Nanshan formed by partial melting of the subducted slab and crust-mantle magma mixing. This discovery has enriched the study of adakite in China and has great significance for the study of the evolution, geodynamics and metallogenesis of the Qilian orogen.