Geology,Geochemistry and Zircon U-Pb Geochronology of Porphyries in the Dabate Mo-Cu Deposit,Western Tianshan,China:Petrogenesis and Tectonic Implications

摘要

The Dabate Mo-Cu deposit is a medium-sized porphyry-type deposit in the Sailimu Lake region, western Tianshan, China. We present the geology, geochemistry and zircon U-Pb geochronology of granite porphyries from the Dabate district with the intent to constrain their tectonic setting and petrogenesis. Porphyries in the Dabate district include granite porphyry I(gray white color with large phenocrysts), granite porphyry II(pink color with small phenocrysts) and quartz porphyry. Granite porphyry II is the Cu and Mo ore-bearing granitoid in the Dabate deposit. LA-ICPMS zircon U-Pb analyses indicate that granite porphyry II was emplaced at 284.2±1.8 Ma. Granite porphyry I and II have similar geochemical features and are both highly fractionated granites:(1) They have high SiO_2 content(70.93–80.18 wt% and 72.14–72.64 wt%, respectively), total alkali(7.58–8.95 wt% and 9.35–9.68 wt%, respectively), mafic index(0.95–0.98 and 0.93–0.94, respectively) and felsic index(0.79–0.94 and 0.89–0.91, respectively);(2) They are characterized by pronounced negative Eu anomaly, "seagullstyle" chondrite-normalized REE patterns and "tetrad effect" of REE;(3) They are rich in Rb, K, Th, Ta, Zr, Hf, Y and REE, but depleted in Sr, P, Ti and Nb. The magma of granite porphyries in Dabate can be interpreted to have been generated by partial melting of the upper crust due to mantle-derived magma underplating in a post-collisional extensional setting.