Geochronology, Geochemistry of Dagenbieli Neoproterozoic Granites in the Yili Block, and Its Geological Implications

Geochemical studies indicate that the Dagenbieli granitic intrusion in the northern margin of the Yili Block is a high calc-alkaline and weak peraluminous granite with high content of SiO₂(76.22%-76.86%), ALK (7.07-7.93) and low content of CaO(0.95%-1.21%) and MgO(0.10%-0.20%). This intrusion has moderate REE concentrations and strong negative Eu anomalies(δEu=0.06-0.19). They are enriched in Rb, Th, Sm, Ga, Nb and depleted in Ba, Ti, Sr. Characterized by average zircon saturation temperature of 759 ℃, consolidation index of 1.07-2.14, differentiation index of 89.98-92.11, the Dagenbieli intrusion is the highly fractionated S-type granites. U-Pb zircon analysis suggests that the intrusions were formed at 942.5±2.6 Ma. The Nb/Ta ratios were close to ratios of the upper crust, Mg# value is 4.52-7.33, and the εHf (t) values of zircon range from -0.41 to +4.34, which indicates that the source rocks were ancient crustal materials with new crustal materials addition. The zircon of intrusion have two-stage Hf model ages of 1.41-1.66 Ga, which means that the Yili Block may be underlain by Middle Proterozoic(>1.7 Ga) crystalline basement. Combined with Neoproterozoic magmatic events in the Tianshan and Tarim area, we suggest that the formation of the Dagenbieli intrusion relate to the Rodinia. This study provides further and important geochemical and age data to support the presence of Precambrian basement in Western Tianshan area and records on the convergence of Rodinia supercontinent.