SHRIMP U-Pb Zircon Geochronology, Geochemistry And Nd-Hf Isotope Of Neoproterozoic Magmatic Rocks In Western Sichuan

Neoproterozoic magmatic rocks are widespread in western margin of the Yangtze block, and their origin and genesis have significant implications for understanding the evolution of the Rodinia supercontinent. At the present time, the genesis and tectonic setting of these magmatic rocks have been an issue of hot debate, i.e., mantle plume vs island arc in origin. On the basis of field investigations, mafic dyke swarms and associated granitoid rocks including the Wasigou complex and the Shimian granite from western Sichuan were selected for comprehensive studies on SHRIMP zircon U-Pb geochronology, petrology, geochemistry and Nd-Hf isotope in this paper, with the aim of revealing their petrogenesis and tectonic implications.Granitoid rocks within the Wasigou complex and Shimian granite from the Kangdian Rift of western Sichuan are of metaluminous I-type. They were emplaced in Neoproterozoic, and experienced the fractional crystallization of plagioclase, hornblende, apatite and Ti-Fe oxides. These granitoids display some geochemical similarities to rocks formed in the island-arc environments such as Nb-Ta depletion. However, they are also depleted in Sr, P and Eu, and high in most other trace elements, which differ from those of typical rocks formed in the island-arc environments, but resemble those of intra-plate granitoids. It is suggested that these granitoid were derived from partial melting of pre-existing, young island arc crust, with contamination of old crust materials during magma ascending and emplacement. Their arc-like geochemical features (such as Nb-Ta depletion) should have been inherited from theprotoliths, rather than inflection of their tectonic setting when the granitoids formed.There are abundant mafic dykes in the Kangding-Luding-Shimian region within the Kangdian Rift, which were spatially and temporally coeval with the Wasigou complex and Shimian granite. In the Wasigou region, mafic dykes show clear phenomena of magma mingling with granitoid wall rocks. Mafic dykes are tholeiite in composition. They expereinced fractional crystallization of olivine, apatite, etc. Field geologic observation and SHRIMP U-Pb zircon data indicate that these mafic dykes were formed at 780 ~ 760 Ma. They are characterized by two-fold geochemical characteristics of intra-plate and island arc basalts. The highest eNd(T) (~8) and eHf(T) (~17) values for the mafic dykes suggest that mafic magmas were derived from depleted asthenosphere mantle source. Modal calculation for the high MgO samples suggests that their melting temperature is about 1450—1470 °C, significantly higher than the potential temperature of 1280—1350 °C for normal asthenosphere mantle. In addition, these high MgO mafic dykes are also high in Fe/Mn ratios, suggesting an anomalously hot mantle plume for origin of these mafic dykes. The mafic magmas were contaminated by young island arc crust to variable degrees while ascending and emplacement, resulting in some "arc-like" geochemical features for some highly contaminated samples.These ca. 780—760 Ma mafic dykes in western Sichuan have a close genetic relationship in time and space to the plume-related ca. 780 Ma mafic magmatic rocks in western North America and ca. 755 Ma mafic dyke swarms in NW Australia. Our results support the reconstruction model of Rodinia in which the South China Block was located between Australia and Laurentia.