1.01-0.98Â Ga mafic intra-plate magmatism and related Cu-Au mineralization in the eastern Jiangnan orogen: Evidence from Liujia and Tieshajie basalts

The Grenvillian-aged mafic magmatism is critical for understanding the regional tectonic settings and testing different tectonic models of South China. However, the geochronology, petrogenesis and geodynamic setting of Grenvillian-aged mafic rocks in the Jiangnan orogen remain unclear. An integrated study on LA−ICP−MS zircon U−Pb geochronology, elemental and zircon Hf isotopic geochemistry has been conducted on the Liujia and Tieshajie basalts in the eastern Jiangnan orogen. Two samples yielded zircon U-Pb ages of ca. 1012-980 Ma. The Liujia and Tieshajie basalts show typical OIB-like trace element patterns, and have high Ti concentrations and positive ɛHf(t) values, implying a fertile asthenospheric mantle as their magma source. The Liujia and Tieshajie basalts have relative low Mg# values (36-58) indicating that their parental magma experienced moderately to strong fractional crystallization of olivine and clinopyroxene. Most samples of the basalts have high Nb/La (1.05-1.39) and Nb/Th (19.04-19.53) values, indicating an insignificant crustal contamination. The 1.01-0.98 Ga intra-plate mafic magmatism argued against the existence of Grenvillian orogeny in the Jiangnan orogen. The geochemical signatures of mafic rocks along the Jiangnan orogen display systematic changes from 1.01-0.98 Ga OIB-like mafic rocks to 0.97-0.83 Ga island arc and arc-like basalts to 0.80-0.76 Ga OIB- and arc-like mafic rocks. The systematic geochemical variations of mafic rocks demonstrate that tectonic switch occurred from an intra-plate regime to an oceanic subduction-related setting to a post-orogenic environment. With respect to Cu-Au mineralization, this study suggests that the Tieshajie deposit is one of the oldest regional mineralization events and the formation of coeval metal-rich juvenile crust is a critical step for the generation of later regional large-scale Cu-Au mineralization.