Middle to late Jurassic felsic and mafic magmatism in southern Hunan province, southeast China: Implications for a continental arc to rifting
Abstract Extensive middle to late Jurassic felsic and mafic magmatism occurred in the southern Hunan province, southeast China. SHRIMP zircon U–Pb dating, mineral chemical, element geochemical and Sr–Nd–Hf isotopic data have been determined for these rocks. The results indicate that the middle Jurassic (178–170 Ma) Changchengling and Ningyuan basaltic rocks belong to tholeiitic series and alkaline series, respectively, and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab-released fluids of the subducted Paleo-Pacific plate. The late Jurassic (152–146 Ma) Daoxian basalts and Guiyang lamprophyre dikes are low-Ti, high-Mg potassic rocks. They were derived directly by partial melting of shallower (60–100 km) lithospheric mantle composed of amphibole- and phlogopite-bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc. The middle Jurassic (164 Ma) Tongshanling granitic rocks belong to calc-alkaline series and are strongly peraluminous. They were formed by partial melting of Palaeoproterozoic metasedimentary basement in the lower-crust plus additional input from coeval basaltic magma. The late Jurassic (156 Ma) Jinjiling and Xishan granitic rocks show A 2 subtype affinity. They were generated by partial melting of granulitized Palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low-Ti, high-Mg potassic magmas. A moderate degree (56–58%) of fractionation of these pure crustal melts could account for more felsic end-member of granitic rocks. Detailed petrologic and geochemical data for the middle to late Jurassic felsic and mafic rocks from the southern Hunan imply that during the middle Jurassic time SE China on the southeast of the Shi-Hang zone was a continental arc coupled with the subduction of the Palaeo-Pacific plate and since the beginning of late Jurassic time an intra-arc rift has been formed along the Shi-Hang zone as a consequence of slab roll-back.