Neoproterozoic Bimodal Volcanic Rocks from Dingyuan Formation in Western Dabie Area, Northern Margin of Yangtze Block, China: Geochemistry, Petrogenesis and Geological Implications

Neoproterozoic bimodal volcanic rocks from the Dingyuan Formation within the western Dabie terrain consist of rhyolite, rhyolitic tuff and basalt. Elemental, whole-rock Sr-Nd and zircon Hf isotopic analyses were carried out in this study for these volcanic rocks, in order to better understand their petrogenesis and geodynamic processes along the northern margin of Yangtze block. The rhyolite is characterized by high concentrations of SiO2 and Al2O3, belongs to the calc-alkaline series, and is peraluminous. It is enriched in light rare earth elements (LREEs) and shows unapparent Eu anomaly. The basalt has SiO2 of 48.26% to 51.71%, Mg# values of 0.32 to 0.59, and is weakly enriched in light rare earth elements. The rhyolite has initial 87Sr/86Sr ratios ranging from 0.703 5 to 0.707 7, and negative εNd(t) values of -10.6 to -6.5, with calculated two-stage Nd model ages of 1.69 to 2.00 Ga, while the basalt has (87Sr/86Sr)i of 0.706 1 to 0.708 1, εNd(t) of -6.6 to +3.4, and the Nd model ages (TDM) values of 1.44 to 2.02 Ga. In situ zircon Hf isotopic analyses show that the εHf(t) values of zircons from the rhyolite vary from -19.2 to -7.2, with calculated two-stage Hf model ages of 1.77 to 2.59 Ga. The geochemical and Sr-Nd-Hf isotopic signatures suggest that the rhyolite has a close affinity to S-type granite, and is derived from partial melting of the ancient crust of the Yangtze block. The basalt is most likely derived from an enriched lithospheric mantle, with fractional crystallization and assimilation of crustal materials during the evolution of the magma. It is proposed that the Neoproterozoic bimodal volcanic suites from the Dingyuan and the Qijiaoshan formations are erupted coevally, but have different sources and magmatic processes. The bimodal volcanic rocks in this study provide evidences for an enriched lithospheric mantle beneath the Dabie terrain at ca.740 Ma, and a rifting extensional setting for the north margin of the Yangtze block. © 2021, Editorial Department of Earth Science. All right reserved.