Geochronology, petrogenesis and tectonic significance of two episodes of Neoproterozoic diabasic magmatism in South China: from orogenesis to intracontinental rifting

Neoproterozoic mafic magmatic rocks are widely distributed in the Jiangnan Orogenic Belt. Understanding the petrogenesis and tectonic settings of these mafic rocks has significant implications for the regional tectonic evolution and the assembly and breakup of supercontinent Rodinia. In this research, two Neoproterozoic diabase samples (Baotan and Huaihua) were investigated by zircon and baddeleyite U–Pb dating with the ages of 805 Ma and 788 Ma, respectively. The Baotan samples are depleted in HFSE, enriched in LILE and have enriched Sr–Nd isotope compositions (⁸⁷Sr/⁸⁶Sr_(i) = 0.731620 to 0.745440, ε_Nd(t) = – 3.7 to – 7.6). We suggest that they were derived from an enriched lithospheric mantle in a convergent tectonic setting, which was metasomatized by contemporaneous slab–derived hydrous fluids. On the contrary, the Huaihua diabase samples have trace element patterns similar to oceanic island basalts but little HFSE depletion and slightly enriched Sr–Nd isotopes (⁸⁷Sr/⁸⁶Sr_(i) = 0.703101 to 0.705006, ε_Nd(t) = – 0.1 to – 1.6). We infer that they originated from partial melting of a convective asthenospheric mantle under an extensional environment, which was previously metasomatized by slab–derived melts. In combination with other geological observations, we proposed a geodynamic evolution model for the South China Block from the mid–Neoproterozoic orogenesis to the late Neoproterozoic intracontinental rifting.