Petrogenesis and tectonic significance of Neoproterozoic meta-basites and meta-granitoids within the central Dabie UHP zone, China: Geochronological and geochemical constraints

摘要

A combined geochemical (whole-rock elements and Sr-Nd-Pb isotopes, zircon trace elements and Hf isotopes) and geochronological (zircon UePb ages) study was carried out on the relatively low-grade meta-basites and meta-granitoids from Longjingguan within the central Dabie ultrahigh-pressure (UHP) metamorphic zone, east-central China. Zircon investigations indicate that the meta-basites were formed at ~772Ma and subsequently experienced granulite-facies metamorphism at ~768 Ma and a later thermal overprint at ~746Ma, while the meta-granitoids recorded three groups of zircon ages at ca. 819 Ma, 784 Ma and 746 Ma. The meta-granitoids can be subdivided into low-Si and high-Si types, and they were derived from mid-Neoproterozoic partial melting of the Neoarchean and Paleoproterozoic metamorphic basement rocks of the South China Block, respectively. These Neoproterozoic zircon ages are consistent with the protolith ages of the Dabie Triassic UHP meta-igneous rocks. In addition, the lowgrade rocks have bulk-rock Pb isotope compositions overlapping with the UHP meta-igneous rocks. Therefore, the low-grade meta-basites and meta-granitoids could be interpreted as counterparts of the UHP meta-igneous rocks in this area, suggesting the same petrogenesis for their protoliths in the Neoproterozoic. Trace element patterns indicate that the low-grade rocks have better preserved their protolith compositions than their equivalent UHP rocks, and thus they are more suitable for elucidating the Neoproterozoic evolution of the northern margin of the South China Block. Zircon ages combined with geochemical features strongly suggest that the protoliths of the meta-granitoids and meta-basites were formed in a magmatic arc and a continental rifting setting, respectively. More specifically, the granitoids derived from partial melting of Neoarchean and Paleoproterozoic basement materials at ~819Ma in a magmatic arc setting, whereas the precursors of the meta-basites are products of a continenta