Petrogenesis and geodynamic implications of the Xiema and Ziyunshan plutons in Hunan Province, South China

To understand the Mesozoic tectonic settings of the South China Block, samples collected from the Xiema and Ziyunshan plutons were analyzed with respect to zircon age, whole-rock and biotite chemical composition, whole-rock Sr-Nd and zircon Hf isotopic composition. Petrographic analysis and zircon LA-ICP-MS dating reveal that the Xiema pluton and the outer portion of the Ziyunshan pluton are composed of Late Triassic (Indosinian) biotite granodiorites and quartz monzonites with contemporaneous mafic microgranular enclaves (hereafter MMEs), whereas the inner part of the Ziyunshan pluton consists of Early Jurassic (Yanshanian) biotite granites and two-mica granites. Elemental and isotopic geochemical characteristics demonstrate that the Late Triassic granites are I-type granites and the Early Jurassic granites are S-type granites; furthermore, the data suggest that the two types of granites originated from different sources. The Late Triassic granites were mainly derived from the partial melting of Proterozoic meta-greywacke and meta-basalts at temperatures of 758-814 °C and log(fO2) values of −13 to −12. The residual mineral phases contain plagioclase, clinopyroxene/orthopyroxene, and quartz. At melting temperatures of 738-772 °C and log(fO2) values ranging from −15 to −13, the Early Jurassic granites originated from the partial melting of Proterozoic greywacke, with K-feldspar, ilmenite, clinopyroxene/orthopyroxene, and zircon serving as residues in the source. Geochronology and geochemistry data suggest that the Late Triassic granites were formed under the extension environment caused by the Early Mesozoic post multi-plate convergence tectonic setting, whereas the Early Jurassic granites formed in an intracontinental extensional setting.