Geochemical constraints on the nature of magma sources for Triassic granitoids from South Qinling in central China

Abstract

A combined study of zircon U-Pb ages and Lu-Hf isotopes, whole-rock major-trace elements and Sr-Nd isotopes as well as mineral chemistry and O isotopes was carried out for Triassic granitoids from the South Qinling orogen in central China. Model calculations were also performed to examine the trace element fractionation during partial melting of crustal rocks. The results provide insights into the nature of magma sources for these granitoids. LA-ICPMS zircon U-Pb dating yields concordant ages of 208±2 to 216±3Ma for these granitoids from the Shahewan (SHW), Caoping (CP) and Zhashui (ZS) plutons, and no relict zircon cores are identified by the CL imaging and U-Pb dating. The SHW and CP granitoids contain hornblende and are metaluminous with A/CNK ratios of 0.84 to 0.93. They exhibit relatively low SiO2 contents (62.88–69.04wt.%) but high contents of FeOT, MgO and TiO2, and slightly to negligibly negative Eu anomalies (δEu=0.79–0.89). Zircons from them show mantle-like δ18O values of 4.71 to 5.72‰. In contrast, the ZS granites contain no hornblende and are metaluminous to weakly peraluminous with A/CNK ratios of 0.99 to 1.03. They show relatively high SiO2 contents (69.32–75.94wt.%) but low FeOT, MgO and TiO2 contents, and moderate negative Eu anomalies (δEu=0.63–0.81). They have slightly low zircon δ18O values of 4.60 to 4.83‰. All of these granitoids show arc-like trace element distribution patterns with enrichment in LREE and LILE (e.g., Rb, K and Pb) but depletion in HFSE (e.g., Nb, Ta and Ti). Geochemical comparison and modeling indicate that these granitoids are different from adakitic rocks originating from the thickened lower continental crust. Compared with the composition of felsic melts produced by petrological experiments of various lithologies, it appears that these granitoids are derived from dehydration melting of metabasaltic sources at normal lower crustal depths, and experienced varying degrees of fractional crystallization. The SHW and CP granitoids were crystallized under the conditions of variably low pressures of 0.7–2.3kbar, temperatures of 628–766°C and higher fO2 than the ZS granites. On the other hand, these granitoids show moderate whole-rock initial 87Sr/86Sr ratios of 0.7045 to 0.7055 and slightly negative εNd(t) values of −4.0 to −1.5 as well as slightly negative to positive zircon εHf(t) values of −1.3 to 3.2. They have two-stage Nd model ages of 1.05–1.38Ga and Hf model ages of 1.04–1.39Ga. These whole-rock εNd(t) and zircon εHf(t) values are comparable with those for Neoproterozoic Yaolinghe metabasalts and mafic-ultramafic intrusions in South Qinling but inconsistent with those for Neoproterozoic Bikou metavolcanics and mafic-ultramafic intrusions in the northern margin of the Yangtze craton. Together with their non-adakitic features, it is suggested that they were derived from partial melting of the mafic lower crust in South Qinling rather than the subducted Yangtze continental crust beneath South Qinling. The continental collision between the North China Block and South China Block in the Late Triassic would have brought about reworking of the mafic lower crust for the granitoid magmatism in the Qinling orogen.