Neoproterozoic peraluminous granites in the western margin of the Yangtze Block, South China: Implications for the reworking of mature continental crust

Abstract

Significant widespread melting of the mantle and juvenile mafic lower crust is known to have occurred along the western margin of the Yangtze Block during the Neoproterozoic, but melting of the mature continental crust remains poorly understood. Peraluminous granites can provide vital insights on the reworking of mature continental crustal materials. We present zircon U-Pb-Hf isotopic, whole-rock geochemical, and Sr-Nd isotopic data from the Neoproterozoic peraluminous granites in the western Yangtze Block, South China. Zircon U-Pb dating displays concordant crystallization ages of ca. 840 Ma for the Kuanyu granites and ca. 835 Ma for the Cida granites. These peraluminous granites have high SiO2 (66.9–75.6 wt%), K2O (4.61–7.29 wt%) contents, as well as high K2O/Na2O (1.44–3.25) and A/CNK (molar ratio of Al2O3/(CaO + Na2O + K2O)) (1.04–1.18) ratios. The samples are enriched in Rb, K, Th, U, and Pb, and depleted in Nb, Ta, Ba, Sr, and Ti, indicating a middle-upper crustal affinity. They are also characterized by high (87Sr/86Sr)i (0.7099–0.7217) and negative εNd(t) values (−5.1 to −2.9), which resemble the isotopic features of an evolved continental crust source. Furthermore, these peraluminous granites possess variable CaO/Na2O (0.09–0.65) and Al2O3/TiO2 (25.3–88.4) ratios, moderate Rb/Ba (1.68–3.86) and Rb/Sr (0.32–0.85) ratios, as well as high molar Al2O3/(MgO + FeOT) (2.04–5.23) and low molar CaO/(MgO + FeOT) (0.15–0.48), implying that they predominantly originate from heterogeneous metasedimentary sources (metapelites + metagreywackes). Scarce evidence of hybridization processes between crust- and mantle-derived components indicates that their heterogeneous zircon Hf isotopic compositions (εHf(t) = −7.75 to +3.31) may be caused by disequilibrium partial melting of heterogeneous metasedimentary sources, similar to peraluminous granites from the Altai area and Jiangnan orogen in China. In combination with previously reported results, we suggest that the Kuanyu and Cida peraluminous granites represent melting of mature crustal material in an evolved middle-upper crust source during the early stages of the Neoproterozoic subduction process. The western margin of the Yangtze Block underwent not only melting of the juvenile mafic lower crust but also reworking of the mature continental crust during the Neoproterozoic.