Petrogenesis of the Neoproterozoic low-δ18O granitoids at the western margin of the Yangtze Block in South China

The western margin of the Yangtze Block in South China preserves voluminous arc-like intrusive and extrusive igneous rocks which were formed in an active continental margin during the Neoproterozoic time. Rocks from the ca. 800 Ma Moutuo pluton are calc-alkaline I-type granitoids which have high SiO2 (67.13-77.96 wt%), K2O (2.08-4.32 wt%) and low MgO (0.16-1.81 wt%). They show concave chondrite-normalized rare earth element patterns, and are rich in large ion lithophile elements (e.g. Rb, Ba, Th, U and Pb), but depleted in high field strength elements (e.g. Nb, Ta and Ti). The magmatic zircons have low delta O-18 (3.29-4.80 parts per thousand) and high epsilon Hf(t) values (+6.10 to +13.16) with two-stage Hf model ages (T-DM2) of 0.87 to 1.33 Ga. These lines of evidence suggest that the juvenile mafic crust, protolith of the low-delta O-18 granitoids, was derived from the sub-arc mantle that was modified by slab-derived materials (similar to 90% altered oceanic crust and similar to 10% oceanic sediments). Partial melting of the newly formed mafic crust followed by fractional crystallization played an important role in the petrogenesis of the Moutuo granitoids. Based on the available oxygen and hafnium isotopic data for the Neo-proterozoic magmatic zircons in this region, the low-delta O-18 character of the I-type granitoids in active continental margins was probably inherited from the sub-arc mantle that was mixed with the altered oceanic crust and oceanic sedimentary materials.