Geochronology and Geochemistry of the 890 Ma I-Type Granites in the Southwestern Yangtze Block: Petrogenesis and Crustal Evolution

The tectonic evolution of the southwestern Yangtze Block during the Early Neoproterozoic period is still controversial because of the limited quantities of 1 000-860 Ma magmatic rocks. In this study, our new LA-ICP-MS zircon U-Pb dating results demonstrate that the Yanbian granodiorites in the southwest Yangtze Block were emplaced at 894.6 +/- 7.4 Ma, representing the product of an 894 Ma magmatism. The Yanbian granodiorites are metaluminous to weak peraluminous with A/CNK values of 0.8-1.1, resembling I-type granitoids. They are characterized by right-inclined REE patterns with moderate to insignificant negative Eu anomalies (delta Eu=0.6-0.9). Their primitive mantle-normalized trace element patterns are characterized by depletion of Nb, Ta and Ti and weakly enrichment of Th. Considering the positive whole-rock epsilon(Nd)(t) (+5.8 to +6.8), we propose that these granodiorites originated from the partial melting of juvenile mafic lower crust. The Yanbian I-type granitoids have low Y and Nb contents similar to volcanic arc igneous rocks in the Y-Nb plot for tectonic discrimination. In conclusion, Early Neoproterozoic Yanbian granodiorites have generated in a compression setting in an active continental margin. Together with previous studies from the southwestern Yangtze Block, we suggest that the 894 Ma subduction-related Yanbian granodiorites represent the early stage of subduction at the southwestern margin of the Yangtze Block.