Neoproterozoic adakitic plutons in the northern margin of the Yangtze Block, China: Partial melting of a thickened lower crust and implications for secular crustal evolution

Numerous Neoproterozoic felsic and mafic-ultramafic intrusions occur in the Hannan region at the northern margin of the Yangtze Block. Among these, the Wudumen and Erliba plutons consist of granodiorites and have SHRIMP zircon U-Pb ages of similar to 735 Ma. The rocks have high K2O (0.8-3.6 wt.%) and Na2O (4.4-6.4 wt.%) and low MgO (0.4-1.7 wt.%). They also have high Sr/Y (32-209) and (La/Yb)(n) ratios (4.4-38.6). Their epsilon Nd values range from -0.41 to -0.92 and zircon initial Hf-176/Hf-177 ratios from 0.282353 to 0.282581. These geochemical features are similar to those of adakitic rocks produced by partial melting of a thickened lower crust. Our new analytical results, combined with the occurrence Of Voluminous arc-related mafic-ultramafic intrusions emplaced before 740 Ma, lead us to propose that the crustal evolution in the northern margin of the Yangtze Block during Neoproterozoic involved: (1) rapid crustal growth and thickening by underplating of mafic magmas from the mantle which was modified by materials coming from the subducting oceanic slab from similar to 1.0 to similar to 0.74 Ga, and (2) partial melting of the thickened lower crust due to a thermal anomaly induced by upwelling of asthenosphere through an oceanic slab window, producing the similar to 735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 kin at the northern margin of the Yangtze Block at similar to 735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region. (C) 2008 Elsevier B.V. All rights reserved.