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

Abstract

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 ∼ 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 εNd values range from - 0.41 to - 0.92 and zircon initial 176Hf/177Hf 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 ∼ 1.0 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 ∼ 735 Ma adakitic Wudumen and Erliba plutons. Our model suggests that the crustal thickness was more than 50 km at the northern margin of the Yangtze Block at ∼ 735 Ma, and rule out the possibility of a mantle plume impact causing the > 735 Ma magmatism in the region. © 2008 Elsevier B.V. All rights reserved.