Upper paleozoic basalts in the southern Yangtze block: Geochemical and Sr-Nd isotopic evidence for asthenosphere-lithosphere interaction and opening of the Paleo-Tethyan Ocean

摘要

Basalts in the Youjiang Basin of the southern Yangtze block. South China, consist of Upper Devonian (D-3) tholeiitic volcanics grading to Lower Carboniferous (C-1) alkali basalts. Alkali basalts are generally enriched in incompatible elements relative to tholeiites, having higher Zr. Nb, Hf, Th, LREE, Ti and P abundances, a La/Sm-CN, range of 2.3-2.8, a La/Yb-CN range of 5.9-7.5, a Sr-87/Sr-86(i) range of 0.7050-0.7053, and epsilon(ND)(t) values of -1.4 to -0.6. Tholeiites possess a La/Sm-CN range of 1.8-2.1. a La/Yb-CN range of 3.3-4.6, a Sr-87/Sr-86(i) range of 0.7047-0.7054, and epsilon(Nd) (1) values of -3.1 to -2.2. Both basaltic suites have EM1-like Sr-Nd isotopic signatures; however, their distinctly different La/Nb, Zr/Nb, and Th/Nb values suggest heterogeneous mantle sources. Higher La/Nb (1.03-1.31), Tb/Nb (0.22-0.24) and Zr/Nb (8.2-8.5) ratios in the D-3 tholeiites indicate the importance of enriched lithospheric mantle in the petrogenesis, whereas strong OIB-type trace element signatures-e.g., low La/Nb (0.74-0.87), Th/Nb (0.14-0.15) and Zr/Nb (5.3-5.9) in the C-1 volcanics necessitates an important contribution of an OIB-type asthenosphere. Increasing epsilon(Nd)(t) and La/ Sm values coupled with decreasing La/Nb ratios grading from D-3 to C-1 basalts suggest that the D-3 tholeiites were produced at relatively larger melting degrees with higher proportions of enriched components, and that the C-1 alkali basalts were generated at smaller melting degrees with lower proportions of enriched components. Such trends reflect a continuous lithospheric thinning event attending asthenosphere-lithiosphere interaction. Progressive rifting episodes explain the petrogenesis of these Upper Paleozoic basalts, suggesting that the southern Yangtze block had become a passive continental margin following the opening and spreading of the Paleo-Tethyan Ocean.