Geochemistry of the Longsheng Ophiolite from the southern margin of Yangtze Craton, SE China

摘要

Geochemical and Sm-Nd isotopic data are presented for basaltic volcanic rocks from the Longsheng Ophiolite, which were crystallized at 977 +/- 10 Ma and tectonically emplaced into the Danzhou Group (Banxi equivalent) along the southern margin of the Yangtze Block, SE China during the Neoproterozoic. The volcanic rocks have undergone low-grade alteration and metamorphism displaying high Na2O contents (>5%) and a large variation of mobile elements, but a relatively small range of immobile elements. With enrichment of Th and LREEs, relatively low high-field-strength elements and other REEs, and the Ta-Nb-Ti negative anomaly, the volcanic rocks are geochemically similar to basalts forming in subduction-related environments, typical of the ''supra-subduction zone'' ophiolites. Nd isotopes of the volcanic rocks preclude a MORE-like depleted mantle source for the Longsheng Ophiolite, though their Sm-Nd isotopic systems have been rehomogenized during the lat Neoproterozoic alteration. The Longsheng volcanic rocks, therefore, are interpreted to have been generated either by extension and thinning of a continental are, or at a backarc side of continental are during the early stage of the backarc spreading.