Tectonic setting of the Beiwu basalts from Yangla,northwestern Yunnan: An approach

The Beiwu basalts constitute a large tectonic block in the Jinshajiang ophiolitic mélange zone in northwestern Yunnan. The integration of detailed field investigation and lithogeochemical study shows that the Beiwu basalts have the features of low-K tholeiitic basalts,low differentiation,remarkedly enrichment of large ion lithophile elements (LILE)relative to the high field strength elements (HFSE),low total REE amount,slightly enrichment of light rare earth elements (LREE). The Beiwu basalts are not assigned to the typical mid-oceanic ridge basalts (MORB)or island-arc basalts (IAB),but to a transitional type between them or to the MORB-like basalts. The similarity of the Beiwu basalts to the island-arc basalts suggests the existence of the subduction during the diagenetic processes. It is concluded from lithologic association,geological background and tectonic setting that the Beiwu basalts may be assigned to the front arc basalts (FAB)which are formed in the initial subduction of intra-oceanic arc environments.