Geochemical Characteristics and Genesis of Pillow Basalts from the Lanong Ophiolite in Tibet,China

The Lanong ophiolite,located in Jiangcuo,Bange,Tibet,is a key element within the Baila-Lanong ophiolite belt.Characterized by well preserved pillow lavas,the Lanong ophiolite consists of blocks of peridotites,gabbros,diabases and pillow lavas.Trace elements analysis indicates that the pillow lavas are enriched in Sr,Ba and Th,delpleted in Nb,Ta and Ti,clearly displaying compositional characteristics of the island-arc volcanic rocks.On the Chondrite-normalized trace element diagram,all lava samples show flat patterns,with (La/Yb)N and (Ce/Yb)N ratios similar to those of N-MORB.In conclusion,the Lanong ophiolite is of SSZ affinity,and should be the relics of a back arc basin.Combining the terranes' distribution with the discrimination results of geochemical diagrams,a simple tectonic evolution model is suggested:the Neotethys oceanic crust to the north began to southwardly subduct in middle-late Jurrasic,then a spreading sub-centre was formed on the obducting slab south of the subdction zone,which developed into a back arc basin subsequently.As the subdction going on,the back arc basin closed and only portions of the back arc crust were preserved by accretion.