A model for multi-stage of the Early Palaeozoic Danfeng ophiolite in Qinling orogen belt: From arc to inter-arc basin
秦岭造山带早古生代蛇绿岩的多阶段演化: 从岛弧到弧间盆地

The origin of the Early Palaeozoic Danfeng ophiolite in Qinling orogen is the key for understanding the tectonic evolutionary processes of proto-Tethyan Ocean in China.Several models have been presented to clarify the evolutionary of the Early Palaeozoic Danfeng ophiolite,e.g.the model of arc,ocean island and mature oceans,etc.But these simple model fail to explain two facts:(1) the diachronism of different mafic-rock type(N-MORB and IAT) ;(2) almost every mafic-rock type shows the characteristic of the superposition of different tectonic setting(arc,BABB and ocean ridge).After studying the Yingezui ophiolite of the Early Palaeozoic Danfeng ophiolite in Shaanxi Province,we discovered that the mafic-rocks(including basalts and gabbros) can be divided into two geochemical group:HTI(TiO2:1.21% ～ 1.56%) and LTI(TiO2:0.09% ～ 1.07%).The two groups cover nearly all of the mafic-rock types in the Early Palaeozoic Danfeng ophiolite.The HTI group with E-MORB affinity is characterized by depletion LREE without negative Nb-Ta anomaly.The LTI group with IAT affinity is characterized by slight enrichment of LREE and with negative Nb-Ta.We propose a multi-stage modal for the origin of the Early Palaeozoic Qinling ophiolite.StageⅠ:ca.524Ma,the ocean possibly started to subduct towards the north.With the dehydration of the subduction slab,an immature arc system being composed of LTI type mafic-rock was formed in the south rim of the North Qinling terrane.Stage Ⅱ:ca.524 ～ 474Ma,the immature arc extended to an arc basin,leading to HTI type mafic-rock(E-MORB).Stage Ⅲ:after 474Ma,the arc basin started to close and plus to the immature arc system,leading to the MORB affinity with IAT.A multi-stage evolution characterize of Tethyan Tectonic Region appear once more in the early Palaeozoic ophiolite in Qinling orogen.