Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area,West Qinling

The mafic-ultramafic assemblages,which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen,Lijiahe,Lubangou and Gaojiahe area,consist mainly of meta-peridotites,gabbros and basalts.The meta-peridotites are characterized by high SiO_(2) and MgO contents,lowΣREE,as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite.The basalts from the Yuanyangzhen,Lijiahe and Lubangou area are characterized by relatively high TiO_(2) content,low Al_(2)O_(3) content and Na_(2)O>>K_(2)O.Above all,it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB.In comparison,the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting.From studies of the regional geology,petrology,geochemistry,geo-chronology and all above evidence,it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite,which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean.It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading.This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault,and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.