The define and geological significance of Permian-Triassic magmatic arc at Dangjiangrong, south of Duocai ophiolite mélange zone in the Zhiduo area, Qinhai, China

Duocai ophiolite melange zone,a part of the Xijir Ulan lake-Jinsha river ophiolite zone,located in the binding site of jinsha suture zone and ganzi suture belt of the northern Tibet Plateau.Through the detailed field investigation of the S-N directional cross section of Dangjiangrong volcanic rocks zone,located in the south of the Duocai ophiolite meglange zone,we found that the pre-existed main rock assemblage was andesite,rhyolite porphyry,rhyolitic tuff and andesitic tuffaceous lava.The results from XRF analysis and ICP-MS analysis on systemic sampling show that the Dangjiangrong volcanic rocks are rich in SiO2(56.92%~75.71%) and Na2O(Na2OK2O),poor in TiO2(0.15%~0.77%),belonging to calc-alkaline type rocks.The chondrite-normalized REE-patterns are characterized by LREE enrichment with variable degree of negative Eu anomalous values(δEu=0.58~0.89) and the primary mantle-normalized trace elements-parterns by HFSE depleted as well as P element.It is proved that the volcanic rocks were formed in an island-arc environment by the evidence of petrochemistry and geochemistry of trace and rare element,which provides the corroborating evidences to the further define of subduction polarity and tectonic setting of Duocai ophiolite melange zone.