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 mélange 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(Na2O>K2O), 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 mélange zone.