Subduction of the Paleozoic intra-oceanic arc terrane as an important mechanism for the formation of the South Qiangtang accretionary complex: evidence from Riwanchaka intra-oceanic arc

Subduction and accretion of intra-oceanic island arc and microcontinent are important mechanisms for the formation of accretionary complex.The present paper deals, on the basis of field measurements of the Riwanchaka Formation in south Qiangtang, with sedimentary facies, fossils, detrital modal analysis and detrital zircon dating. According to regional comparison, we conclude that: (1) the Riwanchaka Formation is not derived from adjacent terranes surrounding Longmu Co-Shuanghu Paleo-Tethys, but from the underlying volcanic rocks of Wangguoshan Formation. (2) Although the coral fossils in the Riwanchaka Formation are abundant, the differentiation is extremely low, suggesting that its depositional position should be a relatively isolated location. In the light of the geochemical signatures of intra-oceanic arc of the Wangguoshan Formation, which should belong to an intra-oceanic system with the SSZ ophiolites with similar age, we suggest that they are originally a Devonian-Carboniferous ancient arc terrane that developed by intra-oceanic subduction.In terms of the similarity of the detrital zircondistribution patterns, we suggest that the part of the South Qiangtang accretionary complex in Maoershan region should be derived from the Riwanchaka Formation.Therefore, the Riwanchaka intra-oceanic arc terrane should have experienced subduction and accretion during the formation of south Qiangtang accretionary complex. The frontal offscrapping resulted in the deformation of the Riwanchaka Formation and Wangguoshan Formationinin the shallow part of the accretionary wedge. While in the deeper part of it, the Riwanchaka Formation was underthrustedand experienced high-pressure metamorphism, and exhumed by later tectonic process.Therefore, the subduction and accretion of the ancient intra-oceanic arc terrane played an important role in the formation of the accretionary complex.