Abstract <p>Oceanic island fragments preserved within the accretionary complex serve as an outstanding medium for investigating the subduction and accretion process. This paper presents a multidisciplinary study on early Carboniferous sediment in the south Qiangtang accretionary complex to better understand its tectonic setting and evolution process. Through the comparison with the detrital zircon ages of adjacent terranes, the detrital zircons of the Riwanchaka Formation are considered to come from the underlying magmatic rocks of the Wangguoshan Formation. The sandstone petrology and cumulative proportion age of detrital zircons indicate that the source tectonic setting of the Riwanchaka Formation is a convergent island arc. The rapidly accumulating sandstone, coral with low differentiation, the limestone breccia, slump limestone, and slump sandstone of the slope facies all further suggest that the Riwanchaka Formation has been accumulated within an isolated basin flanked by an intra-oceanic volcanic arc. Based on the similarity of the age distribution patterns and Th/U ratios of detrital zircons, this research infers that the Maoershan metamorphic rocks are sourced from the intra-oceanic arc composed of the Riwanchaka Formation and the Wangguoshan Formation. This provides important evidence for the existence of the Longmu Co-Shuanghu Paleo-Tethys Ocean and confirms that the subduction of the Paleo-Tethys Ocean in the Early Carboniferous was intra-oceanic subduction.</p>