Provenance and tectonic setting of the Triassic clastic deposits in the Napo basin, South China: evidence from petrography, whole-rock geochemistry and detrital zircon U-Pb geochronology

The provenance and tectonic setting of the Lower-Middle Triassic clastic sediments from the Napo basin, South China, have been examined here using detrital modes, whole-rock geochemistry and detrital zircon U-Pb ages. Field investigations indicate that these sediments consist of fan delta, slope and turbidity fan facies with dominantly southward palaeocurrent directions. Detrital modes and geochemical characteristics of the clastic rocks indicate that they were derived from mixed magmatic arc and Palaeozoic successions in a continental island arc setting, with no significant sediment recycling. The U-Pb age spectra of sandstone detrital zircons from different stratigraphic positions are similar, with one major group (300-230 Ma), two subordinate groups (400-320 Ma and 480-420 Ma, respectively) and two scattered groups (1200-800 Ma and 2000-1700 Ma, respectively). Thus, we consider that the north late Permian - Middle Triassic volcanic rocks and the uplifted Palaeozoic sedimentary/volcanic sequences constituted the predominant sources. The detritus derived from the late Permian Emeishan mafic rocks is subordinate and limited. The pre-Devonian zircons are likely sedimentary-recycled or magmatic-captured instead of directly derived from the early Palaeozoic orogen (e.g. Yunkai massif) and Neoproterozoic Jiangnan orogen because of the topographic barrier of a magmatic arc and carbonate platform. Considering the spatial and temporal distribution characteristics of the volcanic arc and ophiolite, we suggest that the Triassic Napo basin was a fore-arc basin within a continental island arc setting, which developed in response to the northward subduction of the Babu-Cao Bang branch ocean beneath the South China Block.