Detrital records for Upper Permian-Lower Triassic succession in the Shiwandashan Basin, South China and implication for Permo-Triassic (Indosinian) orogeny

Abstract

Upper Permian to Lower Triassic siliciclastic succession in the Shiwandashan Basin, South China, accumulated in response to a Permo-Triassic orogeny (often referred to as the Indosinian orogeny). The petrology, geochemistry and geochronology of this succession, along with north and northwest-directed paleocurrent indicators, reveal an evolving provenance related to erosion and reworking of Precambrian, Early Paleozoic and Permian to Early Triassic units exposed to the south of the basin. The Upper Permian to Lower Triassic sandstones within the basin are quartz dominated, which along with their high Th/Sc and Zr/Sc ratios indicate a multi-cycled source. Sandstone clasts in the Upper Permian conglomerates display age patterns similar to nearby Silurian strata. Other clast types (limestone, mudstone and cherts) are from the Early Paleozoic strata within or adjoining the basin. Detrital zircon age spectra of the strata display prominent age groups at 1200–800Ma, 650–500Ma and 460–420Ma, and are inferred to have been derived from basement units similar to those exposed in the Yunkai Massif to the south and southeast of the Shiwandashan Basin and/or from reworking of the Paleozoic units around the basin. The Lower Triassic strata also contain 260–240Ma zircons that were likely derived from magmatic rocks located to the south of the basin. The Late Permian marks a significant change in the paleogeography of the Shiwandashan Basin from an older deep marine chert succession to a terrestrial to shallow marine environment receiving an influx of clastic detritus related to uplift and erosion to the south of the basin. The Lower Triassic units within the basin record a further pulse of sediment influx including detritus derived from approximately syn-sedimentary magmatic activity. Yunkai Massif, located in the southeast of the basin, underwent uplift in Late Permian and provided majority detritus for the basin in the Late Permian to Early Triassic. Integration of provenance data with regional geological information, magmatic and metamorphic records to the south of the Shiwandashan Basin suggests the basin was converted from a pre-Late Permian deep marine extensional basin to a Late Permian to Early Triassic foreland basin. Conversion to a foreland basin reflected collision between the South China and Indochina blocks.