| The Middle Triassic Pingxiang basin, located in the suture zone between the Indochina and South China blocks, records the processes of the closure of the Paleo-Tethys and the collision of the Indochina and South China blocks. Based on careful observations along a single transect across the Pingxiang basin and detailed analysis of sedimentation and provenance of this basin, its tectonic setting are systematically studied in this paper. The results presented in this paper show that the sedimentations in the Pingxiang basin are dominated by deep-water basin facies and turbidite fan facies, with minor amounts of deposits of delta facies. In particular, tectonic rises and deep valleys were developed during the Middle Triassic associated with the subduction and collision. The deep-water basin is dominated by dark-grey mudstone facies with the interbeds of debris flows, is formed on the steep slope of the tectonic rises. The turbidite fan facies is characterized by flute cast, normal-graded bedding, climbing ripple lamination, convoluted lamination, bidirectional cross bedding, lenticular bedding and parallel bedding. Commonly, the delta facies is spatially associated with the tectonic rises, and characterized by large tabular cross bedding, tidal creek deposits, thick lenticular sandbody and brick red mudstone, in great agreement with the Jamacia-type or active-type fan delta that is always developed in the subduction-related tectonic settings and occurs on the gentle slopes of the tectonic rises. Sedimentation mechanism of the deposits in the Pingxiang basin includes debris flow, turbidity current, and bottom current. Paleocurrent analysis performed within the Pingxiang basin indicated northward and southward directed paleo-sediment dispersal patterns. Petrography features suggest that the sandstones, which have low compositional and textural maturity, were derived from recycled orogen or collisional orogen. The geochemistry of fine-grained sandstones and mudstones are characterized by moderate S1O2contents (61.71%-74.85%) that are close to the upper crust, high K20/Na20ratio (6.50-0.51) and high Fe2O3t+MgO contents (7.29%-10.31%). The Fe2O3t/K2O ratio (2.05%-5.54%) shows low stabilities of materials for these rocks. The REE patterns of analyzed sediment rocks in the Pingxiang basin are similar to those of the upper crust and typical Archean shales, and show enriched LREE and flat HREE with obvious Eu-and Ce-anomalies. The CIA index (generally71-88) and Th/U ratios (3.68-9.53) indicate that the sediments in the basin may have undergone high degree of weathering. Geochemistry demonstrates that the detritus in the Pingxiang basin were derived predominantly from felsic volcanic rocks source, and an active continental margin setting in which the Pingxiang basin developed. In summary, the Pingxiang basin is a tectonically modified remnant forearc basin, temporally and spatially associated with a volcanic arc (mapped as the Beisi Formation and dominated by rhyolites). This arc-forearc basin configuration presents supporting evidence of northward subduction of the oceanic lithosphere of the Paleo-Tethys that started in the late Permian and finally terminated in the Middle Triassic when the Indochina block collided with the South China block. |
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