Carboniferous-Triassic subduction and accretion in the western Kunlun, China: Implications for the collisional and accretionary tectonics of the northern Tibetan Plateau

摘要

A newly defined, 250 km by 500 km, Carboniferous-Triassic subduction-accretion complex, the Mazar accretionary prism in the western Kunlun, comprises two subduction complexes and a forearc-basin succession. (1) The Bazar Dara subduction complex contains imbricated blocks of sandstone, arenite, limestone, and metavolcanic rocks in a matrix of weakly metamorphosed Triassic deep-sea turbidites. The metavolcanic rocks include basalt, diabase, spilite, and andesitic porphyry. Trace element geochemistry shows that pillow and amygdaloidal basalts are oceanic-island tholeiites. This zone has Ordovician to Permian fossils and is situated on the older, more highly deformed and metamorphosed side of the prism adjacent to the Sailiyak magmatic arc. (2) The Heweitan subduction complex is composed of blocks of limestone, turbidite, and radiolarite in a slate-phyllite matrix intercalated with calc-alkalic volcanic rocks. This complex has Permian to Triassic fossils and is situated on the younger, less deformed, and metamorphosed side of the prism adjacent to the suture zone. (3) The Qitai forearc basins are infilled with turbidites (Late Triassic) intercalated with carbonates; these rocks overlie the accretionary prism. The accretion-related structure is dominated by large-scale northeast-dipping thrusts and subvertical cleavage stitched by 215–190 Ma granites. There is an overall decrease in metamorphic grade and deformation intensity from the arc to the suture zone across the Bazar Dara and Heweitan subduction complexes. The Mazar accretionary prism formed by subduction-accretion processes during closure of the Paleotethyan Ocean and the final docking of the Gondwanan Karakoram-Qiangtang block to the Cathaysian (Eurasian) Kunlun block.