Provenance and tectonic settings of late Triassic-Jurassic deposits in the Southwestern Yangtze block: evidence from the geochemistry, SW China.

Abstract

During the Late Triassic to Jurassic, the western Yangtze Block transitioned from marine carbonate deposits to terrestrial detrital deposits. There are different views on the orogen evolution of the western margin of the Yangtze Block, such as whether the Longmenshan Thrust Belt was uplifted in the Late Triassic or Early Jurassic, and when the Yangtze Block began to receive the source from Yidun Terrane. In this paper, whole-rock elemental compositions and zircon U-Pb ages and Hf isotope data from the Upper Triassic to Jurassic successions are introduced. The whole-rock elemental compositions reveal that the clastic rocks were deposited in a collisional setting and were derived mainly from intermediate-felsic magmatic rocks and recycled sediments. The Upper Triassic zircon sample shows a single age peak at ~ 831 Ma. The three Jurassic samples show similar age patterns with four main age populations (e.g., 200-500 Ma, 788-834 Ma, 1863-1875 Ma, and 2462-2531 Ma). The results revealed that the Kangdian Palaeo-land was the main provenance area in the Late Triassic. In contrast, in the Early Jurassic (~ 198.7 Ma), the Longmenshan was massively uplifted, and the recycled sediments from the Longmenshan, Songpan-Ganzi Terrane and Yidun Terrane provided large amounts of detrital material to the southwestern Yangtze Block. Volcanic rocks from the Yidun Terrane also provided a partial source for the southwestern Yangtze Block. In the Middle Jurassic, owing to the weathering and erosion of the Longmenshan Thrust Belt in the Early Jurassic, the provenance supply in Longmenshan decreased, and the provenances from the Yidun Terrane and the Songpan-Ganzi Terrane increased. In the Late Triassic to the Early Jurassic, the tectonic setting of the southwestern Yangtze Block may have changed from a passive continental margin to a foreland basin.