Sedimentary-Tectonic Evolution From Permian To Triassic In Luoji, Zhongdian County

The Luoji Area in Shangri-La County belongs Ganzi-Litang suturezone of Sanjiang tectonic belt in southwest China. Due to its specialtectonic significance, it has attracted wide attention. However, majority ofprevious studies focused on middle and northern section of Ganzi-Litangbelt, while the southern section draw little attention. Based on theresearches of rock assemblages, the sedimentary environments of cherts,tectonic environment of basalt, the characteristics of zircon provenance,ophiolite and its geochronology, the sedimentary-tectonic evolution inPermian and Triassic will be discussed.During this research, the ultrabasic rocks, gabbros, pillow basalt andradiolarian cherts have be discovered in Luoji Area, they composed to bethe different units of the ophiolite which is a dismembered theconstruction of ophiolitic mélange. The ultrabasic rocks have thecharacteristic of cool emplacement, the nascent oceanic crust formed at253.3±7.5Ma by the study of zircon U-Pb age of ultrabasic rocks from theophiolitic mélange, and the initial small ocean basin formed at the LatePermian.The sedimentary-tectonic evolution from Permian to Triassic ofLuoji Area experienced three stages.â‘ The intracontinental rift basinstage (from Early Permian to Middle Permian). In this stage, the basinwas in the intracontinental extension. From the lower to the upper, therock assembles changed from the shallow marine volcanicformationâ†'the shallow to sub-deep sea flysch formationâ†'sub-deep seabasic volcanic formation.â‘¡S hort initial ocean basin stage (late Permian).In this stage, the basin reached its maximum scale and the deep sea basicvolcanic formation was deposited.â‘¢The oceanic subduction stage (end of Permian-Early Triassic). In this stage, the tectonic background changedfrom extension to compression and a small amount volcanic arc weredeposited, finally the terrigenous clastic sediment deposited in theremnant ocean basin. Detrital zircon U-Pb isotopic age of Pingzi Group is254.78±0.81Ma, it imply that sedimentary material of Early Triassiccame from the late Permian magma source, and the sediment have thecharacteristics of near-source deposition.