Sedimentology And Detrital Zircon Geochronology Of Jurassic Strata In The Eastern Gaize,Tibet

The research area is located at the center of the Tibetan Plateau,within the area of the Gaize city of the Ali region.This area is at the then contacting area between the Qiangtang and Lasha blocks.We studied the lithology,chronostratigraphy,and biostratigraphy of the Jurassic strata within the Northern Gaize city based on the regional geology background,measured section,and field investigation.The sedimentary characteristics and basin evolutionary history of this region are investigated.The LA-ICP-MS U-Pb dating technique of zircons is employed to date the detrital zircons sampled from the Shamuluo Formation,Mugagangri Group,and Sewa Formation.Such ages are then compared with those from the adjacent Qiangtang and Lasha blocks to study the implications for regional tectonic evolution.The main finding of this work is summarized as the following:(1)Characteristics of rock association and their location within the Bangonghu-to-Nujiang suggest that the Mugagangri Group were developed in a accretionary basin environment.The basalts and limestone exposed within this group formed before or contemporarily with the ocean island deposition.(2)The first section of the Shamuluo group shows characteristics of shallow ocean environment on formation evaluated from the rock association and organic materials.The material source was neritic shelf deposits such as carbonatites,and bioherms occasionally had occurred within this area.The second section of the Shamuluo group formed as flysch in slope environment.(3)The Sewa Formation exhibits upwardly coarsened sediments indicating formed in regressive depositional environment.The lithological and biological characteristics suggest that the coarsening was caused by a combination of ocean level changes and distances of material sources.The fine clastic rocks within the first section of the Sewa group are dominated by biogenic limestone,indicating they formed in a wide continental shelf environment.Sedimentary fans and flysch in the second section of the Sewa formation suggest that this section formed at a continental slope environment.The medium-coarse quartz sandstone in the third section of the Sewa group represents a proximal offshore environment.(4)The morphology and Th/U ratios of the detrital zircons suggest that most of the zircons studied here are magmatic origin only a minor of them are metamorphic zircons.(5)The detrital zircons sampled from the Jurassic strata resulted in three groupsof ages.Comparison among their distributional map suggests that two groups of ages formed in Permian-Triassic and late Neoproterocoic,and also two groups of ages of early and late Paleoproterozoic area.(6)Comparison among the detrital zircons sampled from the Jurassic strata and those from the Anduo micro continental,western Qiangtang block,central and southern Lasha block,Tethys Himalaya,Himalaya,and western Austrilia suggests that the Sewa Formation at Gaize the southern Qiantang block has similar zircon ages with the whole Qiangtang area.Both the Mugagangri Formation and the Shamuluo Formation exhibit clastic sources from the Qiangtang block and minor sources from the Lasha block to the south.(7)Ages of the detrital zircons sampled from the Mugagangri,Shamuluo and Sewa formations all show similar four peak ages of 0.2~0.5 Ga,0.7~0.8 Ga,1.8~1.9 Ga and 2.4~2.5 Ga,which may represent at least four major magmatic activity and continental growth events.These peak ages are related tectonism within the blocks and surrounding block.The detrital zircons that have ages of 2.5~2.4Ga were probably originated from the Qiangtang block indicating that this block is related with the Proterozoic crystalline Indian basement.Moreover,this age suggest that the Tibetan Plateau,especially the Qiangtang block recorded pre-Cambrian geology activity: the 1.8 ~ 1.9 Ga ages might be associated with the convergence of the Columbia super continent;the 0.7~0.8 Ga age is consistent with the magmatic activity postdating the Grenville event and the breakup of the Rodinia super continent The ~0.5 Ga peak age is similar with the contemporaneous sedimentary age in the Indian-Himalaya area,suggesting that they have similar material sources further supporting that the Bangonghu-Nujiang suture zone was not the northern border of the Gondwana land.We suggest that the 240~280 Ma age corresponds to the closure of the ancient Tethys ocean and the opening of the new Tethys,and the material source was from the Qiangtang block.