Subduction And Closure Of The Paleo-theys Ocean:Constraints From Permo-triaassic Granitoids In The Norhthern Lancangjiang Zone

The Qiangtang block is located in the north-central Tibetan Plateau,recently,geologist gradually reach a consensus that the Qiangtang terrane is no longer a single block as previously thought,it should be devided into the Southern Qiangtang and Northern Qiangtang by the Shuanghu-Longmu Suture Zone.However,the eastward of the Shuanghu-Longmu Suture Zone still remain controversial,and the plate property affinity of the Northern Qiangtang terrane is also the research hotspots among the geological circles.Our research field is located in the west margin of Changdu terrane,northern Lancangjiang zone,it extends to the west into the Shuanghu-Longmu Suture Zone,connects to the Changning-Menglian suture zone in the south,that has a great significance.Large volumes of Permo-Triassic granitoids are exposed along the northern Lancangjiang zone,we conducted detailed geochronological and geochemical analysis of the granitic plutons that crop out along the northern Lancangjiang magmatic belt,probing their magma origin and tectonic setting,providing insights into the tectonic evolution of the Paleo-Tethys ocean.The ganitoids in the northern Lancangjiang zone could devided into 2 phases:the early phase is range from 267 to 257 Ma including the Xiaochangdu and Chaya pluton,while the late phase(the Chala pluton)is around 230 Ma.The early phase granitoids possess low SiO2 contents(58.1-63.0%)and A/CNK values(0.87-0.92),are rich in metalumious minerals(titanite,hornblende),and exhibit weak Eu negative anomalies and high LILE/HFSE,depleted in HFSEs(e.g.Nb,Ta,P,Ti),have low(87Sr/86Sr)i ratios and high nearly positive ?Nd(t)(-0.26-7.71)and?HKt)(0.68-10.57)values,all these suggesting the arc geochemical feartures of the early phase granitoids.Whereas the late phase granitoids(late Triassic Chala pluton)have distinct geochemical feartures from the early phase granitoids,they possess high SiO2 contents(71.3-74.1%)and A/CNK values(0.98-1.18),and exhibit pronouced Eu negative anomalies,they have constant negative ?Nd(t)(-2.78?-2.59)values,these geochemical features suggesting that the Chala pluton belong to the prealumious S-type granites,combined with regional research data and previous research results,our research considering the Chala pluton as the products of the post-collisonal events,their petrogenisis favors a slab breakoff model.The arc-like and post-conllisional granitoids in the northern Lancangjiang zone have similar geochronological and geochemical features to those in the Central Tibet and Changning-Menglian area,these successive Permo-Triassic magmatic rocks parallel to the Longmu Co-Shuanghu-Changning-Menglian Suture Zone,corresponding to the subductiton and closure of the Paleo-Tethys ocean.The main peak ages(mainly concentrated in 2.45-2.57 Ga,1.8-1.9 Ga,740-880 Ma,410-470 Ma,and 215-300 Ma)of the detrital zircon in Changdu area is coincide with those from the northern Qiangtang,Simao and Indochina terrane,which reflecting the common tectonic evolution events in these area.Combined with the distinct sedimentary formation,palaeontological assemblage and basement features in the opposite sides of the Longmu Co-Shuanghu-Changning-Menglian Suture Zone,our research surpporting a single northern Qiangtang-Simao-Indochina terrane.