The Associations,Geochronology And Geochemistry Of Igneous Rocks In Lapeiquan,North Altyn Tagh And Its Tectonic Significance

The Altyn Tagh,located in the northeastern margin of Tibet Plateau and between the Tarim and Qaidam basin,is an important part of Central orogenic belt of China.Its particularities of regional structure,crust-mantle structure and tectonic evolution have always been concerned.Hongliugou-lapeiquan ophiolite belt is an important tectonic boundary of the Altyn Tagh,and its tectonic evolution has an important significance to the tectonic evolution process of the Altyn Tagh.This thesis focuses on field,petrology,geochemistry and geochronology of igneous rocks from Lapeiquan area in the North Altyn Tagh.In Combination with previous studies of high-ultrahigh pressure metamorphism and magmatism in Hongliugou area,the results of this study provide new evidence for the tectonic evolution process of the North Altyn ocean.The main results are summaried as follows:1?The plagiogranite and gabbro-diabase,result from incipient rifting of the ocean in the early stage of the oceanic evolution,identified in age of 517-500Ma.The plagiogranite is characterized with low K2O/Na2O ratios(0.03?0.05%),high Mg#values(36?55),slight LREE enrichment and high positive sHf(t)values(0.18?7.69).Combined with experimental petrological data,these geochemical features indicate that the plagiogranite originated from partial melting of oceanic crust during migration.The age and geochemical features of Lapeiquan plagiogranite is consistent with the Hongliugou plagiogranite(Gao et al.,2012,Gai et al.,2015);Gabbro and diabase fall into the area between E-MORB and with-in plate basalt in the geochemical discrimination diagrams,suggesting that they might result from incipient rifting of the oceanic basin.Therefore,we suggest that the North Altyn ocean was extended to the Laipeiquan area,and opened about ca.518 Ma.2?Providing further constraints to the oceanic crust subduction during 480?475 Ma in the Laipeiquan area.Based on the detailed petrology,geochemistry and whole-rock Sr-Nd-Pb isotope analysis,the generation of the diabase and pillow basalt is related in a back-arc basin induced by the subduction of Lapeiquan Ocean.The age of the diabase is 480Ma;The Baijianshan granodiorite with age of 475 Ma is the result from partial melting of basic rocks in the low crust triggered by the dehydration of subducted oceanic crust.3?K-feldspar granite with age of 468 Ma,provides the age of the closed event of ocean basin in the Laipeiquan area.K-feldspar granite belongs to A-type granite,characterized with low Sr,high Yb and negative Eu anomaly.K-feldspar granite falls into the area of volcanic arc granite and with-in plate granite in the geochemical discrimination diagrams,suggesting that they might result from the extending stage,after the oceanic crust closing and continental collision stage.4?Combined with achievements of high-ultrahigh pressure metamorphism and ophiolites.The Altyn tectonic evolution stages of ocean basin can be divided into three stage:(1)the ocean expansion happened in the age of 479Ma or 518Ma at least;(2)the subduction of oceanic crust stage,which happened during 513Ma-475Ma;(3)post-collision extension stage,which happened between 474 Ma and 431 Ma.