Spatial-temporal Distribution And Tectonic Implications Of The Of Granitoids From Qilian Block

The Qilian Block is located in the northeastern margin of the Tibetan Plateau.It is surrounded by the North Qilian orogeny to the north,North Qaidam-West Qinling belt to the south,the Ordos block to the east,and the Tarim block to the west.The Qilian Block is an important part of the tectonic evolution of China.This paper reports zircon U-Pb ages and whole-rock major and trace elemental for Early Neoproterozoic intrusive rocks(the Baihuagou,Yamacha,and Xiangheer,plutons)and Early Palaeozoic intrusive rocks(the Dongjiazhuang,Xindian,Xiaogaoling,Yuanzhecun,Xishangen plutons)in the eastern part of the Qilian Block.Combined previous works,we have established the geochronological frame of the intrusive rocks in the Qilian Block and have discussed the affinity of Qilian Block and Early Neoproterozoic to Early Paleozoic tectonic evolution of the Qilian orogen.The main results obtained are as follows:1.Zircon ages from published in previous research and eight newly acquired in this paper showed that magmatism in the Qilian Block mainly occurred in four distinct periods: the Paleoproterozoic-Mesoproterozoic(1765?1192Ma),Neoproterozoic(943?750Ma),Early(490?423Ma)and Late(419?402Ma)Paleozoic.2.The Mesoproterozoic Maxianshan granitoids are monzogranite,with calaalkaline,peraluminous,S-type affinity.3.Neoproterozoic magmatism mainly occurred in three distince periods: first period(943?910Ma)mainly found in Xianghe,Baokuhe,Maxianshan,Baihuagou,and Yamacha,and dominated by I-type,minority are S-type.The second period(899?846Ma)mainly found in Datong and Tuole area,are I-type.The third period(826?738Ma)mainly found in Riyueshan,Gaihai,and Haiyan area,are characterized by an evolution from I-type to S-type.The tectonic setting of the Neoproterpzoic Qilian Block from continent margin arc transform to continent rift.4.Paleozoic granitoids are magmatism mainly occurred in three distince periods: first period(490Ma)mainly found in Yemananshan Wuergelate Pluton with I-type affinity and Adakide.The Wuergelate Pluton part melting from the thickened lower crust.The second period(472?427Ma),I-type granite(459?427Ma)of east part mainly found in Jishishan,Heishishan,Shichuan,Bamishan,Niangniangshan,Xining,Riyueshan,Xiaogaoling,and Gangcha area.S-type granite(454?427Ma)of east part mainly found in Xindian,Dongjiazhuang,Huangyaun,Ledu,Yuanzhecunarea.I-type granite(469?434Ma)of west part mainly found in Yemanjiyan,Dadaoerji,Shibaocheng,Sangewatang and Zhazigou area.S-type granite(462Ma)of west part mainly found in Bagadeer.The third period(423?402Ma): I-type granite(419?417Ma)of east part mainly found in Shenchuan;S-type granite(417Ma)of east part mainly found in Xiahangen;and A-type(416?402Ma)of east mainly found in Maxianshan area.I-type granite(415?413Ma)of west part mainly found in Subei and A-type(416Ma)mainly found in Sangewatang.The generation of Paleozoic granitoids of east part Qilian Block are result from the subduction and collision between Qilian and Qaidam Block,and the west part are from the subduction and collision between Qilian and Quanji Block.5.Neoproterozoic to Paleozoic tectonic evolution of the Qilian is mainly divided into the following two stages:(1)Neoproterozoic: ca.943?910Ma Qilian Block as a part of South craton in continent margin arc during the assembly of Rodinia supercontinent.The fore-arc basin had developed in Qilian Block.Ca.899?846Ma,out-continent was subducted and arc-remnant fore-arc basin.Ca.826?738Ma,the rift of Rodinia supercontinent by super plume,during this time the South craton rift away from superconent.(2)ca.600?500Ma,ancient ocean plate on both sides of the Qilian block were developed.Ca.470?446Ma the south ocean crust subducted northward,leading to the generation of subduction related magmatic rocks in the Qilian and ca.479?467Ma Nouth ocean crust subducted northward;ca.446?440Ma ancient ocean plate on both sides of the Qilian Block closed then continent-(or arc-)continent collision occurred;ca.430?402Ma,slab break-off occurred melting of continental crust after obvious crustal thickening,delaminatioa of the lower crust took place in the Qilian Block.