The Granitoids In Zongwulong Tectonic Zone On The Northeastern Margin Of The Qinghai-Tibet Plateau And Its Tectonic Significance

For a long time,the zongwulong(ZWL)belt has been thought to be part of the Northern Qaidam Tectonic Zone or belong to the South Qilian Orogenic belt.Recent studies,however,have indicated that a composite tectonic zone characteristic of complex structures,diversified materials and long-term evolution exists in between the Qaidam massif and the South Qilian Orogenic Belt,which mainly includes the Northern Qaidam Tectonic Zone and the ZWL belt.Deposition construction,ophiolite suite and arc volcanics in ZWL belt represent the structure result land ftracture in Neopaleozoic,ocean basin formation and ocean crust underthrust.Whether underthrust ganite going with arc volcanics and collision ganite exist is a problem not been resolved.Quite a few granitic plutons are emplaced along the south and north sides of the ZWL belt.In the past,there were frequent findings of Proterozic and Early Paleozoic granites in the area and almost no Indosinian intrusion was reported.Thus,it has been significant to find the Indosinian granites for evidence to argue the orogenic characteristics of the ZWL belt.The granitoid intrusions in the ZWL belt include the Tianjunnanshan feldspar granitoid,the Qinghaihunanshan monzogranite and the Erlangdong biotite granitoids. They belongs to high-K calc-alkaline granitoid.Qinghaihunanshan granitoid is metalumlnous,Tianjunnanshan granitoid and Erlangdong granitoid are peralumlnous.Geochemically,Tianjunnanshan granitoid and Qinghaihunanshan granitoid areâ… -type granitoid and enriched in LILE and LREE,and relatively depleted in HFSE, suggesting that they were formed in an island-arc or active continental margin environment.According to Barbarin,these intrusions are similar to those of ACG (Amphibole-rich Calc-alkaline)type,the granitoid represent subduction phase in orogenesis.Erlangdong granitoid deplete Ba,Eu,Sr,Ti,rich in Rb,Th,K,Pr,Ta.The granitoid is A-type granitoid formed in late orogenic,has characteristic as CPG, inosculating epiorogenic in orogenics. There plutons had a simlar composition in isotopic:Erlangdong granitoid isε_Nd(t)=-6.94,T_DM=17.7Ga;Tianjunnanshan granitoid isε_Nd(t)=- 8.3,T_DM=1.34Ga; Qinghaihunanshan granitoid isε_Nd(t)=- 7.48,T_DM=1.13Ga.The Pb isotopic of these granitoid is ²⁰⁶pb/²⁰⁴pb=18.359～18.715,²⁰⁷pb/²⁰⁴pb=15.634～15.648.Not only Sr-Nd isotopic but also Pb isotopic represent the characteristics of crustal granitoid. By contrasting with Sr-Nd-Pb isotopic of early Triassic Xiangride granodiorite and late Triassic Chahanno granodiorite in Qaidam Basin,the granitoids may come from high-K Basalt in Qaidam Basin Quanji team.The Zircon LA-ICP-MS dating for the Tianjunnanshan granitoid yields 246±1.5Ma(2σ).In addition,the dating for the Qinghaihunanshan granitoid and the Erlangdong granitoid are 238±1.5Ma(2σ)and 215±1.2Ma(2σ)evidently an important Indosinian orogenesis.Besides,Qaidam granitoid in the ZWL belt western also be discussed,it belong to granitoid emplaced in the late orogenic Eopaleozoic.Geochemical features and forming date of these granitoids,together with late Carboniferous ophiolites and early Triassic intermediate-felsic volcanic rocks indicate: the rift appear in the Early Devonian;the ZWL ocean was opened up in Late Carboniferous;the subduction of the oceanic crust commenced in Early Triassic and formed the intermediate-acid volcanic arc;collisoinal orogenesis started in 238 Ma～215 Ma,from north to south;Post-orogenic extensional regime promoted the emplacement of the marie dykes and A-type granites.The ZWL belt experience evolutionary process which is crack-subduction- collision- apotectonic extend,it is an independent orogenic belt neither belong to the Northern Qaidam Caledonian Tectonic Zone nor the South Qilian Orogenic Belt.