Study Of Multiple Orogeny And Metallogenesis In Yushu Area, Northern Segment Of Sanjiang

Qinghai Yushu area,with huge metallogenic potential,is located in the northern segment of the famous Sanjiang copper polymetallic mineralization belt.This thesis's objective is to research the division of tectonic units and tectonic evolution of late Palaeozoic early Mesozoic through zircon LA-ICP-MS U-Pb chronology and geochemical analyses for volcanic rocks in North Qiangtang block and granites in Yushu suture zone,as well as study on tectono-magmatic activities and petro-graphic features.Based on the study of Cenozoic evolution and deformation in Yushu- Nangqian structural belt,combining metallogenic age determination of molybdenum(copper) porphyry deposits and lead zinc deposits,we explore the relationships betweeen Cenozoic structural characteristics and metallogenesis in Yushu area.Multiple superimposion was experienced because of two key geological processes,which are subduction of oceanic crust and collision orogeny of late Palaeozoic-early Mesozoic.The Yushu volcanic complex was formed during late Palaeozoic-early Mesozoic;the Pero-Triassic epicontinental arc volcanic rock belt was formed on the basis of North Qiangtang Block.To the north of the volcanic rock belt developed the Bayankala turbidite belt;Cenozoic collisional transformation orogen developed the strike-slip fault and large-scale thrust nappe.The volcanic rock belt in Yushu has experienced strong contraction deformation,thus the green schist phase of low-degree metamorphism was widely developed and the fanlike nappe overlapped the tectonic units on both sides.Within the belt,folds,lens,structural foliation and small faults are developed and indicates the syn-collision contraction.Besides,strongly deformed mafic dikes and granodiorite intrusions are also widely distributed in the belt;while the syn-collision granits occur both in the volcanic belt and Bayankala turbidite belt.In contrast,granodiorite rarely occurs in the Qiangtang Block.Such distribution of granodiorite generally supports the southwards subduction of Jinshajiang Ocean and tectonic division.The Permian subduction produced the Narigongma-Ziqu epicontinent arc volcanic belt in the North Qiangtang Block.The LA-ICPMS U-Pb age of the zircon in these volcanic rocks are 275.4 Ma,273.3Ma. Geochemical features demonstrate that these rocks are of arc volcanic by being rich in large ion lithophile elements(LILE),and poor in high field strength elements(HFSE).These rocks are also rich in ¹⁷⁶Hf/¹⁷⁷Hf andεHf(t),which indicate that the source region of magma may have materials from defective upwelling asthenosphere mantle.The epicontinental activities resulted in volcanic and magmatic arc granites.The zircon ages of volcanic rocks in Dongmozhazhua diposits are 236.6±1.9Ma and 238.3±4.0Ma.The epicontinental arc volcanic feature was indicated by the characteriscs of geochemical data,REE elements and Hf isotope of zircon.Magmatic arc volcanic intruded the Yushu volcanic belt and the zircon ages of two biotite granites are 235.2±1.2Ma and 228.7±1.2Ma.Most of the specimens generally belong to the calc-alkaline series, which are rich in Silicon and alkaline components and poor in P₂O₅å’ŒTiO₂.The major elements feature of three granite specimens are closely related and suggest obvious crystallization differentiation.The distribution characteristics of trace elements and rare earth elements for granites form are similar with magmatic arc granites.From diorite through quartz diorite,granodiorite to biotite granite,rare earth elements show increasing enrichment in LREE,gradually depleting in HREE,LREE fractionation characteristics become increasingly evident that the granite should be products of the subduction of Jinshajiang ocean. The epicontinental arc volcanics related to the Permo-Triassic activities suggest the subduction of the Jinshajiang oceanic crust,and the ocean-continent tectonic regime transformation and the northward accretion of continental crust.Within the Yushu area,these is neither large quantity of rocks which are similar to the "bimodal" volcanic rocks formed in the back-arc spreading environment such as those in the middle-southern segment of Sanjiang tectonic belt,nor volcanic rocks in the environment of large-scale orogenic collision and post-collision extension,which maybe be unfavorable influence for metallogenesis in late Paleozoic-early Mesozoic.Cenozoic collisional transformation orogen is expressed in two deformation styles,i.e.strike-slip spreading and compressional convergence.The former resulted in strike-slip faults,pull-apart basins and alkaili-rich intrusive rocks,potassic to ultra-potassic volcanic rocks;and the compressional convergence produced the large-scale thrust faults system.This system could be divided from the south to the north into root thrust zone,middle thrust zone and front thrust zone,which are separated by main thrust faults. Corresponding to these two deformation styles,two ore-forming system are developed in the studied area. One is late-collision porphyry ore-forming system(40.86±0.85Ma in age of Re-Os),which is controlled by strike-slip faults,produced the Narigongma porphyry molybdenum-copper deposits and the ages of porphyry intrusions and mineralization ages become increasingly young from the north to the south in Sanjiang area.The other is hydrothermal ore-forming system which is controlled by thrust faults system and produced Dongmozhazhua and Mohailaheng lead-zinc deposits.These two deposits occurs in the front thrust zone of the thrust faults system at 35.03±0.89 Ma and 34.0±1.1Ma(in age of Rb-Sr) respectively and their orebodies are formed in fracture zones or back thrust areas in the upper or lower plate of the imbricate thrust faults.