Study On Timing And Tectonic Setting Of Shiquanhe Ophiolitic Mélange In Tibet

The Shiquanhe ophiolitic mélange,located in the north of Shiquanhe Town,is the western part of the Shiquanhe-Yunzhug-Namco ophiolitic mélange belt tectonically,and its rock suits is relatively complete.But the petrogenesis and the evolutionary process of Shiquanhe ophiolitic mélange remain uncertain.In order to investigate the petrogenesis and the tectonic setting,we collected the regional geological datas,and the Shiquanhe gabbros,diabases and quartz diorites were selected for petrology,zircon U-Pb dating,in situ Lu-Hf isotopic and whole-rock geochemical analyses.Shiquanhe ophiolitic mélange is mainly consist of mantle peridotite,basalt,gabbro,diabase,plagiogranite and diorite.The rock samples collected in this paper are mafic rocks?gabbro,diabase?and quartz diorite,which are occurred as blocks.Zircon LA-ICP-MS U-Pb dating results show that the formation ages of mafic rocks are 164.9±2.3 Ma and 165.8±2.3 Ma?gabbro and diabase,respectively?,and the formation ages of high-Mg diorites are 163.8±1.3 Ma,159.7±1.3 Ma,158.7±1.5Ma and 159.0±1.3 Ma.The above age results show that two different lithologies have roughly the same age results,which are similar to those obtained by predecessors in this region.Thus,we conclude that the formation age of Shiquanhe ophiolitic mélange is Late Jurassic.The results of mafic rocks'whole-rock geochemical analysis show flat REE patterns and negative Eu anomaly,while the trace element characteristics are that the samples are enriched in LILEs?Rb,Ba,Th,U?,but depleted in HFSEs?Nb,Ta,Ti?.Combined other geochemistry features,the mafic rocks have the geochemical features of IAB?island arc basalt?and N-MORB?normal mid-ocean ridge basalt?.Thus,we infer that Shiquanhe mafic rocks are formed in an immature intra-oceanic back-arc basin environment,but the depleted mantle source was mainly metasomatized by slab fluids rather than oceanic sediment-derived melts.Shiquanhe quartz diorites have high?2.64-4.03wt.%?MgO and Al₂O₃?15.12-16.73wt.%?contents and high Mg#?50.37-55.44?,which is in consistent with the characteristics of the high-Mg diorite?HMD?.The Shiquanhe high-Mg diorites are enriched in LILEs?U,Th,Rb?,but depleted in HFSEs?Nb,Ta,Zr,Hf?as well,and it may be a product of the interaction between subducted sediment-derived melts and mantle peridotites.Combined the above results of petrology,geochemistry and chronology researches with previous studies,we consider that Shiquanhe-Yunzhug-Namco ophiolitic mélange zone is an independent and autochthonous mélange belt.Ohiolites in Shiquanhe-Yunzhug-Namco ophiolitic mélange belt are probably formed in back-arc basins,while the maturity of ophiolites in the east is higher than those in the west.From about 166Ma to 164 Ma,the intra-oceanic southward subduction of Bangong Meso-Tethys caused the rapid spreading of the Shiquanhe immature back-arc basin.From about 164Ma to 159 Ma,the interaction between subducted sediment-derived melts and mantle wedge led to the formation of high-Mg diorites.