Geochemical Characteristics Of The Paleozoic Volcanic Rocks In The Jinshajiang Zone, NW Yunnan And Jianshui, SE Yunnan, And Evolution Of Paleo-Tethys

The study areas are located in the Jinshajiang zone,NW Yunnan and SW part of the Shizong-Mile zone (Jianshui),SE Yunnan respectively,and tectonically belong to the east part of the Paleo-tethys tectonic domain. The Jinshajiang zone in NW Yunnan was a joint terrain between the Yangtze and the Gondwana blocks,and the Shizong-Mile zone delineates a suture line between the Yangtze and Cathaysian blocks. The studies on the volcanic rocks from these two areas provide new evidences for understanding the Paleo-tethyan evolution.Based on the geological,stratigraphic and geochemical characteristics of the ophiolitic melange,island-arc and rift volcanic rocks from the Jinshajiang zone,an archipelagic tectonic model with multi-rift and multi-subduction is proposed to explain the evolution of Jinshajiang Paleo-tethys.The ophiolitic melanges of the Jinshajiang zone are mainly outcropped at Baimangxueshan,Tongduo,Gongka,Dongzhulin and Jiyidu in the Deqin County. The geochemistry of major and trace elements of the Jinshajiang ophiolite shows its geochemical characteristics are similar to those of the MORE and island-arc volcanic rocks,but different from those of the typical N-MOPJB. Thus,they might be derived from a back-arc basin with an archipelagic system in Paleo-tethys,instead of a large ocean basin. The isotopic geochemistry shows that the Jinshajiang ophiolitic melange has higher e Nd(t) values(+7.42-+5.44),suggesting that it may be derived from a LREE depleted mantle source without significant contamination by the continental crust,but affected slightly by a fluid from a subducted ocean crust. The Pb isotopic features of the Jinshajiang ophiolitic melange display higher 208Pb/204Pb and 207Pb/204Pb ratios,implying a DUPAL anomaly,i.e. mantle signatures of Paleo-tethys. Combining with the studies of isotopic chronology,stratigraphy and paleontology,the ophiolites from the Jinshajiang back-arc basin may be generated during early Carboniferous- Permian. The geochemical characteristics of gabbros from the Jinshajiang ophiolitic melange indicate that they were derived from a mantle source different from that of volcanic rocks,which may be a product in the ocean rift in the earlier stage.The geochemical data of the Benzilan and Pantiange volcanic rocks from the Jinshajiang zone yielded characteristics similar to those of island-arc volcanic rocks:intensive enrichment in LILE and different depletion in HFSE,Nb,Ta,Ti,P and Zr,Hf. Their lower E Nd(t) values(-4.03--11.29) indicate that they are derived from an EMU mantle source or a mixing source between DM and EMU respectively,but which are affected violently by subduction materials. The Pb isotopic ratios show the characteristic of DUPAL anomaly and influence of ocean sediment to the source. Based on our studies,the N-MORB volcanic rocks were occurred in the Pantiange area.The Shigu Permian volcanic rocks of the Jinshajiang zone display the geochemical characteristics similar to those of rift volcanic rocks. The isotopic evidence indicates that they might be derived from a depleted mantle source,and probably contaminated slightly by continental materials. Pb isotopic features imply that its source is similar to that of the Indian ocean MORB,and a DUPAL anomaly. In order to explain the spatial and temporal distribution of volcanic rocks with different ages and tectonic regimes,one archipelagic model is proposed.The Shizong -Mile zone in SE Yunnan was composed of different tectonic blocks with different ages,lithological features and provenances,and contacted each other by a shear zone. This important tectonic zone parallel to the isotopic geochemical steep zone,which represented a suture line between the Yangtze and Cathaysian Blocks. The geochemical evidence of the Jianshui volcanic rocks from the SW part of Shizong-Mile zone shows that their lithologic types are tholeiites with the comparative flat REE patterns and enrichment in Th,depletion slightly in Nb,Ta,and flat HFSE. Thus,they may be formed in a back-arc basin with the characteristics between MORB an...