Geochronology,Geochemistry And Tectonic Implication Of Early Paleozoic Granitic Gneisses In The Dinggye Area,southern Tibet

The early Paleozoic Orogeny extended widespread in Himalayan Orogen,but little has been studied in Dinggye Area,central Himalaya.However,to decode the early Paleozoic orogenic event in Himalaya,study on the early Paleozoic granitic gneisses in Dinggye Area play an important role as a bridge.SHRIMP zircon U-Pb isotopic dating,oxygen isotopic analysis and whole rock geochemical analysis were done of the granitic gneiss in Great Himalayan Crystalline Sequence in Dinggye Area,central Himalaya.Zircons of the granitic gneisses have magmatic concentric oscillatory zoning,except several grains have core-rim structures with inherited cores and metamorphic rims.Zircon 206Pb/238U ages from 504?482 Ma has been obtained form six granitic gneiss samples,hinting that the protoliths of the gneiss was early Paleozoic granite.We explicitly pointed to the existence of the Early Paleozoic magmatism in Dinggye area,central Himalaya.The ages of the inherited cores range from 800?2500 Ma,showing that the protolith granite derived materials from old crust.The zircon metamorphic rims gave the ages of 20?26 Ma,32?38 Ma and 45?60 Ma,which suggest the district experienced three tectonic thermal events in Cenozoic.Geochemical data show that the rocks are characterized by high SiO2,K2O,Al2O3,with aluminum saturation index(ASI)of A/CNK>1.1,and enrichment in Rb,Th,U,depletion in Ba,Nb,TA,Sr,Zr,and strong negative Eu anomalies.These features suggest that the protoliths of thegneisses are high potassium calc-alkaline and peraluminous S-type granites.The range of ?18O values of the zircons is from 7‰ to 10‰,combine with the limited whole rock Sr-Nd and zircon Hf isotopic data have published in adjacent areas,suggesting the protoliths of the gneisses originated from partial melting of argillaceous components in the old crust injected by minor mantle components.The protolith granites of the granitic gneisses resulted in a in a syn-collisional or post-collisional setting,related to the Andean-type orogeny with the subduction of the Proto-Tethyan Oceanic lithosphere toward to the northern margin of east Gondwana continent.The mafic magmatic underplating,triggered by the subduction,rollback and the break off of proto-Tethyan Ocean slab,resulted in partial melting of the crust in the GHC.The Cambrian-early Ordovician granitic geneisses in Dinggye Area,central Himalaya,represent the southward continuation of the early Paleozoic granitic belt that extended along the northern margin of east Gondwana.The range of orogensis belt is from Turkey,central Iran,Afghanistan,northwest Pakistan,northwest India,Nepal,south Tibet to Tengchong and Baoshan areas in southwest China.The proto-Himalaya formed as a result of the Early Paleozoic orogenic events,and the building of the present-day Himalaya may have been initated as early as the Early Paleozoic.