Geochronology And Geochemistry Of The Kangmar Gneissic Granite And Mafic Dykes In Southern Tibet

This paper reports new zircon LA-ICP-MS U–Pb age and Hf-isotope,whole-rock major and trace element,and Sr–Nd isotope data from Cambrian metavolcanic rocks in the Kangmar Dome of Tethyan Himalayan sedimentary sequence.These rocks form a bimodal-like volcanic suite consisting mainly of silicic gneiss leucogranite rocks with subordinate mafic amphibolite in vein,missing the rocks with SiO₂=53?71%.Four silicic gneiss granite samples dated at ca.490-501 Ma and two mafic amphibolite samples yielding a mean ²⁰⁶Pb/²³⁸U age of 492.4±5.2 Ma and 490.8±8.1 Ma respectively indicate that mafic and silicic invasion were contemporaneous.The mineral association of amphibolite are Amp?45?55%?+Pl?35?40%?+Qz?<5%?+Ep&Bi?<5%?,and a little Sph,Ga,The mafic amphibolites with blastophitic structure are mostly medium-K calc-alkaline?TiO₂=1.21-1.54%,Al₂O₃=13.89-17.02%,MgO=5.21-6.51%,Mg^#=46-51,limited Cr,Ni?,enriched in Th,U,Pb,LREEs,and depleted in Nb,Ta,Ti,Zr,and Hf,geochemically resembling the E-MORB,except KM1306 with T-MORB feature.The amphibolites samples have negative whole-rock?_Nd?t?values?-2.6 to+0.4?and varying zircon?_Hf?t?values?-5.6 to+0.6?and(⁸⁷Sr/⁸⁶Sr)_i?0.7046-0.7091?,differing significantly from those of the silicic metavolcanic rocks,which yield negative whole-rock?_Nd?t?values of-8.1 to-7.6 and varying zircon?_Hf?t?values?-7.5 to-1.3?.The metabasaltic rocks are interpreted as resulting from partial melting of an enriched lithospheric mantle source that was metasomatized by subduction-related components,whereas the S-type gneissgranite with high-K calc-alkaline and low in Nb and Zr,were derived from basaltic melt-induced anatexis of the crustin the northernmost margin of the India Craton with inherited mantle melt signatures in Hf isotopic composition.The age and geochemical characteristics of the Kangmar rocks are similar to those of the basic rocks in Middle Okinawa Trough and the Cambro-Ordovician Shao La rocks further north,which were interpreted as produced in a back-arc setting.Stable element structure diagram get the same conclusion.The generation of bimodal-like metavolcanic rocks in Kangmar Dome may be a result of slab rollback driven by continued subduction of the proto-Tethyan Ocean lithosphere and gravitational sinking.In the slab rollback causing back-arc extension geodynamics background,enrichment mantle decompression melting and subduction fluid/melt metasomatism promotes molten.The magmatic arc may lay at microcontinents or terranes?e.g.,westernQiangtang and Amdo?located outboard of the continental margin,analogous to the Lhasa Terrane paleographically located in the Australian proto-Tethyan margin represent an early Paleozoic Andean-type magmatic arc facing the proto-TethyanOcean.