Petrogenesis Of The Cuonadong-Lhozag Leucogranites And Implication For Tectonic Evolution And Be-W-Sn Metallogeny In Southern Tibet

Himalayan leucogranite,the petrogenesis and tectonic significance of which has always been a hot scientific issue for scholars all over the world,is the key to understanding the regional magmatism,tectonic evolution and mineralization.The Cuonadong-Lhozag area is located in the eastern of the Himalayas.Since the Cenozoic,the Lhozag region experienced three periods of magmatism and generated the early 17.8 Ma Lhozag tourmaline granite,the middle 15.1 Ma Lhozag two-mica granite,and the late~12 Ma Khula Kangri two-mica granite.The Cuonadong area experienced two stages of magmatism and generated weakly oriented two-mica granites and garnet-bearing muscovite granites at~20 Ma in the early stage,and garnet-bearing two-mica granites at~17 Ma in the late stage.The Sr-Nd-Pb-Hf isotope indicates that the source rock of the leucogranite in the Cuonadong-Lhozag area is metapelite of high Himalayan crystalline basement.The Lhozag tourmaline granite,the Lhozag two-mica granite,and the Cuonadong weakly oriented two-mica granite have relatively low Eu/Eu~*,CaO,Nd/Nd~*,LREE,Sr and Th contents and relatively high Rb,Rb/Sr ratio(>2)and the zircon Ti saturation temperature indicate that they are formed by dehydration melting of muscovite.The Khula Kangri two-mica granite in the Lhozag region has a large variation concentrations of CaO,Sr,Rb as well as Rb/Sr and Eu/Eu~*ratios,with high Th,LREE content and Nd/Nd~*ratio as well as low zircon Ti saturation temperatures,which indicates that the Khula Kangri two-mica granite is formed by the fluid-present melting of muscovite,and experienced fractional crystallization.The Cuonadong garnet-bearing muscovite-granite and garnet-bearing two-mica granite show high Si,Al,low Ca,Fe,Mg,Ti,and abnormally high Rb/Sr ratio(>18.6),as well as the tetrad effect of the whole rock and monazite and the twin elements Y-Ho,K-Rb,Zr-Hf,and Nb-Ta deviating from the chondrite ratio,which indicates they are highly fractionated leucogranites that interact with fluids containing F.The high degree of crystallization differentiation and fluid exsolution make the incompatible elements Be,W and Sn enriched and migrated with the fluid.The two-stage highly fractionated magmatism promotes the accumulation of ore-forming elements and forms the Cuonadong Be-W-Sn polymetallic deposit.The partial melting mechanism of the metapelite in the eastern Himalayas evolved from the early 20-15 Ma muscovite dehydration melting to the late 12 Ma fluid-present melting associated with the tectonic evolution from the early N-S extension to the late E-W extension.During the N-S extension,the pressure decreasing caused muscovite-dehydration melting and generated the Lhozag tourmaline-bearing granite,the Lhozag two-mica granite,and the Cuonadong weakly oriented twomica granite;During the E-W extension,the permeation of the fluid from deep crust caused fluid-present melting and generated Khula Kangri two-mica granite.