Geochronology And Geochemistry Of The Cuonadong And Cuora Leucogranites In Cuona Area, South Tibet

The Himalayan leucogranites have always drawn much attention of the geologists all over world.This study presents first comprehensive investigations of the Cuonadong and Cuora leucogranites exposed in the North Himalayan gneiss dome of southern Tibet,including in-situ Secondary Ion Mass Spectrometry?SIMS?zircon U-Pb ages,⁴⁰Ar/³⁹Ar dating by laser stepwise heating,chemical compositions of major and trace elements,as well as Sr-Nd isotopic analysis,to determine the formation ages and investigate the sources and mechanisms of the Cuonadong and Cuora leucogranites.Zircon U-Pb ages of the Cuonadong leucogranite range from 34.1 to 16.0 Ma,suggesting protracted melting of the mid-crust.In other words,the forming process of the crustal-derived magma might had taken a long time.We interpreted the youngest and concordant age of 16.0 Ma as the emplacement age of this pluton.The muscovite ⁴⁰Ar/³⁹Ar laser stepwise heating analyses yield essentially flat age spectrum,exhibiting closed K-Ar system behavior of Ar release pattern.The ⁴⁰Ar/³⁹Ar dating results reveal that the Cuonadong leucogranite cooled down to 450? at 14 Ma.SIMS U-Pb age and the muscovite ⁴⁰Ar/³⁹Ar age suggest that the Cuonadong gneiss dome experienced rapid emplacement and exhumation with cooling rate of 1190C/Myr during 16?14 Ma.SIMS U-Pb zircon dating of the Cuora leucogranite yield ages between 14.6 Ma and 18.4 Ma.The muscovite ⁴⁰Ar/³⁹Ar dating yield 8.7 Ma and biotite yield 7.8 Ma,indicating that the Cuora leucogranite experienced rapid cooling at 9?8 Ma with cooling rate of 111?/Myr.Quartz ⁴⁰Ar/³⁹Ar dating by crushing in vacuum yields 4 Ma,suggesting that the Cuona area has been affected by extension of the North-South trending rifts of the eastern Himalaya at?4 Ma.Both Cuonadong and Cuora leucogranites are characterized by high SiO₂ contents>71%,high alkali?K₂O + Na₂O?>8%,strongly peraluminous ASI>1.2,high initial 87Sr/86Sr ratios?0.72?0.79?and low ?Na?t??-15.0?-11.6?.They are enriched in Cs,Rb,K,Pb,U and light rare earth element?LREE?,but depleted in Nb,Ta,Ti,Zr, Ba,Sr,heavy rare earth element?HREE?and Y.Meanwhile,obvious fractionation between light and heavy rare earth?La/Yb?N>7 can be seen in chondrite-normalized rare earth element patterns.Almost all the sample has negative Eu anomalies.Based on previous studies of the Himalayan leucogranites,initial 87Sr/86Sr ratios and Nd model ages indicate the Cuonadong and Cuora leucogranites were generate from the metapelites of the Greater Himalayan Crystalline Complex.the characteristics of high Rb/Sr,low Sr/Ba ratios,and negative Eu anomalies suggest they formed under the condition of fluid-absent melting of muscovite.