Geochronology And Geochemistry Of The Kongbugang Leucogranite In Cuona,South Tibet

There are two belts of leucogranites laying west-east along the Himalayan orogen,divided by the southern Tibetan detachment system（STDS）:the Tethys Himalayas leucogranites and the High Himalayan leucogranites.The research object of this paper,the Kongbugang rock mass,is located in Cuona region of southern Tibet,close to the STDS,belonging to the High Himalayan leucogreanites,mainly including double-mica leucogranites and garnet leucogranites.Its major mineral composition contains quartz,potassium feldspar,plagioclase,muscovite,and biotite,with common auxiliary minerals of garnet,zircon,titanite,and monazite.While for major elements,it has the properties of high Si,Al,low Mg,Fe,high alkali content,high ratio of K₂O/Na₂O,belonging to peraluminous high potassium calcium alkaline series.For microelements,it is rich of LILE,while lack of HFSE.Based on different distribution formations of REE,Kongbugang leucogranites there could be divided into two kinds:one contains low ratio of（Gd/Yb）_N,comparatively rich in HREE,equipped with tetrad effect,which belongs to high differentiation leucogranites;the other is with high ration of（La/Yb）_N and HREE loss heavily,which is made of muscovite dehydration melting.Mostly,the initial temperature of the leucogranites magma stays 690⁷40℃,measured by Zr saturation thermometer and Ti thermometer,laying in the temperature range of muscovite dehydration partial melting.Besides,temperature calculation results concentrates in the range of 750⁷80℃as well,under which water partial melting or decompression dehydration melting could happen to biotite.The Kongbugang leucogranites has been formed in 14.9¹6.4Ma,seemed as the result of the High Himalayan magmation during the Miocene,the same period as STDS which share closed genetic relationships with the Kongbugang leucogranites.The extensional tectonic movement of STDS creates decompression environment,leading to the dehydrated partial melting of muscovite and biotite in deep earth crust;Meanwhile,STDS provides extended channels for the upward migration of hypomagma,which means the magma would have longer time to be fully crystallization differentiation and sufficient magmatic evolution.