Geochronology And Geochemistry Of Leucogranite In Yadong Area,Southern Tibet

The Himalayan tectonomagmatic belt provides an excellent field laboratory to study leucogranites,while the High Himalayan belt in southern Tibet has the world’s largest-development area of Leucogranites,widely exposed Miocene Leucogranites,recording abundant information of the Indo-Asia continental collisional orogenics.This paper researches the characterristics of petrology,zircon geochronology,trace elements and Hf isotopes,whole rock geochemistry,Sr-Nd isotopes in Leucogranites from Yadong region,discussing its petrogenesis and the relationship with the orogenesis of Himalayas.The studied Gaowu and Dingga intrusives located in Yadong and north of the county.It is tectonically locating in the intersection portion of Southern Tibet Detachment System（STDS）and Yadong-Gulu rift,eastern sector of the Higher Himalayan Leucogranite Belt.The rocks emplaced in the Higher Himalayan crystalline rock sequence,the rock types are tourmaline leucogranite（Tg）and two-mica leucogranite,the Tg subdivides into tourmaline muscovite granite and tourmaline two-mica granite.The mainly rock type of Gaowu and Dingga is tourmaline muscovite granite of coarse-grain weak gneissic-like structure and medium-fine grained tourmaline two-mica granite respectively.Tourmaline is disseminated or can form giant crystal pulse output in rock mass.The rocks are light gray and white,coarse grain structure,massive-gneissic-like structure,lithology is relatively homogeneous.Rock mass has surrounding-rock xenoliths.The leucogranites have high content of SiO₂,Al₂O₃,（Na₂O+K₂O）,Rb/Sr and(⁸⁷Sr/⁸⁶Sr)_i,low MgO,CaO/Na₂O ratio and(¹⁴³Nd/¹⁴⁴Nd)_i.The CIPW result indicate that all rock samples have corundum（c）appeared（>1%）,Alumina supersaturated indexes are higher than 1.0,belong to high-K calc-alkaline series peraluminous to strongly peraluminous S-type granites.Their total Rare Earth Element（∑REE）are 25.87～167.15ppm,show LREE enriched,LREE and HREE moderate differentiation（La/Yb）_N=4.3～12.3,Chondrite-normalize（REE）display strong negative Eu anomalies.Yadong intrusive body was formed in the Miocene period,age of Gaowu and Dingga Leucogranites Body is 12.9±1.1Ma～17.3±0.3Ma and 14Ma respectively,almost the same age.Inherited and surrounding rock metamorphic zircon have wide range span,show the crest value of 450～500 Ma,830 Ma and 1140 ma,also a small amount inherited zircon of 2100～2700Ma.Their source rocks are ancient inhomogeneous crustal materials,experienced multiphase metamorphism magmatism.The comprehensive feature of Yadong Leucogranite reveals that Yadong leucogranite originated in the Higher Himalayan crystalline rock series,it is the result of muscovite dehydration melting in metapelites at lower temperature,represents for the pure crust source typically,reflects the collision orogenic process by the Earth’s crust thickening collapse to extended thinning process.