The Chronology,Geochemistry And Petrogenesis Of Granites In Duilongdeqing Area,South Tibet

There are amounts of Mesozoic-Cenozoic volcanic rocks and associated granitoids in the southern Lhasa Terrane which located between the Yarlung Zangbo Suture Zone and the Milashan Fault Zone,their petrogenesis and geological setting is still ongoing debate up to now.In this dissertation,we carried out systematic field investigations,petrography,chronology,bulk-rock major element,trace element of the granitoids in the Duilongdeqing region of south Lhasa Terrane,Tibet,and combined with literature data to discuss the petrogenesis,magma source and geological setting,aiming to provide new evidence for the geological evolution process of the Southern Lhasa Terrane.The granitoid samples obtained from Duilongdeqing region of south Tibet in this paper are mainly biotite granites and monzogranites.They have granitic textures,massive structures,and occasionally plagioclases with band features.The zircons LA-ICP-MS U-Pb dating results show that the ages of biotite granite and monzogranite are 60.6±0.3Ma and 60.4±0.3Ma respectively,which can represent the diagenetic age of rock and time,the ages of both samples are Paleocene.The zircon?Hf(t)value ranges from 7.57 to 10.76,and the Hf isotope crustal model age varies from 0.4 to 0.7Ga.The age statistics of the existing magmatic rocks in the south Lhasa terrane since the Mesozoic show that there is a magmatism peak period of?60 Ma in the southern Lhasa terrane,which coincide with the main collision period(65-40Ma)of the collision between India and Eurasia.The geochemical characteristics of the granitic samples in this paper show that they are calc-alkaline to high-K calc-alkaline series,which belong to the quasi-aluminous to weak peraluminous I-type granite.The major element concentrations of biotites and hornblendes show that the characteristics of rich magnesium,and belong to magnesium series.All the samples are enriched in light REE and depleted in heavy REE,most rock samples show medium to weak negative Eu anomalies(?Eu=0.65?0.85).The samples are relative enriched in large ion lithophile elements(LILE)and depleted in high field-strength elements(HFSE),and show well-developed negative Nb,Ta and Ti anomalies.The samples with high Mg~# value(36-41.22)and low Sr/Y value indicate that the granitoids have typical an island-arc-type geochemical affinity.Previous studies have shown that the Linzizong Dianzhong group volcanic rocks which distributed in southern Lhasa Terrane may be formed by northern subduction of the Neo Tethys oceanic lithosphere.The genesis of the Paleocene granites which contemporaneous in this paper has similar characteristics.Geochemical data show that the magma source area is mainly new crust with the participation of mantle-derived materials,showing the characteristics of mixed crust and mantle.This may be the end of the northward subduction of the Neo-Tethys Ocean,The fluids carried by the subducting slabs cause partial melting of mantle wedge,and causing fractional crystallization during the magma upwelling and evolution.