The Genesis Of Granite From The West Side Of The Eastern Himalayan Syntaxis:Constraints From Zircon Geochronology And Geochemical

The eastern Himalayan syntaxis links the collision and lateral strike-slip of India–Eurasiacontinents. The study area is located on the west side of the eastern Himalayan syntaxis, aroundNyingchi County and Lulang Town. Five typically granitic intrusives(Baxiliegu,Demula WestNyingchi1st, West Nyingchi2ndand West Nyingchi3rd) were studied in this thesis. A systematicstudy, including field geological，zircon U-Pb geochronology using LA-ICP-MS,petrology andelements geochemistry were applied in discovering the petrogenesis type of the batholiths,thegeological significance and its constraints to the aggregation events occurred in the easternHimalayan syntax and the surrounding block.The main research results obtained as follow:(1) Based on the granite zircon U-Pb LA-ICP-MS study results: The formation age ofBaxiliegu granite body is82Ma, it is from magma events of the ocean-continentconvergence stage prior to the India-Eurasia collision; The formation age of West Nyingchi3rdgranite body is complex, they are,respectively,47,50, and52Ma. There are varying degrees Pbloss in the zircons which was bornd in50Ma. The magma, which belongs to the magmatismcaused by the remelting of thicken crust in India-Eurasia collision stage,was pulse-invasive.The final diagenetic age is46.96±0.52Ma.;The formation ages of Demula granite body andWest Nyingchi1st，2ndgranite body are both27Ma.They are the products of intracontinentaladjustment in late India-Eurasia collision.(2) The lithology of the granite formed in the Neo-Tethys ocean oceanic crust subductionstage is porphyritic monzogranite.The rocks are with higher levels of K-feldspar in, andbasicallydoes not contain enclaves; The the granite formed in the India–Eurasia continent collisionperiod is mainly composed of biotite granite and biotite granodiorite; The the granite formed inthe post India-Eurasia collision adjustment phase is chiefly fine-grained biotite granite,medium-grained biotite granitediorite and biotite monzonitic granite. The rock mass have a highcontent of plagioclase, containing a large number of enclaves, and biotite collection concern withlocal hydrothermal is common..(3) The master and trace element analysis results of the whole rock show that the granitein study area are peraluminous, high-K calc-alkaline series. Syn-collisional granite inheritedtrace element characteristics of its source rocks with island arc properties。Post-collisionalgranite which show the nature of the adakite is from the partial melting of the deep subductedslab or basalt from the bottom of thick crust.(4) This study show that: the Gangdese granite in the west of the eastern Himalayan syntaxis nearly completely record the India-Eurasia collision occurred in the region: Baxiliegugranite body formed in the oceanic crust subduction stage, it reveal that in this area the oceaniccrust has not completely disappear to the Late Cretaceous; West Nyingchi3rd granite bodyformed in the collision stage, it show that the study area is characterized by the strong collisionof India plate towards to Eurasian plate and remelting of the crust thickening in Eocene; Demulagranite body and West Nyingchi1st, West Nyingchi2nd granite body formed in thepost-collision stage, and they all suffered the Miocene geological events.As a magmatic event,occurred in post-collision intracontinental adjustment stage.Show that,by further subduction ofIndia plate towards to Eurasia, East Himalayan syntaxis area’s nuclear Department-Indian landoccurs slippage escape in the Oligocene, and it accompanied by a strong adakite magmatism.