Geochronology And Geochemistry Of The Samba Granitoids In North Gangdese

In the southern Tibet Plateau, Gangdese belt bounded by the Indus-Yarlung Zangpo suture zone (IYSZ) in the south and the Bangong-Nujiang suture zone in the north, is a huge tectonic - magmatic belt, which includes 80 % area of granoitoid within Tibet. The Gangdese betholith which parallel to the IYSZ, is divided into three subzones (north, middle and south, from north to south). They represent the magmatism corresponding to the subduction of Neo-Tethyan oceanic crust pre- and synchronous to the India-Asia collision. However, its lithospheric structure, impact on previous subduction, the growing histories and the contact with Northern Qiangtang Terrane have resulted in confusion and dispute. The existing research mainly concentrated in the central and southern Gangdese and The research in the northern of Gangdese is less. On the basis of recently finished 1: 250000 regional geological mapping work,Sangba pluton,lie in the east of northern Gangdese in Tibet,were studied in this thesis. A systematic study,including U-Pb isotope chronology, petrology, mineral chemistry, major and trace elements and Sr-Nd-Hf isotopic geochemistry ,were invovlved in in discover the petrogenesis of the batholiths. the purpose of this study endeavors to bring up the geological constraints for the structural activities and magmatic evolution of the Northern Gangdese and Bangong–Nujiang Tethyan Ocean.The 24 representative samples, in terms of lithology, are divided into 20 biotite granites and 4diabases. The method of Zircon U-Pb dating reports the age of biotite granites 110¹18Ma, diabase age may refer to the south of allgovites in Dongzhongla area, is 117.0±1.0Ma. The biotite granite samples from Sangba pluton compositionally characterize itself by high content of SiO₂(71.65% 76.19%), K2O(4.50% 5.43%)and Na2O(2.76% 3.66%), whereas the contiguous diabase bears the chemical compositions of SiO₂ (47.85% 48.10%),K₂O 1.32% 1.08%, MgO(5.27% ⁷.00%), it belongs basite. whose all saturation index (A/CNK) of felsic sample is in the range of 1.11¹.19, With regard to their trace element features, the biotite granite samples are detected to be enriched in lithophile element but severely depleted of high appearance elements, which is categorized into S-type granite due to the fact that it is peraluminous and has been highly fractionated. Comparing with the mature continental crust-oriented materials in Gangdese emphasized by the heavily peraluminous granite from Ningzhong in Early Jurassic, Sangba pluton displays theεNd(t) value -7.3 -5.7 and(87^Sr/86^Sr)t value 0.7102 0.7504, But the measurement of zircon Hf isotope in the biotite granite exhibits wide range ofεNd(t) values (-10.1⁺3.4),and the relatively old crustal model ages (1.5¹.7Ga) ,it says that crustal material likely that in the process of Sangba pluton diagenesis plays an important role. The southward subduction of Bangong–Nujiang Tethyan oceanic lithosphere is set as the initial geodynamic movement for the formation of the Sangba pluton. The slabs of the oceanic crust were subjected to breaking off during the subduction, which caused the local soft flow pressure rose and melt circle. at the same time this kind crust-derived substances were mixed by its upper crust of baking acidic magma molten income. Sangba pluton come form crystallization differentiation formation of mixed magma.