The Geochronology And Geochemistry Of The Mesozoic And Cenozoic Granitoids From Southwestern Gangdese Belt, Tibet

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. On the basis of recently finished 1: 250000 regional geological mapping work, six granitic intrusives, including Sewazhahai-Zhaqiongle and Kezha in the middle of the Gangdese Magmatic Belt; Gaer, Shiquanhe west, Gaerbuda and Gangrenboqi in the south of the GMB, were studied in this thesis. A systematic study, including petrology, mineral chemistry, major and trace elements and Sr-Nd-Pb isotopic geochemistry, were involved in discover the petrogenesis of the batholiths.The rocks are composed of syenogranite, monzogranite, quartz monzonite, quartz monzodiorite, alkali feldspar syenite, syenite, monzonite, monzodiorite and granophyre. The rocks are granitic texture, porphyritic structure, block structure. The main minerals are quartz, grid twinning potassium feldspar, perthite, plagioclase (An: 12~68), amphibole, biotite and aggregates of Am and Bi. The results of electron probe prove that plagioclase, amphibole and biotite have the geochemical characteristics of mixing crust and mantle. SHIRIMP U-Pb daing resoults show that the granitold for the middle parts range from 121.7 Ma to 107 Ma, while the ones from the south parts ranges from 80.3 Ma to 42.5 Ma.The south intrusives of the Gangdese magmatic belt have lower SiO2,CaO,Al2O3, higher alkali then the middles. In the Hark plots, there are obvious linear correlations between SiO2 and other main elements, indicating a magma-mixing processes. Further, the granitoid in the south belt, have higherΣREE, stronger fractionation degree in LREE then the middles. The rocks have negative Eu anomaly, enriched in Th, K, Pb and Nd, and negative anomaly of Ba, Nb, Sr, P and Ti. The intrusives mainly formed in VAG and COLG tectonic settings. The Sr-Nd-Pb isotopic compositions (εNd(t)= +2~ -8, 87Sr/86Sr(t)= 0.704460 ~ 0.715147) suggest that the source regions of the granitolid in the study area are composed of mantle component. Comparing to the batholith in the central Gendese (such as Quxu betholith), the rocks from this work have very similar age and geochemical features with those of the middle Gangdese, impling the granitoids in the south of Gangdese which relate to collision between India and Asia are homogeneous along a more than 1000 km west-east extension in the southern Gangdese. The Pb composition in this work are significant higher than that in the central parts, suggesting that the basement in the western parts have added to the source region, while in the central parts, much high percentage of mantle component or juvenal arc-accretion crustal materials have played a very important role in the granites.