Chronology,Geochemistry And Petrogenesis Of Paleocene Granitoids In The Zhebangwo Area,Central Lhasa Terrane

The Qinghai-Tibet Plateau is the largest and highest orogenic belt in the world,Large-scale magmatic rocks.Recording the history of magmatism since the Paleozoic,the generous and type-rich magmatic rock developed in the Lhasa area on the north side of the Indus-Yaluzangbu suture zone is a part of the Burma-Gandese-Ladakh-Kohistan rock base with a length of about 3000km,containing important information about the crustal accretion and the evolution of the orogenic belt.Based on field research,the method of petrology,mineralogy,geochronology and whole-rock geochemistry of the Zhebangwo pluton in the southern Lhasa terrane is used to systematically studied.And compared with the Linzizong volcanic rocks in the corresponding period,the genesis of rock and its geological significance for the collision of Neo-Tethys as well as the collision process of India-Eurasia are discussed in this paper.The lithology of the Zhebangwo pluton studied in this paper is granodiorite,which is mainly composed of plagioclase,quartz,amphibole,biotite and other minerals.The plagioclase has a positive ring structure,mainly medium feldspar.The An value is53.37%²8.82%,and the An value from the core to the edge decreases.The amphibole has a simple twin crystal structure with a MgO content of 11.97%¹3.27%,which is a magnesia;the biotite MgO content is 10.51%to 11.21%,is a magnesium biotite.In this paper,the lithology of the Zhebangwo pluton studied is granodiorite.The results of LA-ICP-MS zircon U-Pb dating indicate that the Zhebangwo pluton invaded in the Paleocene?61Ma?and the Linzizong volcanic rocks,which is regional contemporaries,constitute the main body of the Paleozoic-Eocene magmatism in the Lhasa terrane in the south.Geochemical characteristics show that it has higher SiO₂?67.33%?,Al₂O₃?15.21%?,K₂O?3.04%?content,enrichment of large ion lithophile elements?such as Rb,Th,U,etc.?and LREE,relative depletion of high field strength elements?Nb,Ta,Ti?and HREE.It's significant differentiation of light and heavy rare earth elements??La/Yb?_N=6.1?,with significant Eu negative anomalies??Eu=0.14?,belonging to aluminum-type I-type granites.The Zhebangwo pluton similar geochemical characteristics with the volcanic arc granite,low Cr?16.304ppm?,high Ni?7.35ppm?content,and low Sm/Nd value?0.2?,indicating that the magma source zone is the lower crust.High Si?67.33%?,high K?3.04%?,high Th/U?12.01?and Nb/Ta ratio?10.39?and significant negative Eu anomaly are similar to magmatic rocks formed by remelting crustal material caused by mantle source magma intrusion;The rock consolidation index is SI=13.9 and the magnesium iron index MI is82.64,La/Sm?5.13?the ratio of La/Yb?8.47?indicates that it has undergone a low degree of separation crystallization.In addition,the average content of MgO in biotite of granodiorite is 10.96%,the average content of FeO is 19.79%,the value of hornblende Mg^#is66.28⁷3.19,and the average value is 69.68,all of which show the characteristics of crust-mantle mixed source.According to the comprehensive study,formed by the northward subduction of Neo-Tethys Ocean.the Paleocene granitoids of the Zhebangwo area may be are the result of the invading of basaltic magma which caused by partial melting of the mantle wedge material,is causing remelting of the lower crust which then undergoing a low degree of separation and crystallization.