Geochronology,Geochemistry And Geological Significance Of Granitic Rocks In Balang In Southern Tibet

The Lhasa terrane,which is located in the southern part of the Tibet Plateau,is a giant tectonic-magmatic rock belt with a large number of Late Cretaceous-Paleogene magmatic activities.These magmatic activities have well recorded important processes related to the closure of the Neo-Tethys ocean,Asian-Indian collision,and the tecctonic evolution of the plateau.It has become the focus of many geologists of the world.In the paper,we present field geology,petrography,geochronology,zircon Hf isotopes and geochemistry for the granite exposed in the Balang area of the eastern region of the southern Lhasa terrane,with the aim of constraining the petrogenesis and origin of the granite.And compared with the granites formed in the same period and Linzizong volcanic rocks,the main research results are as follows:1.Balang area are mainly Biotite monzonitic granite and granodiorite.The zircon U-Pb ages of the two biotite monzonitic granites in the study area show that they are products of Paleocene magmatic activity,and their formation ages are 60.54±0.59 Ma(sample HGB1701-1)and 61.63±0.65 Ma(sample HGB1702-1).In terms of zircon trace elements,the samples were depleted in light rare earth elements and enriched in heavy rare earth elements,with significant Ce positive anomaly and weak Eu negative anomaly.2.The results of major and trace elements of grantoid samples in the study area show that the value of Si O₂ range of Paleocene granites from 67.57%?73.08%,and having K₂O/Na₂O=0.77?1.52,and the Ritman index are less than 3.3(?=1.59?2.77).They are dominantly high-potassic calc-alkaline series.The A/CNK=0.88?1.03,belonging to metaluminous-peraluminous rocks.The distribution of rare earth element and trace element shows that the granitoid samaples are relative enriched in light REE and large ion lithophile elements(LILE)(Rb?Th?U),depleted in heavy REE and high field-strength elements(HFSE)(Nb?Ta?P?Ti),and REEs fractionate evidently((La/Yb)_n=3.38?7.03).The weak negative Eu anomaly(Eu/Eu*=0.70?0.88)shows that the magma may have undergone fractional crystallization of plagioclase in the upward intruding process.The values of?Hf(t)of the samples in the Balang area are between-2.3?+4.8,and have a relatively old crustal model age of 821?1279 Ma.3.Based on regional geological data and previous research,this dissertation believes that the Paleoocene granitoid bodies in the Balang area were formed at the northward subduction of the Neo-Tethys oceanic slab.The source area of the granites was affected not only by the partial melting of the new crustal material but also by the ancient crystallization basement,and formed after a certain degree of separation and crystallization...