Zircon U-Pb Age And Tectonic Setting Of Granitoids In East Gangdese Belt

In the Tibetan Plateau,the Gangdese belt locating in the most south of the Asina continent.The Gangdese belt has experienced the subduction of Tethyan oceanic lithosphere and the India-Asia continental collision in the processes of which widespread magmatic activities occurred.The Gangdese belt is the most distinctive landform on our planet.Based on the plate tectonic and collision orogenic theory,in this paper,we present field geology,petrography,LA-ICP-MS zircon U-Pb geochronology,geochemistry compositons for the Gangdese granitoids,with the aim of constranining the petrogenesis and the tectonic evolution of the Gangdese belt.The main research results are as follows:1.Six granitoid samples from the east of Gangdese belt were selected for in situ zircon U-Pb analyses.According to the zircon U-Pb data as well as previous study,the magmatic activities are divided into four intrusive stages:(1)Late Triassic(Sample JM23-1;207.5±5.3Ma);(2)Early Jurassic(Sample JM24-1,192.6±3.7Ma;SampleJM25-1,194.9±3.1Ma);(3)Late Cretaceous(Sample JM27-1,87.4±2.4Ma);(4)Paleocene(Sample JM22-1,62.6±1.7Ma;Sample JM22-2,61.4±1.2Ma),which means the magmatic activities occurred throughout the Mesozoic.2.The Late Triassic granite porphyry is strongly peraluminous,with A/CNK=1.22.The granite porphyry is are relative enriched in large ion lithophile element(LILE)Rb,high field-strength elements(HFSE)Th?U and show negative Ta?P?Nb anomalies,indicating that the granitoids have an island-arc-type geochemical affinity.P and Ti show negative means that the fractional crystallization of ilmenite and apatite have some kind of relationship with that.3.The two Early Jurassic granite porphyries are metaluminous-peraluminous granitoids(A/CNK=0.996-1.23).They have K20/Na20=1.54-1.94.The value of Si02 rang from 76.44-76.97%.These granitoids are dominantly high-potassic calc-alkaline series.These granitoids are relatively show negative LILE and Nb.(La/Yb)N=4.15-7.57 which means the Early Jurassic granite porphyry is island-arc-type geochemical affinity.Based on the study of the Late Triassic and Early Jurassic igneous rock and their zircon U-Pb geochronology and the petrogenesis in the Gangdese belt,the beginning time of the Neo-Tethyan oceanic slab subduction is not later than Early Jurassic.The Neo-Tethyan Ocean has long lasting tectonic evolution.4.The Rangdese batholith are composed mainly of the Paleocene granitoids.They are high-potassic calc-alkaline series.The A/CNK=1.098 shows they are metaluminous granitoids.The granite porphyry is are relative enriched in LILE(Rb?Th?K)and light rare earth element(LREE)and show negative HFSE(Nb?Ta?P?Ti)anomalies.That means the magmas were mainly derived from partial melting of juvenile crust with ineorporation of the ancient crustal material deriving from the Middle Lhasa terrane.