Geochemistry,Zircon U-Pb Dating and Hf Isotope Compositions of Early Cretaceous Magmatic Rocks in Yongzhu Area,Northern Lhasa Terrane,Tibet,and Its Geological Significance

Zhang Shiqi; Qi Xuexiang; Wei Cheng; Chen Songyong

Abstract：The study on the petrogenesis and tectonic setting of the Early Cretaceous magmatic rocks in the northern Lhasa is important to define the geodynamic evolution for the Lhasa terrane.In this paper,it is reported of petrology,petrogeochemis-try,zircon U-Pb ages and zircon Hf isotopic comopositions of Early Cretaceous magmatic rocks from Yongzhu area in the northern Lhasa terrane.Zircon U-Pb ages for biotite-monzonitic granite,rhyolite and andesite are 118±1.0 Ma,121±0.8 Ma and 115±0.8 Ma respectively,representing their intrusion and eruption period.Biotite-monzonitic granite,granite porphyry and rhyrolite show similar geochemical characteristics.They are high K calc-alkaline and weakly peraluminous-strongly peralu-minous granites(A/CNK=1.01—1.35).In primitive mantle-normalized spider diagrams,these rocks are characterized by enriched large ion lithophile elements Rb,K and radioactive elements U,Th,and negative anomalies in Nb,P,Ti,Ba and Sr. Chondrite-normalized REE patterns show that these rocks are enriched in LREE,nearly flat HREE and negative Eu anomalies. Above chemical natures suggest that they are island-arc igneous rocks and formed in continental margin arc setting.The Hf iso-topic compositions in the biotite-monzonitic granite and rhyolite are —1.21 to 3.01 and —0.68 to 5.35,respectively,and two stage model ages are 0.99—1.26 Ga and 0.84—1.22 Ga,respectively,which suggests mixed source of crust and mantle.In con-trast,the andesite shows slightly different geochemical characteristics.They are characterized by(1)high K calc-alkaline;(2) negative anomalies in Nb,Ta,P,Ti,U and Sr,and enrichment of Rb,K and Th in primitive mantle-normalized spider dia-grams;(3)chondrite-normalized REE patterns show that these rocks are enriched in LREE,nearly flat HREE,and slight neg-ative Eu anomalies;(4)formation in the continental margin arc setting.It is proposed that the Early Cretaceous magmatic rocks in Yongzhu were formed in the continental margin arc setting.During southern subduction of Bangonghu-Nujiang Tethy-an oceanic basin,dehydration melting of the subduction oceanic plate produced the high thermal molten mass,which induced partial melting of the mantle wedge and formation of mafic magma.Then upwelling of mafic magma induced partial melting of the lower crust material and formation of acidic magma.During ascent process of the mafic magma and acidic magma,the two types of magma mixed in different proportion,and formed volcanic and plutonic rocks.