藏东类乌齐地区晚白垩世花岗岩成因及其地质意义

Petrogenesis and geological implication of Late Cretaceous granite in Leiwuqi area,East Tibet

文章对类乌齐地区的花岗岩进行了全岩地球化学、锆石U-Pb年代学和Lu-Hf同位素研究,表明类乌齐花岗岩成岩年龄为(73.75±0.58)Ma。岩石具有高硅[w(SiO_(2))=71.02%]、富碱[w(K_(2)O+Na_(2)O)=8.09%]、FeO^(T)/MgO比值高等特征。富集Rb、U、K等大离子亲石元素,亏损Nb、Ta、Zr、Hf等高场强元素。样品具有强烈的轻、重稀土元素分馏特征(LREE/HREE=13.39),稀土球粒陨石标准化配分模式图为轻稀土元素富集型,表现出Eu负异常,配分曲线呈右倾,样品锆石ε_(Hf)(t)值主要介于-1.1~-7.9之间,对应的二阶段模式年龄(T_(DM2))为1640~1202 Ma,指示其由中元古代壳源物质熔融形成。岩石地球化学特征表明类乌齐花岗岩属于晚白垩世A型花岗岩,其岩浆来源于中元古代下地壳变质杂砂岩部分熔融。综合区域大地构造背景与前人研究,文章认为类乌齐地区晚白垩世岩浆活动与岩石圈拆沉有关。

In this study,whole-rock geochemistry,zircon U-Pb dating and Lu-Hf isotope studies are carried out on the granite in the Leiwuqi area,and the diagenetic age of the Leiwuqi granite is(73.75±0.58)Ma.The rock has the characteristics of high silicon(SiO_(2)=71.02%),rich alkali(K_(2)O+Na_(2)O=8.09%),and high FeO^(T)/MgO value.It is enriched in large ion lithophile elements such as Rb,U,K,and depleted in high field strength elements such as Nb,Ta,Zr and Hf.The rock has strong fractionation characteristics of light and heavy rare earth elements(LREE/HREE=13.39).The chondrite-standardized partition pattern is enriched in light rare earth elements,showing negative Eu anomaly,and the distribution curve is right-sloping.The sample zircon ε_(Hf)(t)values are mainly between -1.1 and -7.9,and the corresponding two-stage model age(T_(DM2))is 1640 Ma to 1202 Ma,indicating that it is formed by the melting of Mesoproterozoic crustal material.The petro-geochemical characteristics show that the Leiwuqi granite belongs to the Late Cretaceous A-type granite,and its magma originates from the partial melting of the metamorphosed sandstone in the lower crust of the Mesoproterozoic.Based on the regional tectonic background and previous studies,this study considers that the Late Cretaceous magmatic activity in the Leiwuqi area is related to the lithosphere detachment.