吉林中部大西山二长花岗岩U-Pb年代学和岩石地球化学特征构造

U-Pb geochronology,geochemical feature and tectonic evolution of monzonitic granites from Daxi mountain,central Jilin Province

为揭示吉林省中部黄金村西部的大西山似斑状二长花岗岩的源区性质及构造背景,研究了二长花岗岩LA-ICP-MS锆石U-Pb年代学与岩石地球化学。结果表明,大西山似斑状二长花岗岩形成时代为172.7±2.0 Ma,即中侏罗世。该岩体富硅、铝,贫钙、镁、铁,属准铝质高钾钙碱性系列。稀土元素配分曲线为右倾型,显示强烈的Eu负异常,微量元素富集Rb、K、Th等大离子亲石元素、亏损Ba、Sr等强不相容元素和Nb、Ti、P等高场强元素。大西山岩体为高分异I型花岗岩,其形成背景可能为古太平洋板块向欧亚板块俯冲时,吉中地区的弧后伸展加厚的下地壳部分熔融环境。高分异I型花岗岩与吉黑地区中生代矿床存在十分密切的伴生关系。

This paper deals with LA-ICP-MS zircon U-Pb chronology and petro-geochemistry obtained for the porphyritic monzonitic granites from Daxi mountain,west of huangjin village,in central Jilin Province,with the aim of discussing the origin and tectonic setting of the rock mass.The results of zircon U-Th-Pb age data indicate that the porphyritic monzonitic granites formed in middle Jurassic with a weighted mean age of 172.7±2.0 Ma.The porphyritic monzonitic granites are rich in Si,Al,poor in Ca,Mg,Fe,belonging chemically to metaluminous High-K calc-alkaline series.The distribution curve of rare elements is right-inclined,showing strong negative Eu anomalies.In the trace elements spider diagram,all the samples are relatively enriched in large ion lithophile elements(such as Rb,K and Th),and relatively depleted in strong incompatible elements(such as Ba and Sr)and high field strong elements(such as Nb,Ti and P).The comprehensive analysis shows that the Daxi mountain plutone classified as high-fractionated I-type granite resulted from thickened lower crust and partial melting of back-arc extension during the subduction of the ancient Pacific plate to the Eurasian plate,accompanied by the close association between high-resolution I-type granites and Mesozoic deposits in Jihei area.