吉南中侏罗世花岗闪长岩的锆石U-Pb年龄、地球化学及Hf同位素组成

Zircon U-Pb Geochronology,Geochemistry and Hf Isotopic Composition of the Middle Jurassic Granodiorite in South Jilin Province

古太平洋构造体系控制了东亚大陆边缘的形成和演化,但关于古太平洋板块俯冲的开始时间以及精细过程仍存在争议。和龙市石人村位于吉林南部,地处华北板块北缘东段,是研究古太平洋演化的有利区域。本文对研究区内花岗闪长岩进行了锆石U-Pb年代学、全岩地球化学和Hf同位素研究。结果表明:锆石LA-ICP-MS U-Pb定年结果为(173±1)Ma,属中侏罗世;岩石具有高Mg^(#)(65.23~65.92),异常高w(Sr)((1657~2467)×10^(-6)),低w(Y)((7.50~7.98)×10^(-6))、w(HREE)的特征,非常类似于低硅埃达克岩,反映了其初始岩浆来源于板片熔融交代的地幔楔。该类型岩石的发现暗示在173 Ma左右区域存在非常热的俯冲作用,可能是斜向俯冲或洋脊俯冲所致,也意味着中侏罗世早期古太平洋板块向欧亚大陆的俯冲方向发生了偏转或俯冲方式发生了转变。然而,石人村花岗闪长岩又不同于典型的低硅埃达克岩,其w(SiO_(2))=61.13%~62.31%,明显大于埃达克岩的56%,在野外和镜下均观察到岩浆混合作用的痕迹,反映岩浆在侵位过程中经历了岩浆混合作用开放体系的演化过程。

The paleo-Pacific tectonic system controls the formation and evolution of the East Asian continental margin,but there is still controversy over the onset of the subduction of the paleo-Pacific plate and its detailed evolution process.Shiren Village in Helong Lity is located in south Jilin Province,in the eastern part of the northern margin of the North China plate,which is a favorable area to study the evolution of the paleo-Pacific Ocean.This paper presents a synthesis study of zircon U-Pb geochronology,geochemistry and Hf isotopes on the granodiorites.The studied rocks yield an average zircon LA-ICP-MS U-Pb age of(173±1)Ma,suggesting a Middle Jurassic magmatic event.The rocks are characterizd by high Mg^(#)(65.23%-65.92%),abnormally high w(Sr)((1657-2467)×10^(-6)),and low w(Y)((7.50-7.98)×10^(-6))and w(HREE),very similar to the low Si adakite defined by Martin et al.(2005),reflecting that the initial magma is derived from the mantle wedge metasomatized by plate melting.The significance lies in the existence of very hot subduction in this area around 173 Ma,which may be caused by an oblique subduction or oceanic ridge subduction,and also means that the subduction direction of the paleo-Pacific plate towards Eurasia deflected or the subduction mode changed in the Early Middle Jurassic.However,the studied rocks are different from the typical low Si adakites,while their w(SiO_(2))values(61.13%-62.31%)are significantly greater than 56%.The traces of magmatic mixing were observed in both the field and under the microscope,reflecting that the magma experienced the evolution process of magmatic mixing in an open system during the emplacement process.