东昆仑高压变质带榴辉岩年代学、地球化学及其地质意义

Zircon U-Pb Geochronology and Geochemistry and Their Geological Significances of Eclogites from East Kunlun High-Pressure Metamorphic Belt

近年来在东昆仑造山带中发现出露多处榴辉岩,由夏日哈木-苏海图、大格勒、宗加、尕日当(浪木日上游)、温泉、加当等多个榴辉岩、榴闪岩高压变质地体组成,呈透镜体或条带状产于金水口岩群中,构成了一条长达530km的高压变质带.从榴辉岩的岩石学、地球化学、同位素年代学等方面进行系统梳理,结果表明岩石类型复杂,主要可分为榴辉岩、退变榴辉岩、榴闪岩,岩石地球化学显示东昆仑榴辉岩SiO2含量为41.58%~59.00%,平均值为50.19%,Al2O3含量为11.27%~18.54%,平均值为14.66%,TiO2含量为0.76%~1.59%,平均值为1.03%.稀土配分曲线主要为轻稀土富集型,微量元素配分主要介于E-MORB与N-MORB之间.获得加当榴闪岩变质年龄为440±13Ma,原岩年龄为934±15Ma,同时结合东昆仑地区榴辉岩锆石年龄对其进行分析,锆石单点206Pb/238U年龄在直方图上显示出丰富的信息,变质峰期年龄出现明显3个年龄峰,分别为451Ma、432Ma和412Ma,原岩年龄出现峰值934Ma,其中515~440Ma记录了板块俯冲时段的岩浆热事件;440~420Ma为陆壳俯冲-碰撞的记录;420~390Ma是榴辉岩在折返过程中退化变质的反映.东昆仑榴辉岩变质时代与东昆仑原特提斯洋构造演化密切相关.

Eclogites have been recognized in areas of Xiarihamu-Suhaitu,Dagele,Zongjia,Garidang(the upper reach of Langmuri),Jiadang,and Wenquan,respectively,in East Kunlun orogenic belt.The eclogites in various sizes occur in Jinshuikou metamorphic rocks and different terranes of eclogites constitute the East Kunlun high pressure metamorphic belt with a length of more than 530 km.This paper presents a research on the petrology,geochemistry and isotope chronology of the eclogites.The results show that eclogite types are complex which can be further divided into eclogites,retrograded eclogites and amphibole-eclogites.The eclogite geochemical analyses show that SiO2 contents range from 41.58% to 59.00% with an average of50.19%;Al2O3 contents range from 11.27% to 18.54% with an average of 14.66%;TiO2 contents range from 0.76% to1.59% with an average of 1.03%;REE distribution patterns mainly show enrichment in LREE and the trace element distribution patterns show the characteristics between the E-MORB and N-MORB.440±13 Ma metamorphism age and 934±15 Maoriginal age of amphibole-eclogites in Jiadang were obtained.And the histogram of eclogite zircon age shows a variety of information including three peak ages of the metamorphic ages at 451 Ma,432 Ma and 412 Ma respectively,one peak age of the original rock age at 936 Ma,515-440 Ma records the subduction of the magma thermal event,440-420 Ma recording subduction and collision of high pressure metamorphic and 420-390 Ma as a reflection of eclogite degradation.It is concluded that the metamorphic age of eclogite is closely related to the tectonic evolution of East Kunlun.