东昆仑高压变质带榴辉岩和榴闪岩地球化学特征及形成动力学背景

大陆造山带的高压—超高压变质带是古板块汇聚边界及古大洋俯冲和大陆碰撞造山的重要标志，榴辉岩作为高压变质作用形成的岩石，历来被广为关注。根据近几年东昆仑新发现的加当、苏海图、温泉、浪木日上游、宗加、五龙沟西（大格勒）等榴辉岩及榴闪岩，厘定出一条超过500 km的高压变质带，它是青藏高原北部一条重要的古板块接合带。在野外调查基础上，综合大量前人资料，从岩相学、矿物学及岩石地球化学等方面对东昆仑榴辉岩进行了系统的分析和研究。结果表明:东昆仑高压变质带榴辉岩可分为两种类型，即榴辉岩和退变质榴辉岩；石榴石矿物化学成分均表明榴辉岩为C型；榴辉岩主要为拉斑玄武岩系列，球粒陨石标准化稀土元素配分模式均为轻稀土元素富集型，主要介于富集型洋中脊玄武岩（E-MORB）与正常型洋中脊玄武岩（N-MORB）之间，原始地幔标准化微量元素蛛网图基本一致，显示出洋中脊玄武岩（MORB）特征；榴辉岩、榴闪岩温压条件近于一致，形成压力介于1.2~2.0 GPa或更高，温度不小于575 ℃，具有一个顺时针“发卡”型P-T轨迹，其变质峰期温压达到榴辉岩相；峰期变质年龄出现3个明显峰值，分别为451、432、412 Ma，432 Ma主峰期变质年龄约束了东昆仑早古生代洋盆关闭时间。

Geochemical Characteristics of Eclogites and Garnet-amphibolites in East Kunlun High Pressure Metamorphic Belt and Their Geodynamic Setting

High-ultrahigh pressure metamorphic belt of the continental orogenic belt is the convergent boundary of the paleoplate, and is an important indicator of the palaeo-ocean subduction and the continental collision. Combined with the eclogite and garnet-amphibolite discovered in recent years, such as Jiadang, Suhaitu, Wenquan, Langmurishangyou, Zongjia, West Wulonggou (Dagele), a high pressure metamorphic belt more than 500 km is found, which is an important paleoplate suture zone in the northern Qinghai-Tibet Plateau. Based on field investigation and comprehensive previous data, the petrography, mineralogy and lithogeochemistry of eclogites in East Kunlun were studied. The results show that the eclogite in East Kunlun high pressure metamorphic belt can be divided into eclogite and retrograded eclogite, respectively; the mineral chemical component of garnet indicates that the eclogites are all C-type; the eclogites mainly belong to the tholeiite series; chondrite-normalied REE patterns show that LREE is strongly enriched between E-MORB and N-MORB; primitive mantle-normalized trace element spider diagrams are basically consistent, showing the characteristics of MORB; the temperature and pressure conditions of eclogite and garnet-amphibolite are nearly consistent, the formation pressure is 1.2-2.0 Gpa or higher, and the temperature is not less than 575 ℃; the P-T path is a clockwise “card” type; the temperature and pressure of the peak metamorphic stage are eclogite facies; according to the histogram of peak metamorphic ages of eclogites, there are three peak metamorphic ages, including 451 Ma, 432 Ma and 412 Ma; the main peak metamorphic age of 432 Ma restrains the time of Early Paleozoic ocean basin closure in East Kunlun.