东昆仑东段浪木日上游早古生代榴辉岩的发现及其意义

摘要

Eclogite was recognized recently in the upper reaches of Langmuri in eastern East Kunlun Mountains. In the field, the eclogite occurs in lenticular and massive forms and exhibits fault contact with surrounding rocks. The ecologite-bearing metamor⁃phic zone is 1~3km in width. The lenticular eclogite is distributed in metamorphic rocks of Paleoproterozoic Baishahe Formation and Neoproterozoic granite gneiss.The eclogite facies rock association is composed of eclogite and pomegranate amphibolite, and the eclogite facies mineral assemblage consists of eclogite+omphacite+rutile+quartz. According to eclogite-clinopyroxene mineral temperature and pressure gauge, the temperature and pressure conditions of eclogite rock are estimated as follows: T=650~750℃,p=1.8~2.0GPa, falling into the C-type eclogite zone in garnet Alm+Spess-Pyr-Gross triangular diagram. Zircons in the eclogite CL images show that most zircons have the fan zoning or“fir leaves”structure, with a few weak band structures. Th/U ratios of 31 zir⁃con samples are relatively low (0.06~0.135). The LA-ICP-MS zircon U-Pb ages can be divided into two kinds:a kind of concor⁃dant age is 431.9±2.3Ma, representing the eclogite facies metamorphic peak age;the other concordant age is 487±5.9Ma. Original rock might have resulted from the deterioration that occurred under the influence of tectonic events during the subduction process. In consideration of regional ophiolite and tectonic magmatic events, the authors hold that the peak of high-pressure metamorphism in the study area occurred in late Early Silurian in the East Kunlun subduction-collision environment. Combined with the same type of eclogites found in adjacent areas, the authors restored a Palaeozoic high-pressure metamorphic belt in East Kunlun region.%在东昆仑东段郎木日上游最新识别出榴辉岩，其野外产状为透镜状、块状，与围岩呈断层接触。含榴辉岩的变质带宽1~3km，榴辉岩呈透镜体状分布于古元古界白沙河岩组变质岩及新元古代花岗片麻岩中，榴辉岩相岩石组合为榴辉岩和榴闪岩。榴辉岩相矿物组合为石榴子石+绿辉石+金红石+石英。石榴子石-单斜辉石矿物温压计估算出榴辉岩形成的温压条件为T=650~750℃，p=1.8~2.0GPa，在石榴子石Alm+Spess-Pyr-Gross三角图解中落入C型榴辉岩区。榴辉岩中锆石阴极发光图像显示，大部分具有扇形分带或杉树叶结构，少数具有弱环带结构，31颗锆石Th/U值较低（0.06~0.135），其LA-ICP-MS锆石U-Pb年龄明显分为2类，一类谐和年龄值为431.9±2.3Ma，代表了榴辉岩相峰期变质年龄；另一类谐和年龄值为487±5.9Ma，可能为原岩在俯冲过程中受构造热事件影响发生变质的结果。参考区域蛇绿岩及构造岩浆事件，确定研究区本次高压变质的峰期时代为早志留世晚期，可能发生于东昆仑俯冲消减-碰撞环境，结合邻区发现的同类型榴辉岩，恢复了一条东昆仑地区早古生代的高压变质带。