东昆仑西段夏日哈木-苏海图早古生代榴辉岩的发现及意义

Discovery and its geological significance of Early Paleozoic eclogite in Xiarihamu-Suhaitu area,western part of the East Kunlu

1∶25万区域地质调查在东昆仑西段昆中断裂以北的北昆仑变质基底中发现了呈构造透镜分布的榴辉岩及其退变质的榴闪岩,沿小灶火-苏海图-夏日哈木-拉宁灶火断续延伸长达20km。榴辉岩经历了强烈的退变质作用,大部分已退变为榴闪岩,由峰期变质矿物组合石榴石＋绿辉石＋金红石＋钛铁矿和后期退变矿物组合普通角闪石（绿色）＋斜长石组成,石榴石边缘发育似针状普通角闪石及微粒状斜长石组成的后合成晶结构,形成宽0.15~1cm的＂白眼圈＂结构。石榴石成分为Alm52-53And4.2Spess0.7-1.2Gros21.7-22Pyrope19-20,绿辉石中硬玉分子量30.14mol%~37.7mol%。估算榴辉岩相变质温压条件P≈20kbar、T≈660~700℃,榴闪岩相退变温压条件P≈7kbar、T≈550℃,岩石经历了明显的降温降压过程。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,17颗锆石Th/U极低,在0.007~0.09之间,其（LA-ICP-MS）U-Pb年龄411.1±1.9Ma代表了榴辉岩相峰期变质年龄,结合区域构造岩浆事件,确定本次高压变质的峰期时代为晚志留-早泥盆世,其形成可能与大规模的陆-陆碰撞事件有关,这一发现为研究东昆仑造山带的构造演化具有重要意义。

Eclogite and the retrograde amphibolitic eclogite,occurred as tectonic lenses,are found in the metamorphic basement,western part of the East Kunlun,based on the results of 1∶250000 regional geological survey. They are intermittently scattering about20 km along Xiaozaohuo-Suhaitu-Xiarihamu-Laningzaohuo area. Most of the eclogites are retrograded into amphibolitic eclogites,they consist of the peak metamorphic mineral assemblage（ garnet ＋ omphacite ＋ ilmenite ＋ rutile） and the later stage retrograde mineral assemblage（ hornblende（ green） ＋ plagioclase）. The edge of the garnet grains develops needle-like hornblende and particulate plagioclase symplectic corona texture,forming white eye rims about 0. 15 ~ 1cm in size. The composition of garnet is Alm52-53And4. 2Spess0. 7-1. 2Gross21. 7-22Pyr19-20,Jadeite molecular in omphacite is 30. 14mol% ~ 37. 7mol%. Estimation of eclogite-facies metamorphic conditions shows P≈20kbar,T≈660 ~ 700℃,and the amphibolite facies retrograde metamorphic conditions of P≈7kbar,T≈550℃.The rocks had experienced a retrogressive process of significant cooling and decompression. Most of the CL images of zircons from the eclogites show sector zoning or＂fir leaves＂structure. Th / U ratio ranges in 0. 007 ~ 0. 09. Zircon U-Pb age（ LA-ICP-MS） is 411. 1 ±1. 9Ma（ n = 17）,which represents the peak metamorphism age of eclogite-facies. Combined with the analysis of regional metamorphic and magmatic events,the peak stage of the high-pressure metamorphism is determined to be Late Silurian-Early Devonian,which is probably associated with a massive continent-continent collision event. The discovery is significant for the tectonic study of the Kunlun orogenic belt.