东昆仑大格勒一带基性二辉麻粒岩的变质演化及地质意义  

Metamorphic evolution of mafic two-pyroxene granulites in the Dagele area,East Kunlun

作　　者：李佳斌 吕增[1] 陈振宇[2] 刘小辉 LI JiaBin;L Zeng;CHEN ZhenYu;LIU XiaoHui(MOE Key Laboratory of Orogenic Belts and Crustal Evolution,School of Earth and Space Science,Peking University,Beijing 100871,China;MNR Key Laboratory of Metallogeny and Mineral Assessment,Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]北京大学地球与空间科学学院,造山带与地壳演化教育部重点实验室,北京100871 [2]自然资源部成矿作用与资源评价重点实验室,中国地质科学院矿产资源研究所,北京100037

出　　处：《岩石学报》2023年第9期2636-2650,共15页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(91955202、42072075)资助.

摘　　要：基性麻粒岩作为高温变质作用产物,其矿物组合能够较好地记录区域地壳热流的峰值范围,是揭示造山带演化历史的重要窗口。本文报道了在东昆仑造山带东段大格勒地区金水口岩群副片麻岩中发现的基性二辉麻粒岩露头,它们呈块状-片麻状构造,斑状变晶-细粒变晶结构,主要矿物为斜长石、斜方辉石、单斜辉石、黑云母和普通角闪石。斜方辉石与单斜辉石和高钙斜长石(An=74~81)的平衡结构以及斜方辉石中包裹残余普通角闪石和黑云母的结构表明这些岩石经历了较充分的麻粒岩相结晶作用。填隙状生长的普通角闪石和黑云母可能代表了冷却至固相线附近矿物-残余熔体发生反应的产物。根据二辉石温度计和相平衡模拟计算,获得在0.4~0.7GPa压力条件下的麻粒岩相变质温度为840~910℃。详细的变斑晶斜长石和钛铁矿包体研究表明,麻粒岩相变质之前可能存在一期低温高压变质作用,形成的矿物组合为斜黝帘石+金红石+榍石+普通角闪石±阳起石+黑云母+石英,相平衡模拟确定该阶段的温压条件为0.85~1.2GPa、500~640℃。斜黝帘石在减压阶段的早期转变为低钙斜长石(An=46-51),同时角闪石转变为单斜辉石,可能代表了低压角闪岩相阶段。通过LA-ICPMS锆石U-Pb测年以及锆石包体分析,确定麻粒岩相变质年龄为411.4±2.9Ma(MSWD=1.05)。这些二辉麻粒岩是在金水口以外的东昆仑地区发现的首例晚古生代低压高温变基性岩,其与围岩片麻岩构成一个麻粒岩地体。它们反映了泥盆纪早期东昆仑造山带的异常高温事件,与后碰撞伸展阶段软流圈地幔上涌带来的区域高温热流有关。本研究对解析东昆仑造山带高级变质地体的精细结构以及古特提斯洋形成过程的深部动力学机制具有重要意义。As a representative product of high-temperature metamorphism,the paragenesis of mafic granulites may well reflect the peak values of regional heat flow and thus provides an important window to get insight into the evolution of orogenic belts.This paper reports exposures of mafic two-pyroxene granulites enclosed in paragneisses of the Dagele area,East Kunlun Orogenic Belt.They are massive or gneissic with porphyroblastic or fine-grained textures,and largely consist of plagioclase,orthopyroxene,clinopyroxene,biotite and hornblende.The textural equilibrium between matrix pyroxene and high-Ca plagioclase(An=74~81),and the textures of rare relict hornblende and biotite in orthopyroxene suggest nearly complete granulite-facies recrystallization.Interstitial hornblende and biotite in the matrix may represent the product of melt-mineral reaction approaching solidus during cooling.Based on the two-pyroxene thermometer and phase equilibria modelling,the granulite-facies temperatures are estimated to be 840~910℃at 0.4~0.7GPa.Detailed investigations of mineral inclusion assemblages of porphyroblastic plagioclase and ilmenite support a low temperature-high pressure stage,during which the assemblage clinozoisite+rutile+sphene+hornblende±actinolite+biotite+quartz was formed.P-T pseudosection calculation suggests this assemblage is stable at 0.85~1.2GPa and 500~640℃.During initial decompression,clinozoisite was replaced by relatively low-Ca plagioclase(An=46~51)and hornblende by clinopyroxene,which probably suggest a low-pressure amphibolite-facies stage.LA-ICPMS U-Pb dating and inclusion assemblage analyses of zircons reveal the granulite-facies metamorphic age of 411.4±2.9Ma(MSWD=1.05).These two-pyroxene granulites are mafic high-temperature metabasites of Late Paleozoic reported for the first time outside the Jinshuikou area.Their intimate association with the host granulite-facies paragneisses suggests a granulite-facies metamorphic terrane of the Dagele area in Late Paleozoic.This may reflect a regional-scale unusuall

关 键 词：二辉麻粒岩 P-T轨迹 锆石U-PB定年 低压高温变质作用 大格勒 东昆仑造山带 

分 类 号：P588.347[天文地球—岩石学] P597.3[天文地球—地质学]