石榴子石:麻粒岩地体进变质记录的档案库  

Garnet:The archive of prograde metamorphic records of granulite-facies terranes

作　　者：吴佳林 翟明国[2] Jialin Wu;Mingguo Zhai(Ministry of Education Key Laboratory of Western China’s Mineral Resources and Geological Engineering,School of Earth Science and Resources,Chang’an University,Xi’an 710054,China;State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]长安大学地球科学与资源学院,西部矿产资源与地质工程教育部重点实验室,西安710054 [2]中国科学院地质与地球物理研究所,岩石圈演化国家重点实验室,北京100029

出　　处：《科学通报》2022年第36期4388-4394,共7页Chinese Science Bulletin

基　　金：国家自然科学基金(41890834,41702196)资助。

摘　　要：麻粒岩地体是否可以保留进变质记录是变质岩石学中的一个难点课题,相关研究很是薄弱.这主要源于麻粒岩地体本身的变质记录具有明显信息不对称性:它们的矿物组合和矿物成分一般被认为代表了峰期变质或退变质阶段的信息,几乎没有进变质信息.但是这种习惯性认识需要谨慎对待.来自华北克拉通晋冀交界地区麻粒岩地体的变质作用研究结果表明,石榴子石在一定程度上可以作为“抗温抗压容器”,保留麻粒岩峰期之前的部分进变质记录.通过对泥质麻粒岩的求证性研究和石榴基性麻粒岩的探索性研究,证实了研究区麻粒岩的进变质信息可以像基因片段一样部分地烙印在岩石中石榴子石的结构(如矿物包裹体群的分布型式、组合)和某些成分环带中.有时,即使主量元素生长环带很难保存,某些微量元素环带仍然可以抵抗峰期和退变质阶段的改造.它们促使石榴子石成为麻粒岩地体进变质信息的重要档案库.该研究表明,高温变质作用并不一定可以将麻粒岩地体中石榴子石多世代生长信息完全重置,同时也指示利用石榴子石核部成分计算麻粒岩峰期变质条件需要谨慎.Whether granulite terranes can preserve prograde metamorphic records is a perturbing topic in metamorphic petrology,and related research is rare.This is mainly because the metamorphic records of the granulite terranes have information asymmetry.Generally,the mineral assemblages and compositions of granulite-facies rocks are considered to reflect peak and retrograde metamorphism,rather than prograde metamorphism.However,this assumption must be made with caution.Case studies of granulites from the junction area of Shanxi and Hebei provinces in the North China Craton suggest that garnet could serve as“temperature and pressure resistant containers”under granulite-facies conditions,and retain the evidence of pre-peak metamorphism.For pelitic granulites,their protoliths are supracrustal rocks,and must have undergone prograde metamorphic processes from the Earth’s surface to the deep crust.Therefore,a confirmatory study seeking prograde metamorphic records for pelitic granulites from the Huai’an-Datong area was conducted.In the studied pelitic granulites,inclusions in garnet display well-preserved microstructural zoning,with radical zonal distribution patterns:Crowded quartz grains in the core,a few K-feldspar-kyanite-bearing assemblages in the mantle,and some sillimanite flakes in the rim.The major element zoning(here,major endmember proportions XSps,XAlm,XPyr,and XGrsare used)of garnet is relatively homogeneous.However,the high rare earth elements(HREEs)exhibit bell-shaped profiles decreasing from the core to the rim.This suggests that even if the major elements of garnet are homogenous due to hightemperature diffusion,microstructural zoning and some trace elements with slow diffusion rates could be protected from peak-and retro-grade high-temperature modification.For garnet mafic granulites,their protoliths are considered as gabbro dykes,and there is no robust evidence of prograde metamorphism from the upper to lower crust;therefore,their tectonic significance is unclear.Here,an exploratory study of prograde

关 键 词：麻粒岩地体 进变质 石榴子石 多世代生长 

分 类 号：P588.3[天文地球—岩石学]