内蒙古中西部宝音图群云母片岩的变质P-T轨迹与锆石年代学研究  

Metamorphic P-T path and zircon geochronology of the mica schists in the Baoyintu Group,central western Inner Mongolia

作　　者：唐爽 张晋瑞 姜灵 魏春景[2] 初航[3] TANG Shuang;ZHANG Jinrui;JIANG Ling;WEI Chunjing;CHU Hang(College of Earth Sciences,Jilin University,Changchun 130061,China;School of Earth and Space Sciences,Peking University,Beijing 100871,China;Tianjin Institute of Geology and Mineral Resources,Tianjin 300170,China)

机构地区：[1]吉林大学地球科学学院,吉林长春130061 [2]北京大学地球与空间科学学院,北京100871 [3]天津地质矿产研究所,天津300170

出　　处：《地质学报》2023年第3期705-724,共20页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号42072073,41702049)资助的成果。

摘　　要：位于内蒙古中西部地区的宝音图群,主要由云母片岩、斜长角闪岩、石英岩和大理岩等组成。其中云母片岩中发育典型的巴罗式递增变质带,目前对于该递增变质带的研究非常薄弱。本文对宝音图群的云母片岩开展了系统的岩相学、矿物化学、相平衡模拟以及锆石U-Pb年代学的研究。岩相学研究结果显示,三个云母片岩样品具有不同的峰期矿物组合,分别为石榴子石+白云母+黑云母+斜长石+石英,十字石+白云母+黑云母+斜长石+钛铁矿+石英,石榴子石+蓝晶石+白云母+黑云母+斜长石+钛铁矿+石英。此外,样品LS93和LS07存在晚期红柱石的叠加。相平衡模拟研究限定三个云母片岩的峰期温度压力分别为:577℃、0.73 GPa,620℃、0.78 GPa和670℃、0.8 GPa,具有递增型特征,其峰期温度压力指示的地热梯度为18~20℃/km,为典型的中压相系;此外,三个样品的P-T轨迹均显示早期升温升压直至峰期阶段,随后降温降压的顺时针型,反映地壳加厚和后期抬升的演化过程。云母片岩中的锆石发育典型的变质边结构,变质边的Th/U=0.004~0.02,锆石U-Pb年代学结果显示该变质年龄为398±6 Ma,代表峰期或近峰期阶段的变质年龄。综合分析认为,宝音图群巴罗式变质带可能形成于与古亚洲洋闭合有关的碰撞造山背景。The Baoyintu Group in the central western Inner Mongolia,mainly consists of mica schist,amphibolite,quartzite and marble.Typical progressive metamorphism is developed in the mica schist,which,however,is still poorly studied.In this paper,the mica schist in the Baoyintu Group has been systematically studied in terms of petrography,mineral chemistry P-T paths and zircon U-Pb geochronology.Petrographic study on three mica schist samples shows different peak mineral assemblages,respectively of garnet+muscovite+biotite+plagioclase+quartz in sample LS89,staurolite+muscovite+biotite+plagioclase+ilmenite+quartz in sample LS93,garnet+kyanite+muscovite+biotite+plagioclase+ilmenite+quartz in sample LS07.In addition,the late superposition of andalusite is observed in samples LS93 and LS07.Phase equilibrium modeling defines a progressively trending peak P-T condition for the three mica schist samples,such as 577℃,0.73 GPa in LS89,620℃,0.78 GPa in LS93 and 670℃,0.8 GPa in LS07.Moreover,the peak P-T conditions of the samples yield geothermal gradient of 18~20℃/km,being typical medium P/T series.The P-T paths of the samples show clockwise evolution including an early prograde stage until the peak stage,followed by cooling and decompression,representing a crustal thickening setting.Zircons in mica schist of the Baoyintu Group typically have metamorphic rims with Th/U ratios of 0.004~0.02,and the LA-ICP-MS U-Pb dating of the zircon rims yield a weighted mean of 398±6 Ma,which is interpreted to represent the age of near-peak metamorphic stage.Consequently,we interpreted that the Barrovian-type metamorphic zone of the Baoyintu Group may have formed in a collision orogenic background related to the closure of the Paleo-Asian Ocean.

关 键 词：云母片岩 宝音图群 巴罗式变质带 相平衡模拟 锆石U-PB定年 

分 类 号：P597.3[天文地球—地球化学] P588.3[天文地球—地质学]