北大别饶拔寨石榴辉石岩P-T-t轨迹及其构造意义  

Metamorphic P-T-t path and tectonic implication of garnet pyroxenite from Raobazhai,North Dabie Complex,central-eastern China

作　　者：闫方超 刘庆 马雪盈 何苗 YAN FangChao;LIU Qing;MA XueYing;HE Miao(CAS Key Laboratory of Computational Geodynamics,College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地球动力学重点实验室,中国科学院大学地球与行星科学学院,北京100049

出　　处：《岩石学报》2021年第8期2579-2598,共20页Acta Petrologica Sinica

基　　金：国家重点研发计划“深地资源勘查开采”重点专项(2016YFC0600401);国家自然科学基金项目(41273046);中央高校基本科研业务费专项资金联合资助.

摘　　要：P-T-t轨迹作为变质岩的重要研究方法之一,对揭示岩石的构造演化过程具有重要意义。北大别饶拔寨镁铁-超镁铁岩形成的构造环境和就位过程长期以来尚存争议。本文通过岩相学观察、矿物化学研究和温压计计算,揭示出饶拔寨石榴辉石岩经历了四个变质演化阶段:1)超高压变质阶段(M1):主要根据石榴子石中金红石的出溶,单斜辉石中石英的出溶和磷灰石中不透明矿物的出溶,结合前人研究,认为饶拔寨石榴辉石岩经历过超高压变质阶段(≥2.5GPa);2)高压麻粒岩相阶段(M2):矿物组合为石榴子石(变斑晶)和单斜辉石(基质)+斜长石(基质),记录的温压条件为T=648~700℃,P=1.47~1.94GPa;3)中压麻粒岩相阶段(M3):以石榴子石外围发育的主要由斜方辉石+斜长石组成的内圈“白眼圈”为特征,形成的温压条件为T=781~796℃,P=0.92~0.98GPa;4)角闪岩相阶段(M4):以石榴子石变斑晶周围发育的外圈“白眼圈”为特征,其矿物组合为角闪石+斜长石的后成合晶,形成的温压条件为T=663~685℃,P=0.50~0.58GPa。石榴辉石岩的锆石SIMS U-Pb定年得到了3组不同的交点年龄,分别为208.1~202.1Ma、227.6Ma和817.7Ma。根据锆石包裹体中角闪石+斜长石的组合,推断208.1~202.1Ma代表了角闪岩相变质年龄的下限,227.6Ma则是高压麻粒岩相的下限年龄,而新元古代年龄(817.7Ma)与区域上的变质岩原岩年龄一致,可能代表了其原岩年龄。结合前人研究,饶拔寨石榴辉石岩记录了顺时针的PT-t轨迹,揭示了板片俯冲(超高压变质)-碰撞-折返(降压升温过程,~227Ma)-抬升(降压降温过程,208~202Ma)的完整过程。P-T-t path plays an important role in revealing the tectonic evolution of metamorphic rocks.The debate on the tectonic environment and emplacement process of mafic-ultramafic rocks in Raobazhai have been controversial for a long time.Four generations of mineral assemblages have been recognized in the garnet pyroxenite from Raobazhai,North Dabie.Thermodynamic computation shows that the metamorphic conditions of the garnet pyroxenite are as follows.1)Ultra-high pressure metamorphism stage(M1):the exsolution of rutile from garnet,quartz from clinopyroxene and opaque minerals from apatite,indicated that the garnet pyroxene in Raobazhai has experienced ultra-high pressure metamorphism(≥2.5GPa).2)High pressure granulite-facies metamorphism stage(M2):the temperature and pressure condition is T=648~700℃and P=1.47~1.94GPa,which recorded by the mineral assemblage of garnet(porphyroblast)+clinopyroxene(matrix)+plagioclase(matrix).3)The medium-pressure granulite-facies metamorphism stage(M3):it is characterized by the inner"white-eye socket"which is mainly composed of orthopyroxene+plagioclase developed on the periphery of garnet,and the temperature and pressure condition is T=781~796℃,P=0.92~0.98GPa.4)Amphibolite-facies metamorphism stage(M4):it is characterized by the outer"white-eye socket"developed around garnet(porphyroblast),and its mineral assemblage is amphibole+plagioclase under the condition of T=663~685℃,P=0.50~0.58GPa.SIMS U-Pb dating of metamorphic zircons from the Raobazhai garnet pyroxenite yielded three different intercept ages of 817.7Ma,227.6Ma and 208.1~202.1Ma,respectively.According to the assemblage of amphibole+plagioclase inclusions in zircons,the age of 208.1~202.1Ma is linked to the lower limit of the amphibolite-facies metamorphic age,the high pressure granulite-facies metamorphism began in 227.6Ma,and the Neoproterozoic age(817.7Ma)is consistent with the protoliths of metamorphic rocks in the region.Combined with previous studies,the clockwise P-T-t path of Raobazhai garnet pyroxenite was r

关 键 词：石榴辉石岩 温压计 P-T-T轨迹 北大别 变质作用时代 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]