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

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 metamorphism for garnet mafic granulites from the Huai’anHengshan area was conducted based on the study experience of pelitic garnet.In the investigated mafic granulites,garnet also displays a zoned distribution of various mineral assemblages from its center to rim,including quartz-and titanite/ilmenite-bearing assemblages in the core,rutile-bearing assemblages in the outer mantle to the inner rim,and a resorbed rim.Fortunately,garnet of the mafic granulites displays well-preserved compositional growth zoning patterns,with bellshaped XSps,increasing XPyrfrom core to mantle,and a resorbed rim(XSps).The HREEs(for example,Yb)show bellshaped patterns similar to those of the XSps.Both textural and compositional zoning patterns in the studied mafic granulites documented multiple generations of garnet growth history from the prograde to peak metamorphic stages.These findings provide robust metamorphic evidence for the Paleoproterozoic subduction and assembly of the Jin-Yu Mobile Belt in the North China Craton.The above two case studies suggest that high-temperature granulite-facies metamorphism may not completely reset the multi-generation growth information retained in the garnet.In contrast,minerals in the matrix only record the history of intensive re-equilibration at the peak or rehydration retrogression metamorphism.Thus,garnet is an important archive of prograde metamorphic information.This study further suggests that caution should be exercised when determining the peak P-T conditions of granulites using garnet core compositions.