华北克拉通大同-怀安地区古元古代泥质麻粒岩的矿物学特征及变质作用剖析

An analysis of mineralogical characteristics and metamorphism of Paleoproterozoic pelitic granulites from Datong-Huai'an area,the North China Craton

以往研究认为华北克拉通中北部古元古代泥质麻粒岩主体属于一套中-低压高温麻粒岩。最近我们在大同-怀安地区发现该区泥质麻粒岩早期经历了高压-高温麻粒岩相变质作用,与伴生的石榴基性麻粒岩具有一致的变质条件,但由于后期中-低压高温(超高温)变质条件的叠加,早期高压矿物组合被强烈改造。本文以大同-怀安地区的高压-高温泥质麻粒岩为例,对其进行详细的矿物学特征和变质作用剖析。根据岩相学,如石榴石包裹体、矿物反应结构和矿物化学特征,将研究区泥质麻粒岩划分为四个变质阶段:进变质阶段(M_1)、压力峰期阶段(M_2)、温度峰期阶段(M_3)和冷却阶段(M_4)。M_1主要以石榴石核部包裹体为特征,含大量石英,少量黑云母、金红石和白云母等;M_2以幔部包裹体为特征,包裹体含量少,有石英、蓝晶石、钾长石、金红石、黑云母和锌尖晶石等,有时可见多硅白云母;M_3以基质矿物或石榴石边部包裹体为特征,如石榴石、夕线石、钾长石、石英和钛铁矿等,有或无锌尖晶石、金红石、黑云母等;M_4以石榴石边部的分解反应结构为特征,伴随熔体的结晶。综合矿物温压计、金红石Zr温度计和视剖面图方法获得进变质晚期阶段的条件为~10kbar/661~779℃,压力峰期变质条件为10~15kbar/810~860℃,温度峰期条件为6~10.5kbar/850~925℃,冷却阶段变质条件约为5~8kbar/645~761℃。同时,本文亦根据视剖面图方法模拟并解释了泥质麻粒岩(样品13GS47)中不同变质阶段的反应结构和变质反应,并对其中的石榴石成分环带进行了正演模拟。研究区与泥质麻粒岩伴生的其它岩石在不同程度上均记录了降压和降温的反应结构。最近研究证实部分石榴基性麻粒岩记录了进变质阶段,支持研究区的泥质麻粒岩和石榴基性麻粒岩可能经历了一致的变质历史,它们有可能是在压力峰期变质阶段(ca.1.97~1.90Ga)之前通过造山作用进入下地壳叠置在一起,并非在晚期(ca.1.85~1.80Ga)叠置在一起。

It was suggested that Paleoproterozoic pelitic granulites from the northern-central North China Craton were metamorphosed under low-to intermediate-pressure granulite-facies conditions in early studies.However,our recent studies identified that the pelitic granulites from Datong-Huai’an area had suffered high-pressure-high-temperature granulitie-facies metamorphism.It was comparable to P-T evolution of the spatially associated garnet mafic granulites in the study area.However,the early metamorphic assemblages were intensively and extensively overprinted by following intermediate-pressure sillimanite-bearing granulitie-facies metamorphism.Here,we presented mineralogical and petrological studies of the pelitic granulites from Datong-Huai’an area in detail.Based on the fine mineral inclusions,microstructures and mineral chemistry studies,four metamorphic generation assemblages were divided：prograde（M_1）,pressure-peak（M_2）,temperature-peak（M_3）and cooling stage（M_4）.M_1was characterized by mineral inclusions in garnet core,including a large amount of quartz,minor biotite,rutile and neglectable muscovite.M_2was featured by a few mineral inclusions within garnet mantle,consisting of kyanite,K-feldspar,rutile,biotite,quartz and zinc spinel,sometimes phengite also appeared.M_3was comprised of minerals in matrix and mineral inclusions in garnet rim,including garnet,sillimanite,K-feldspar,quartz,ilmenite,with or without rutile,biotite and zinc spinel.M_4was characterized by retrograde minerals around porphyroblastic garnet.With a combination of mineralogical thermobarometry,Zr-in-rutile thermometry and pseudosection approaches,the metamorphic conditions of M_1to M_4were estimated around^10kbar/661~779℃,10~15kbar/810~860℃,6~10.5kbar/850~925℃and 5~8kbar/645~761℃.Moreover,pesudosection approaches were not only used to model and interpret the microstructures and metamorphic reactions of the pelitic granulite（Sample 13GS47）,but also to forward simulate the evolution of garnet composition.Furthermore,lots of decompression and/or cooling microstructures were also documented in other rocks adjacent to these pelitic granulites.Recently,lines of prograde petrologic evidence were also reported in some garnet mafic granulites,which support the conclusion that the pelitic granulites and associated garnet mafic granulites were possibly experienced similar metamorphic history,and they were probably juxtaposed together in lower continental crustal level before pressure-peak metamorphism（ca.1.97~1.90Ga）during Paleoproterozoic orogeny.