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1、摘要北祁連HP/LT變質(zhì)帶分布在北祁連造山帶中段祁連縣清水溝一百經(jīng)寺一帶,主要由各種綠片巖、藍(lán)片巖、榴輝巖及榴輝巖質(zhì)巖石、變質(zhì)沉積巖和少量超基性巖塊所組成,被認(rèn)為是早古生代與洋殼俯沖有關(guān)的增生楔的深部組成部分。FIP/LT變質(zhì)帶中的榴輝巖一般以透鏡體的形式產(chǎn)在藍(lán)片巖和多硅白云母片巖中,主要由石榴石、綠輝石,藍(lán)閃石、多硅白云母、綠簾石組成,部分樣品還含有硬柱石。根據(jù)藍(lán)閃石、硬柱石、綠簾石等特征變質(zhì)礦物的含量可將本區(qū)榴輝巖相巖石劃分為如下五種類型:榴輝巖、含綠輝石石榴藍(lán)閃片巖,含硬柱石榴輝巖、綠輝石巖、退變榴輝巖.其中含綠輝石石榴藍(lán)閃片巖是榴輝巖相巖石的主體組成
2、部分。詳細(xì)的巖石礦物學(xué)、顯微結(jié)構(gòu)特征分析表明,北祁連低溫榴輝巖相變質(zhì)作用經(jīng)歷四個變質(zhì)演化階段:I峰前藍(lán)片巖相(硬柱藍(lán)片巖相/綠簾藍(lán)片巖相)階段、II峰期榴輝巖相階段、Ill峰后(綠簾)藍(lán)片巖相階段,IV綠片巖相階段。我們利用Grt-Cpx叩he礦物壓力計(gavna鑫ndTerry。2004)和Grt-Cpx礦物溫度計(R8vna.2000)估算蜂期溫壓條件為430-57doC和2.I-2.66Pa。詳細(xì)的尊Ar-‰法和SHRIMP鋯石U-Pb定年分析顯示,北祁連HP/LT變質(zhì)巖的原巖形成時代為502_+161Ja,而發(fā)生榴輝巖相變質(zhì)作用時代為477-Ia-
3、489bla。根據(jù)稀土元素和微量元素的變化特征及標(biāo)準(zhǔn)化配分模式,本區(qū)唧/LT變質(zhì)巖可分為兩類:I類肝/LT變質(zhì)巖以輕重稀土分異不明顯和具有Eu正異常為特征;II類榴輝巖以輕稀土富集和輕微的EIl負(fù)異常為特征。研究表明,I類肝/LT變質(zhì)巖的原巖來源于長期虧損的地幔源區(qū),形成于大洋環(huán)境;而II類/伊/LT變質(zhì)巖的原巖形成過程很可能是混入了陸殼物質(zhì),其原巖可能形成于大陸邊緣或洋陸過渡環(huán)境。關(guān)鍵詞:俯沖一增生雜巖;變質(zhì)作用:變形作用IIAbstractThenorthQilianhigll·pressure(HP)/10w-temperature(LDmetamor
4、phicbelt,whichiscomposedmainlyofgreenschists,blueschists,eclngiticrocks。metasedimentswithminorsliceofultramaficrocks,islocatedbetweenQingshuigouandaaijingsiatthemiddlepartofthenorthQilianOrogenicbelts.Itisbelievedtobeade印unitoftheEarlyPalaeozoicaeeretionarywedgeaszoeiatedwithtIlesi
5、tbductionofoceaniucrusLF_,elogiteocculB私lenseswithinblueschistsandphengite-sc№.Itconsistsofgarnet,omphaeite,glaucophane,phengiteandepidote,lawsonitecouldbeobservedinpartssamples.Theeclogitefaciesrockscouldbedividedintothefollowingfivetypes:oclogite,omphaeite·bearinggarnetbluesehist
6、,lawsoniteeelogim,omphaeitite,retrogradedeclogite.Detailedmineralogyandmicrostructureimplythattheeelogitefaciesrocksunderwentmultiplemetamorphismincludinglawsonitebluesehisl/epidotebluesehistfacies,eclogitefacies,post-poakepidoteblueschistfaciesandgreenschist讎醅.Peakpressureconditio
7、nsestimatedfromomphacite—phengite—garnet∞semblageswere2.1-2.6GPaattemperatureof430-570C.SHRIMPU-Pbdatingofzirconyieldsagesof463-489whichreflectaneclogitefaciesmetamorphism..BasedonREEand仃a∞elementanalysesaswellasSr-Ndisotopicdata,wedividedtheeelogitesintotwotypes.TypeIeclngiteischm
8、毗rizedbynearlyflatREEnorma