冀北赤城蛇纹石化橄榄岩是否为太古宙蛇绿岩残片?  

Is the serpentinized peridotitein Chicheng,northern Hebei,a fragment of Archaean ophiolite?

作　　者：刘卉 张宏福[1,2] LIU Hui;ZHANG HongFu(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;School of Earth Sciences,Zhejiang University,Hangzhou 310058,China)

机构地区：[1]中国科学院地质与地球物理研究所,岩石圈演化国家重点实验室,北京100029 [2]浙江大学地球科学学院,杭州310058

出　　处：《岩石学报》2022年第12期3760-3770,共11页Acta Petrologica Sinica

基　　金：国家青年科学基金项目(42103042)资助.

摘　　要：华北克拉通中部带是一条形成于古元古代之前的构造碰撞拼合带。在中部带的北段赤城地区,出露一套特殊的角闪斜长片麻岩、退变榴辉岩以及蛇纹石化橄榄岩组合,其成因众说纷纭。这套岩石组合归属于红旗营子群,是前寒武纪结晶基底,其中蛇纹石化橄榄岩和退变榴辉岩以构造团块的形式包裹在片麻岩之中。赤城蛇纹石化橄榄岩由纯橄岩、方辉橄榄岩和超镁铁质堆晶岩组成。其中纯橄岩和方辉橄榄岩的全岩主量元素都具有高Mg低Si的特征,大部分方辉橄榄岩中的橄榄石具有很高的Fo值(91.0~92.2),尖晶石具有高的Cr#值(53.9~85.9)。这些特征表明方辉橄榄岩和纯橄岩都经历了高程度部分熔融,它们很可能代表残余的大洋岩石圈地幔,并且可与典型的蛇绿岩套橄榄岩剖面相对比。超镁铁质堆晶岩表现出饱满的岩石特征,代表残余地幔之上的岩浆早期堆晶。橄榄岩Re亏损模式年龄表示,赤城橄榄岩的部分熔融作用发生在~2.5Ga之前。赤城红旗营子群角闪斜长片麻岩和退变榴辉岩的锆石U-Pb年代学揭示其原岩形成于~2.5Ga,经历了~1.85Ga的峰期变质作用和300Ma的退变质作用叠加。角闪斜长片麻岩和退变榴辉岩的原岩分别为大陆火山岩和大洋拉斑质岩石。因此,红旗营子群的橄榄岩、退变榴辉岩和角闪斜长片麻岩这一套岩石组合属于典型的蛇绿岩剖面,它很可能代表了华北克拉通东部和西部陆块之间曾经存在的大洋岩石圈。赤城蛇纹石化橄榄岩属于新太古-古元古代蛇绿岩残片,它可能记录了古大洋的消亡和中部造山带的形成,指示早期板块构造在新太古代已经启动。The Trans-North China Orogen(TNCO)is an accretive orogenic belt formed by collision before the Paleoproterozoic.A suit of special rocks exposed in Chicheng area along the northern margin of the TNCO,which was composed of amphibole plagioclase gneisses,minor retrograde eclogites and serpentinized peridotites,has a debatable origin.This suit of Precambrian basement rocks is attributed to the Hongqiyingzi Group,and the retrograde eclogites and serpentinized peridotites occur as discrete tectonic lenses or boudins within amphibole plagioclase gneisses.The Chicheng peridotites are categorized into harzburgite,dunite and ultramafic cumulate.The harzburgite and dunite are all characterized by high whole-rock MgO and low SiO_(2) contents.Most harzburgites have olivines with high Fo values(91.0~92.2)and spinels with high Cr^(#) values(53.9~85.9).These features indicate that dunites and harzburgites are highly refractory after a large amount of melt extraction,representing oceanic lithosphere mantle,considered as an analogue to typical ophiolitic mantle section.The ultramafic cumulates show fertile chemical composition,thus are likely to represent early ultramafic cumulate phase crystallizing and gravitationally settling to the bottom of the lower crust,above the refractory mantle.The Re-depleted model ages of dunites and harzburgites suggest that the partial melting could occur before~2.5Ga.The zircon U-Pb geochronology of the amphibole plagioclase gneisses and retrograded eclogites from the Hongqiyingzi Group reveals that their protoliths were formed at~2.5Ga,and then experienced metamorphic events at~1.85Ga and 300Ma.The protoliths of the amphibole plagioclase gneisses and retrograded eclogites are volcanic rocks and mafic oceanic basalts,respectively.Thus,the suit of serpentinized peridotites,retrograde eclogites and gneisses is remarkably similar to the sequence of typical ophiolite model,and could represent a missing ocean between the Eastern and Western blocks of the North China Craton.In other words,Chicheng serpen

关 键 词：蛇绿岩 橄榄岩 红旗营子群 中部造山带 华北克拉通 

分 类 号：P541[天文地球—构造地质学] P588.125[天文地球—地质学]