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Ultrahigh-pressure metamorphism in the forbidden zone:the Xugou garnet peridotite,Sulu terrane,eastern China

【摘要】:正The Xugou garnet peridotite body of the southern Sulu ultrahigh-pressure(UHP) terrane is enclosed in felsic gneiss,bounded by faults,and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite.The peridotite is composed of variable amounts of olivine(Fo_(91),enstatite(En_(92-93)),garnet (Alm_(20-23)Prp_(53-58)Knr_(6-9)Grs_(12-18)),diopside and rare chromite.The ultramafic protolith has a depleted residual mantle composition,indicated by a high-Mg number,very low CaO,Al_2O_3 and total REE contents compared to primary mantle and other Sulu peridotites.Most garnet(Prp_(44-58)) clinopyroxe-nites are foliated.Except for rare kyanite-bearing eclogitic bands,most eclogites contain a simple assemblage of garnet(Alm_(29-34)Prp_(32-50)Grs_(15-39)) + omphacite(Jd_(24-36)) + minor rutile.Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns,respectively,but both have flat HREE patterns.Normalized La,Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-pressure crystal accumulation(+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source. Recrystallized textures and P-T estimates of 780-870℃,5-7 GPa and a metamorphic age of 231±11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamor-phism in the P-T forbidden zone with an extremely low thermal gradient(5℃km~(-1)),and multistage retrograde recrystallization during exhumation.Develop of prehnite veins in clinopyroxenite,eclogite, felsic blocks and country rock gneiss,and replacements of eclogitic minerals by prehnite,albite,white mica,and K-feldspar indicate low-temperature metasomatism.

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1 ;Ultrahigh-pressure metamorphism in the forbidden zone:the Xugou garnet peridotite,Sulu terrane,eastern China[A];中国科学院地质与地球物理研究所二○○三学术论文汇编·第一卷(地球动力学)[C];2003年
2 BOR-MING JAHN;;Partial equilibrium of radiogenic and stable isotope systems in garnet peridotite during ultrahigh-pressure metamorphism[A];中国科学院地质与地球物理研究所二○○三学术论文汇编·第一卷(地球动力学)[C];2003年
3 ;Petrography,mineral chemistry and shock metamorphism of Yamato 984028 lherzolitic shcrgottite[A];中国科学院地质与地球物理研究所第11届(2011年度)学术年会论文集(上)[C];2012年
4 MASAKI ENAMI;;Chromian dissakisite-(Ce) in a garnet lherzolite from the Chinese Su-Lu UHP metamorphic terrane:Implications for Cr incorporation in epidote-group minerals and recycling of REE into the Earth's mantle[A];中国科学院地质与地球物理研究所二○○三学术论文汇编·第一卷(地球动力学)[C];2003年
5 ;Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen,China:evidence from mineral Nd-Sr-O isotopic composition[A];中国科学院地质与地球物理研究所第十届(2010年度)学术年会论文集(中)[C];2011年
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