东海地区高压-超高压变质带中变质橄榄岩及其原岩和成因类型的判别——以PP1孔和PP3孔为例  

Metamorphic Assemblage and Protolith Discrimination of Meta-Peridotite in the HP-UHP Metamorphic Belt the in the East Sea Area,Eastern China:Evidence from the PP1 and PP3 Holes

作　　者：王希斌[1] 杨经绥[1] 李天福[1] 陈松永[1] 任玉峰[1] 

机构地区：[1]中国地质科学院地质研究所

出　　处：《地质学报》2009年第7期946-963,共18页Acta Geologica Sinica

基　　金：国家"973"项目(编号2003CB716500);中国地质调查局项目(编号2001CCB00900);自然科学基金(编号40300140)的资助的成果

摘　　要：本文在总结全球地幔橄榄岩岩石学和地球化学特征的基础上,首次提出了一个用于判别HP-UHP变质带中变质橄榄岩原岩及其成因类型的判别图解。该图主要由镁铁总量MgO+<FeO>(%)和一个参数m+f/si比值构成。另用Al2O3和CaO分别与MgO+<FeO>(%)制成两个辅助图解,以示方辉橄榄岩和二辉橄榄岩之间在Al2O3和CaO含量上的分界。通过原岩判别结果和研究表明,PP3孔和PP1孔两者在变质组合、原岩成因类型、地球化学和变质条件方面存在一系列的重大差异。分别代表来自两种极端的地球化学类型和两种不同大地构造环境的UHP变质体。PP3钻孔以Ol+Gt+Cpx+Opx+Sp为变质矿物共生组合的含石榴石纯橄岩,其原岩系来自地幔残余成因的方辉橄榄岩遭受UHP变质作用的产物,它以成分高度均一,富Mg(Mg’=92),极端亏损不相容元素REE(∑REE<1×10-6可称为超亏损型)为特征。在变质相中仍保留原岩的残余矿物铬尖晶石(Sp),其成分显示蛇绿岩地幔橄榄岩的成分趋势。并出现以Gt和Sp共存相为特征的变质相。据实验结果(klemme,2004)表明该共存相的稳定域的P-T条件Cr-Sp可达7Gpa,T1400℃,即形成于200km的地幔深度。综合研究显示该孔变质橄榄岩原岩(方辉橄榄岩)具有大洋岩石圈地幔残余成因的某些印记,而不是同深度原生地幔岩相转变的产物。PP1孔变质橄榄岩是由无水矿物相(Ol+Opx+Cpx+Gt)+含水矿物相(Phl±Chu)组成的石榴石橄榄岩杂岩,其原岩来自两种不同成因的超镁铁岩系列:一为具地幔成因的方辉橄榄岩-二辉橄榄岩系列(可能相当于地幔楔中的Al型橄榄岩),另一部分(少数)来自具岩浆成因的超镁铁岩系列(纯橄岩—异剥橄榄岩—辉石岩组合,可能相当于A2型橄榄岩)。该套变质橄榄岩,以成分高度不均一,极端富集REE(∑REE平均>20×10-6可称为超富集型)和大离子亲石元素(K、Ba、Rb)为特征。这种异常现象并不反映其原岩�Based on summing up of petrography and petrochemisty for mantle peridotite all over the world, we firstly propose a diagram for discriminating the protolith of meta-peridotite of HP and UHP metamorphic terrane. This diagram is constituted by total MgO＋〈FeO〉（%） versus （m＋f）/si ratio. Two additional diagrams, consisting of Al2O3 versus MgO＋〈Fe〉（%） respectively, are used to divide the boundary between harzburgite and lherzolite. By discriminating the protolith of meta-peridotite from the PP3 hole and PPlhole, we found a large sum of differences existing in metamorphic mineral assemblage, protolith genesis, geochemistry and metamorphic conditions among them, which represents two extremely differentgeochemical types and distince tectonic environment, respectively. The meta-peridotite from the PP3 hole, being garnet bearing dunite, has assemblage of Ol＋Gr＋Cpx＋Opx＋Sp. Its protolith is the product of residual mantle harzburgite which suffered UHP metamorphism. The garnet bearing dunite is characterized by highly uniform composition, enrichment in MgO （Mg′= 92）, and extreme depletion in incompatible REE （total REE abundances to lower than 1 ppm）. The residual Cr-sp, still preserved in the meta-peridotite, has the compositional features of typical ophiolitic dunite-harzburgite, with coexistence of garnet and spinel. In view of the experiments by Klemme et. Al. （2004）, spinel coexisting with garnet is stable up to 7GPa at 1400℃, which means the meta-peridotite from PP3 hole is formed no more than 200km. Comprehensive studies show that the meta-peridotite （its protolith to be harzburgite possesses theattribution of oceanic lithosphere mantle, but is not the transition phase of the mantle at 200km depth. The meta-peridotite from PP1 hole, being garnet peridotite complex, has assemblage of Ol＋Gr＋Cpx＋Opx anhydrous phase and Phl＋Ti＋Chu hydrous phase. Based on the discrimination of their protolith, they are found to derive from two different genetic ultramafic series,

关 键 词：超高压 变质橄榄岩 变质组合 原岩类型 判别图解 PP1孔 PP3孔 东海 

分 类 号：P581[天文地球—岩石学]