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

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原有的地球化学特征,它可能是由于在俯冲过程中受到陆壳物质的污染,或壳-幔相互作用所致。据该孔变质相中缺乏Sp相,而以Gt为标志的变质相的事实,推断其形成的压力条件应>7Gpa,即形成的深度应大于200km。上述研究表明在苏鲁UHP变质带中,不仅有来自大陆地幔楔中的地幔残余的UHP变质体,而且首次提出有可能来自大陆俯冲前锋具大洋岩石圈地幔性质的(蛇绿岩型地幔残片)变质体存在,这对揭示该区UHP变质带的形成和演化过程提供了新的信息。

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, one is harzburgite-lherzolite from continental mantle wedge the other is dunite-wehrlite-pyroxenite deriving from magmatism. The garnet peridotite is characterized by highly inhomogeneous composition, extreme enrichment in REE （total REE abundances 20ppm） and LIL. This does not reflecting the geochemical features of their protolith, which iscontaminated by continental crust during subduction, or crust-mantle interaction. According to the fact that the rock from PP1 hole lacks of spinel phase, and symbolizes with garnet phase, we can conclude that the rock is formed at pressures of more than 7GPa, over 200km in depth. From the above results, it shows that in the Sulu UHP metamorphic belts, there exist metamorphic teranes not only from continental mantle wedge, but also from ophiolitic mantle as the front of continental subduction slab. This research provides new information for revealing the formation and evolution of the UHP metamorphic belt.