铬尖晶石和石榴石的相变:大陆科学钻探CCSD-PP3孔超镁铁岩超高压变质作用的证据

Transformation of Chromium Spinel and Garnet: Evidence of CCSD-PP3 Ultramafic Rocks Processed UHP Metamorphism

PP3超镁铁岩主要岩石类型有纯橄岩和石榴石橄榄岩，两者为渐变，主要矿物为橄榄石、铬尖晶石、石榴石、单斜辉石和斜方辉石．铬尖晶石的Cr^#[Cr／（Cr＋Mg）×100]从51～89变化，TiO2和MnO2值分别低于0．26％和0．46％．铬尖晶石矿物表现为4期次演化的特点，反映了从岩浆期、榴辉岩相、角闪岩相和绿片岩相演化特征．随着超镁铁岩的演化，铬尖晶石表现为Cr^#不断增大，而Mg^#[Mg×100／（Mg＋Fe^2＋）]不断减少、氧逸度不断增加的过程．PP3铬尖晶石反映了地幔来源，为大陆岩石圈超镁铁岩特征，后期随折返而演化．从石榴石与铬尖晶石相互转变过程看出，PP3超镁铁岩经历了深度加大的过程，超镁铁岩曾经到达100km以上的岩石圈地幔深处．在绿片岩相一绿片角闪岩相变质过程中，铬尖晶石中Cr、Mg和～减少，Fe相对增加，产生富cr尖晶石变质作用样式．晚期剪切变形等次生变化影响了铬尖晶石矿物成分。

The ultramafic rocks in the CCSD-PP3 （China Continental Scientific Drilling） drill-hole consist of dunite and garnet peridotite with a gradual margin between them. The main minerals in the rocks are olivine, chromium spinel, diopside, endiopside, and/or garnet, orthopyroxene, amphibole and phlogopite. Chromium spinels in PP3 ultramafic rocks can be divided into 4 groups with a varied composition, Cr^# （molar 100Cr/（Cr＋Mg）） from 51 to 89, which consists of four evolution stages of the ultramafic rocks. The 4 stages are partial melting, eclogite facies, amphibole facies and greenschist facies. When the Cr^#of chrome spinels increases, the Mg^# （molar 100Mg/（Mg＋Fe^2＋ ）） of chromium spinels decreases, but oxygen fugacity rises. Compositions of the chromium spinels reflect that the rocks originating from the shallow mantle have subducted to a depth of over 100 km and have then been exhumed to the surface. During the subsequent greenschist-facies and amphibole-facies metamorphism, the chromium spinels lost some Cr, Mg and Al, but gained relatively more Fe.