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

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+Fe2+) ]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-绿片角闪岩相变质过程中,铬尖晶石中Cr、Mg和Al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. 

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

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.