Transformation of Chromium Spinel and Garnet: Evidence of CCSD-PP3 Ultramafic Rocks Processed UHP Metamorphism
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摘要: 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.Abstract: 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.
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Key words:
- garnet peridotite /
- chromium spinel /
- UHP /
- CCSD-PP3 /
- Sulu
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图 1 石榴石橄榄岩(a)和(铬尖晶石) 纯橄榄岩(b)
Fig. 1. Garnet peridotite (a) and chromium spinel peridotite (b)
2A 不同期次的铬尖晶石在岩石中的分布
1.尖晶石呈条带状分布(单偏光); 2.B15R22P1尖晶石与镍黄铁矿(反射光); 3.石榴石与角闪石(单偏光); 4.铬尖晶石裂理(反射光); 5.橄榄石中尖晶石(单偏光); 6.尖晶石中石榴石(单偏光); 7.石榴石边部尖晶石和角闪石(反射光); 8.石榴石中部和边部尖晶石(单偏光)
2A. A Distribution of chromium spinels in different rocks
2B 不同期次的铬尖晶石在岩石中的分布
1.石榴石中尖晶石(SEM); 2.“排骨状”铬尖晶石(SEM); 3.斜方辉石出溶单斜辉石和尖晶石(SEM); 4.角闪石和尖晶石(SEM); 5.尖晶石与石榴石共生(SEM); 6.尖晶石中单斜辉石(SEM); 7.尖晶石的环状结构(SEM); 8.橄榄石外部的尖晶石(SEM)
2B. B Distribution of chromium spinels in different rocks
2C 不同期次的铬尖晶石在岩石中的分布
1.尖晶石的环状结构(SEM); 2.尖晶石的环状结构(SEM); 3.角闪石中尖晶石(SEM); 4.尖晶石边部金云母(单偏光); 5.劈理发育的蛇纹石中尖晶石(SEM); 6.被橄榄石交代的尖晶石(SEM); 7.尖晶石中橄榄石(单偏光); 8.尖晶石中单斜辉石(SEM)
2C. C Distribution of chromium spinels in different rocks
图 4 铬尖晶石Mg#与Fe3+/(Fe3++Fe2+) 关系
Fig. 4. Chromite corelationship diagram of Mg# and Fe3+/(Fe3++Fe2+)
图 5 铬尖晶石成分图解(a)和铬尖晶石Cr2O3与斜方辉石Al2O3关系(b)
图a据Guillot et al., 2001;图b据Kepezhinskas and Defant, 1997
Fig. 5. Composition of chromium spinels (a) and diagram of Cr2O3 of chromium spinels and Al2O3 of orthopyroxene (b)
图 6 橄榄石Mg#和铬尖晶石Cr#图解(Pearce et al., 2000)
Fig. 6. Plot of chromium spinel Cr# against olivine Mg# for the PP3 peridotites
图 7 铬尖晶石Cr、Al和Fe3+含量与铬铁矿比较
Fig. 7. Chromium spinel compositions shown in Cr, Al and Fe3+ ternary diagram
图 8 铬尖晶石矿物Cr#演化变化(据Klemme, 2004修改)
Fig. 8. Evolution of Cr# of PP3 chromium spinels
图 11 PP3铬尖晶石TiO2与Cr#图解(Pearce et al., 2000)
Fig. 11. Diagram of TiO2 and Cr# of PP3 chrome spinel
表 1 铬尖晶石电子探针分析结果
Table 1. Composition of chrome spinels
表 2 32个氧原子时铬尖晶石分子式和参数
Table 2. Molecular formula and parameter of chrome spinels
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