西藏雅鲁藏布江缝合带泽当地幔橄榄岩的矿物化学和铂族元素特征

Mineralogy and PGE features of Zedang peridotites in eastern Yarlung Zangbo suture, Tibet

泽当蛇绿岩位于雅鲁藏布江缝合带东段,由地幔橄榄岩、辉长辉绿岩、火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量的铬铁矿化方辉橄榄岩和透镜状纯橄岩。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石。斜方辉石为顽火辉石,En端员组分变化于87.8~90.3。单斜辉石En组分变化于44.1~50.0,主要为顽透辉石和透辉石。二辉橄榄岩与方辉橄榄岩铬尖晶石的Cr#为17.0~31.8,为富铝型尖晶石。泽当地幔橄榄岩PGE总量为16.67×10-9~32.59×10-9,与原始地幔相似。矿物化学特征显示泽当二辉橄榄岩属于深海型地幔橄榄岩,方辉橄榄岩属于弧前地幔橄榄岩。尖晶石Cr#、橄榄石Mg#的变化以及高Os含量（3.50×10-9~7.75×10-9）表明泽当地幔橄榄岩经历了部分熔融过程;正斜率的PGE配分模式以及较高的Pd/Ir值（1.09~3.94）表明泽当地幔橄榄岩受到了俯冲带环境下地幔交代作用的改造。泽当地幔橄榄岩矿物学特征与铂族元素地球化学特征显示其形成于MOR环境,后受到SSZ环境的改造。

The Zedang mafic-ultramafic body crops out in the eastern Yarlung Zangbo suture zone of Tibet. This massif consists of mantle peridotite, gabbro, diabase and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite and minor harzburgite with chromite. A dike-like bodies of dunite are also present. The Fo values of olivine vary in the range of 89.62-91.84 and all olivines belong to forsterite. The orthopyroxenes are enstatite, and the En values vary from 87.82 to 90.32. Clinopyroxenes are mainly endiopside and diopside, and the En values of clinopyroxene vary in the range of 44.12- 50.02. The spinels have Cr#values from 17.00 to 93.59. The aluminum- rich spinels are of harzburgite and lherzolite. The PGE content of Zedang peridotites（16.67 × 10- 9- 32.59 × 10- 9） is similar to that of primary mantle. The mineral- chemistry indicates that the lherzolite is abyssal peridotite and the harzburgite is fore-arc peridotite. The Cr#of spinel, Mg#of olivine and high Os（3.50×10-9-7.75×10-9）imply that Zedang peridotites were formed by partial- melting. The positive slope of PGE patterns and high Pd/Ir are consistent with the opinion that the Zedang peridotites were influenced by melts/fluids which were enriched with PPGE. The mineralogy and PGE geochemistry of the Zedang peridotites suggest that they formed originally at a mid-ocean ridge（MOR） and were later modified by suprasubduction zone（SSZ）melts.