地幔流体的交代作用——来自碱性正长岩及其深源岩石包体的证据

滇西玉龙县小桥头硅化霓辉正长斑岩中,含有较多镁铁-超镁铁质深源包体岩石。经岩相学和电子探针及扫描电镜分析发现,伴随交代蚀变,寄主岩和各类镁铁-超镁铁质包体岩石中,普遍发育沿粒间和矿物晶体裂隙或解理纹贯入或穿插的黑色不透明物质,主要由微晶硅酸盐矿物和磁铁矿组成。本文研究认为,硅酸盐矿物与磁铁矿在背散射电子图像中表现为熔离特征,这种在透光显微镜下呈黑色不透明的微晶固体,是引发交代蚀变、具熔浆流体特点和超临界流体性质的地幔流体交代作用的一种微观表现。

Mantle fluid metasomatism: evidence from alkali syenite and its deep xenoliths

There are many deep xenoliths in silicified aegirine syenite porphyry of Xiaoqiaotou area,western Yunnan Province.A petrographic analysis combined with electron microprobe analysis and SEM shows that black opaque matters,whose composition is mostly of microcrystalline silicate and magnetite,are widely found running reticulately through mineral particles,crystals and cleavage cracks in association with metasomatic alteration in the mafic-ultramafic xenoliths and their host rocks.It is concluded that microcrystalline silicate and magnetite show characteristics of liquation in the back-scattered electron image.Such a microcrystalline solid,which assumes black opaque state under transmitted-light microscope and might have resulted in metasomatic alteration,is one of the direct microcosmic manifestations of mantle fluid metasomatism with characteristics of melted and supercritical fluids.