There are many deep xenoliths in silicified aegirine syenite porphyry ot Xiaoqiaotou area, 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.