Mantle-derived immiscible characteristics for fluid inclusions in Special xenoliths from Cenozoic alkali-rich porphyry in west Yunnan

摘要

There is a series of Cenozoic alkali-rich porphyries along Jinshajiang river-Ailaoshan fracture belt in western Yunnan province. Quartzic calcite peritecticum, pegmatitic xenolith of quartz-albite, pure quartz peritecticum that includes glass inclusions and iron-rich melt xenoliths, associated with Mafic-ultramafic deep xenoliths, have been discovered in the alkali-rich porphyries. Fluid inclusion studies show that the generation of pure quartz peritecticum, Quartzic calcite peritecticum and pegmatitic xenolith of quartz-albite is bounded with immiscibility, which is caused by continuous decompressing of CO_2- rich fluid. The paragenetic relationship, which glass inclusions were associated with H_2O-NaCl inclusions and CO_2 inclusions, indicates that inclusions have been trapped with immiscibility of melt and fluid. Geochemistry of REE and trace element research exhibites that fluid came from enriched mantle. Reversed abnormality of Zr、 Hf in pure quartz peritecticum and iron-rich melt xenoliths showed the special peritecticum (xenoliths) are products which mantle fluid (melt or fluid) generated through immiscibility of decompression in upwelling. Meanwhile, it indicates a potential truth, which melt transforms to fluid for mantle fluid which was upwelling to crust.