Undex mineral: zoned phlogopite for evolution of carbonatitic magma: an example from the Xueye carbonatite in western Shandong.

The composition of phlogopite presented as phenocrysts and groundmass in Xueye carbonatite was determined by electron microprobe analysis. The cores of the phenocrysts are all biotite with their Mg/(Mg+Fe) ratios less than 0.66, the compositions of the mantle and rim of the phenocrysts are similar to those of phlogopites occurred as groundmass. The abrupt change of compositon between core and mantle and the resorption of core implied that the core and the outer parts of phenocrysts were not crystallized from the same carbonatitic magma, the core biotite is likely exotic biotite entrapped during the ascending of the carbonatitic magma. The resemblance in composition of the mantle and rims of the phenocrysts and the phlogopite in groundmass indicates that the outer part of the phenocrysts and the groundmass were crystallized from carbonatitic magma. In phenocryst phlogopite, Mg/(Mg + Fe) ratio and TiO2 content gradually reduced from mantle to rim which reflected the crystallizing process and the decreasing of CO2/H2O ratio in the magma caused by degassing. The gradually decreasing of Al2O3 from mantle to rim indicated no Al-rich wallrock added into the carbonatitic magma.