RETROGRESSIVE HYDRATION OF CALC-SILICATE XENOLITHS IN THE EASTERN BUSHVELD COMPLEX - EVIDENCE FOR LATE MAGMATIC FLUID MOVEMENT

Two calc-silicate xenoliths in the Upper Zone of the Bushveld complex contain mineral assemblages which permit delineation of the metamorphi c path followed after incorporation of the xenoliths into the magma. P eak metamorphism in these xenoliths occurred at T=1100-1200 degrees C and P <1.5 kbar. Retrograde metamorphism, probably coinciding with the late magmatic stage, is characterized by the breakdown of akermanite to monticellite and wollastonite at 700 degrees C and the growth of ve suvianite from melilite. The latter implies that water-rich fluids (X( CO2) <0.2) were present and probably circulating through the cooling m agmatic pile. In contrast, calc-Silicate xenoliths within the lower zo nes of the Bushveld complex, namely in the Marginal and Critical Zones , also contain melilite, monticellite and additional periclase with on ly rare development of vesuvianite. This suggests that the Upper Zone cumulate pile was much 'wetter' in the late-magmatic stage than the ea rlier-formed Critical and Marginal Zone cumulate piles.