FLUID MINERAL EQUILIBRIA IN PREHNITE PUMPELLYITE TO GREENSCHIST FACIES METABASITES NEAR FLIN-FLON, MANITOBA, CANADA - IMPLICATIONS FOR PETROGENETIC GRIDS

A sequence of regional metamorphic isograds indicating a range from pr ehnite-pumpellyite to lower amphibolite facies was mapped in metabasit es near Flin Flon, Manitoba. The lowest grade rocks contain prehnite pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature-X(CO2) and pressure-tempera ture (constant X(CO2)) diagrams were calculated to quantify the effect s of CO2 in the metamorphic fluid on the stability of prehnite-pumpell yite facies minerals in metabasites containing excess quartz and chlor ite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X(CO2) < 0.002. For X(CO2) > 0.002, epidote + chlorite + c alcite assemblages are stable. Our calculated phase relations are cons istent with regional metamorphism in the Flin Flon area in the presenc e of an H2O-rich fluid and a more CO2-rich fluid in the later fault zo nes. We believe that the potential effects of small amounts of CO2 in the metamorphic fluid should be assessed when considering the pressure -temperature implications of mineral assemblages in low-grade metabasi tes.