Fluid inclusions in shear zones: The case of the Umwindsi shear zone in the Harare-Shamva-Bindura greenstone belt, NE Zimbabwe

A fluid-inclusion study in syn-tectonic quartz veins from the Umwindsi Shea r Zone in the Harare-Shamva-Bindura greenstone belt (Zimbabwe) indicates th e P-V-T-X fluid evolution in relation with the regional deformation. Four f luid types were found: (1) isolated and clustered H2O-CO2 +/- halite inclus ions; (2) intergranular planes of H2O-CO2-CH4, +/- halite +/- graphite incl usions; (3) high-salinity H2O inclusions, and (4) intergranular planes of l ow-salinity H2O inclusions. Type 2 and type 4 fluid-inclusion planes are or iented parallel and normal to the main trend of the shear zone, and were fo rmed in relation to pluton diapirism and pluton expansion, respectively. Tr apping of type 1 and type 2 inclusions occurred under conditions of fluid-f luid immiscibility at 600 degrees-650 degrees C, and 4-6 kbar. The X-CO2/(X -CO2 + X-CH4) ratio of the type 2 inclusions was used to calculate the oxyg en fugacity (f(O2)), and X-H2O of the initial homogeneous fluid (i.e., befo re unmixing). The calculations indicate a log(10)f(O2) Of -19.2 similar to -20.8 (i.e., QFM-0.7 similar to QFM-1) at a temperature and pressure of 600 degrees similar to 650 degrees C and 4 similar to 6 kbar, respectively. X- H2O Of the initial homogeneous aqueous-carbonic fluid was at least 0.9. Re- equilibration of type 1 and 2 inclusions to lower densities implies a retro grade cooling-decompression path.