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

Abstract

A fluid-inclusion study in syn-tectonic quartz veins from the Umwindsi Shear Zone in the Harare-Shamva-Bindura greenstone belt (Zimbabwe) indicates the P-V-T-X fluid evolution in relation with the regional deformation. Four fluid types were found: (1) isolated and clustered H 2 O - CO 2 + or -halite inclusions; (2) intergranular planes of H 2 O - CO 2 - CH 4 + or -halite+ or -graphite inclusions; (3) high-salinity H 2 O inclusions, and (4) intergranular planes of low-salinity H 2 O inclusions. Type 2 and type 4 fluid-inclusion planes are oriented parallel and normal to the main trend of the shear zone, and were formed in relation to pluton diapirism and pluton expansion, respectively. Trapping of type 1 and type 2 inclusions occurred under conditions of fluid-fluid 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 oxygen fugacity (f O2 ), and X H2O of the initial homogeneous fluid (i.e., before unmixing). The calculations indicate a log 10 f O2 of -19.2 approximately -20.8 (i.e., QFM-0.7 approximately QFM-1) at a temperature and pressure of 600 degrees -650 degrees C and 4-6 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 retrograde cooling-decompression path.