Fluid inclusions in shear zones: The case of the Umwindsi shear zone in the Harare-Shamva-Bindura greenstone belt, NE Zimbabwe
Huizenga, Jan M.; Touret, Jacques L.R.
European Journal of Mineralogy Volume 11 Number 6 (1999), p. 1079 - 1090
published: Nov 29, 1999
manuscript accepted: Jun 4, 1999
manuscript received: May 25, 1999
ArtNo. ESP147051106016, Price: 29.00 €
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 H2O-CO2 ± halite inclusions; (2) intergranular planes of H2O-CO2-CH4 ± halite ± graphite inclusions; (3) high-salinity H2O inclusions, and (4) intergranular planes of low-salinity H2O 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°-650°C, and 4-6 kbar. The XCO2/(XCO2 + XCH4) ratio of the type 2 inclusions was used to calculate the oxygen fugacity (fO2), and XH2O of the initial homogeneous fluid (i.e., before unmixing). The calculations indicate a log10fO2 of-19.2~-20.8 (i.e., QFM-0.7 ~ QFM-1) at a temperature and pressure of 600° ~ 650°C and 4 - 6 kbar, respectively. Xmo 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.