Constraints on the P-T conditions of high-pressure metamorphic rocks from the Inyoni shear zone in the mid-Archean Barberton Greenstone Belt, South Africa

The metamorphic P-T conditions and processes of the mid-Archean Barberton Greenstone Belt (BGB), South Africa were examined with petrological and thermodynamic data from metamorphosed banded iron formation (BIF) and garnet amphibolite in the Inyoni shear zone (ISZ). Equilibrium mineral assemblages in these two rocks range from Grt + Cpx + Gru + Mag + Qz through Grt + Cpx + Gru + Amp + Mag + Qz to Amp + Gru + Mag + Qz, and from Grt + Cpx + Amp + Pl + Ep + Qz to Cpx + Amp + Pl + Ep + Qz, respectively. The metamorphic P-T conditions of these assemblages were estimated from thermodynamic calculations combined with their whole-rock and constituent mineral compositions. The results show that the metamorphosed BIF and garnet amphibolite samples record three and two different metamorphic P-T conditions, respectively, within a single period of metamorphism. The P-T conditions of the BIF changed from ca. 600-680 °C and >10 kbar through ca. 510-540 °C and 8-11 kbar to less than 500 °C and 4 kbar. The P-T conditions of the garnet amphibolite changed from ca. 9.5-10.5 kbar and 620-660 °C to less than 5 kbar and 500 °C. These new results combined with previously published data suggest that the geothermal gradients of the peak and retrograde metamorphisms were ca. 15-20 °C/km and 20-30 °C/km, respectively, and the possibility that the ISZ underwent slightly earlier prograde metamorphism than the high-pressure BGB. The difference between the gradients of 15-20 °C/km and 20-30 °C/km in the ISZ rocks was probably related to a shift from oceanic plate subduction to continental collision at ca. 3.23 Ga.