Constraints on the form of the P- T loop in the Southern Marginal Zone of the Limpopo Belt, South Africa

Abstract

A generalised, clockwise P- T path, consisting of a prograde portion, a high-grade isothermal decompression portion and an essentially isobaric cooling portion, has been suggested for the metamorphic evolution of the Southern Marginal Zone. Previous studies have concluded that the high-grade decompression path started in the kyanite stability field; peak-metamorphic mineral compositions have been reset during the equilibrium decompression event; and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field. The present petrographic study from the Bandelierkop Quarry, a locality in the granulite zone of the Southern Marginal Zone, identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700°C to 850°C. During this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite. The maximum temperature melting event is closely followed by two divariant reactions, (Ga + Qz + Sil = Cord and Ga + Qz = Cord + Hy), which occur in response to a period of general isothermal decompression. A subsequent cooling dominated rehydration period, is indicated by a crystallisation-hydration reaction (Cord + Ksp + Melt = Bi + Ky + Qz) which occurred at a minimum temperature of approximately 640°C and pressure greater than 6.5 kbar. The present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion. Additional mineral chemistry data, relating to the second decompression reaction, indicate that the zonation profiles of cordierite and garnet, as well as the growth of small quantities of garnet after the reaction, are in strong support of the suggested P- T path. The present mineral compositions in the Southern Marginal Zone reflect a near total re-equilibration during the high-grade, cooling dominated portion of the P- T path and as a result, reflect P- T conditions considerably lower than those of the peak metamorphism.