Thermobarometry and fluid evolution of enderbites within the Magondi Mobile Belt, northern Zimbabwe

Abstract

Metamorphic conditions within arenaceous, calcareous and argillaceous supracrustal rocks of the Magondi Mobile Belt (Zimbabwe) range from greenschist to granulite facies. Within the high-grade segment, basement gneisses of early Proterozoic age and argillaceous rocks of the Mid-Proterozoic Piriwiri Group are intruded by charnockites and enderbites. Metamorphic mineral assemblages and thermobarometric data for enderbitic granulites of Nyaodza show temperatures of 700–800°C and pressures of 5–7 kbar for the peak of granulite-facies metamorphism. Microthermometry and Raman microspectroscopy reveal that CO 2, associated with minor N 2, has been the dominant fluid phase during granulite-facies metamorphism. The chronology of the CO 2 inclusions and the development of microtextures and mineral assemblages in the enderbites indicates that isolated negative crystal shaped CO 2 inclusions in quartz and plagioclase porphyroclasts entrap syn-metamorphic fluids of medium-high densities (0.88–0.90 g/cm 3). Lower density (0.71–0.77 g/cm 3) CO 2 inclusions in trails and clusters within the same minerals were formed from local re-equilibration and re-entrapment of the former (near-) peak granulitic CO 2 inclusions. As in many other granulites, syn-metamorphic CO 2 is associated with intrusives emplaced near the peak of metamorphism.