Metamorphic Evolution of Neoproterozoic Manganese Formations and their Country Rocks at Otjosondu, Namibia

Abstract Manganiferous chemical sediments of Neoproterozoic age in Namibia were subjected to high-T–low-P metamorphism during the Damara Orogeny and display unique phase assemblages. The manganese formations are embedded in iron formations and siliciclastic country rocks. This sequence is petrographically subdivided into restricted lithotypes which bear specific mineral assemblages and compositions depending on their protolith type. In pure manganese ores the critical assemblage braunite + haematite + jacobsite + rhodonite is frequently developed, whereas interlayered impure silicate ores bear various proportions of spessartine, Mn3+-bearing andradite–calderite and andradite garnets, rhodonite, manganoan aegirine–augite, aegirine, Ba–K–Na-feldspars, barite and rare kinoshitalite. Petrological constraints derived from country rock lithologies indicate peak metamorphic conditions of 660–700°C at estimated pressures of 3·5–4·5 kbar. Numerous Ba-rich pegmatitic veins restricted to the ore horizons testify to the production of partial melts from siliciclastic strata within the manganese formations. They are correlated with peak pressure conditions between 5 and 6 kbar, accompanying the main deformation event and pre-dating the thermal peak. An early H2O-rich generation of fluid inclusions is interpreted as a manifestation of prograde dehydration reactions in the ore horizons. This caused hydraulic fracturing of the ores and, subsequently, triggered the formation of partial melts which intruded the fracture planes in situ. Peak metamorphism then occurred under strainfree conditions allowing equilibrium recrystallization of all minerals to develop. Phase relationships of manganese oxides and silicates modelled in the system Mn–Fe–Si–O reveal variable chemical compositions of braunites, jacobsites and haematites depending on their paragenesis. They indicate very restricted oxygen reservoirs within specific strata of the manganese ores and eliminate a prominent mass exchange even on a small scale. This is supported by δ18O analyses of silicate assemblages which further exclude mass transfer between manganese ores and country rocks, and indicate preservation of the exchange equilibria during cooling. The uplift path of the sequence can be constrained using different decrepitation patterns of H2O fluid inclusions and a syn-to late-metamorphic CO2-rich fluid inclusion population, which indicate high geothermal gradients of 70°C/km and more. The P–T–D evolution of this high-T–low-P metamorphic belt conforms with the palaeotectonic setting of the study area at the southernmost part of the Congo Craton, representing the continental buttress colliding with the Kalahari Craton during the Pan-African orogeny.