METAMORPHIC EVOLUTION OF NEOPROTEROZOIC MANGANESE FORMATIONS AND THEIR COUNTRY ROCKS AT OTJOSONDU, NAMIBIA

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 embedd ed in iron formations and siliciclastic country rocks. This sequence i s petrographically subdivided into restricted lithotypes which bear sp ecific mineral assemblages and compositions depending on their protoli th type. In pure manganese ores the critical assemblage braunite + hae matite + jacobsite + rhodonite is frequently developed, whereas interl ayered impure silicate ores bear various proportions of spessartine, M n3+-bearing andradite-calderite and andradite garnets, rhodonite, mang anoan aegirine-augite, aegirine, Ba-K-Na-feldspars, barite and rare ki noshitalite. Petrological constraints derived from country rock lithol ogies indicate Peak metamorphic conditions of 660-700 degrees C at est imated pressures of 3.5-4.5 kbar. Numerous Ba-rich pegmatitic veins re stricted to the ore horizons testify to the Production of partial melt s from siliciclastic strata within the manganese formations. They are correlated with peak pressure conditions between 5 and 6 kbar, accompa nying the main deformation event and pre-dating the thermal peak. An e arly H2O-rich generation of fluid inclusions is interpreted as a manif estation of prograde dehydration reactions in the ore horizons. This c aused hydraulic fracturing of the ores and, subsequently, triggered th e formation of partial melts which intruded the fracture planes in sit u. Peak metamorphism then occurred under strain-free conditions allowi ng equilibrium recrystallization of all minerals to develop. Phase rel ationships of manganese oxides and silicates modelled in the system Mn -Fe-Si-O reveal variable chemical compositions of braunites, jacobsite s and haematites depending on their paragenesis. They indicate very re stricted oxygen reservoirs within specific strata of the manganese ore s and eliminate a prominent mass exchange even on a small scale. This is supported by delta(18)O analyses of silicate assemblages which furt her exclude mass transfer between manganese ores and country rocks, an d indicate preservation of the exchange equilibria during cooling. The uplift path of the sequence can be constrained using different decrep itation patterns of H2O fluid inclusions and a syn- to late-metamorphi c CO2-rich fluid inclusion population, which indicate high geothermal gradients of 70 degrees C/km and more. The P-T-D evolution of this hig h-T-low-P metamorphic belt conforms with the palaeotectonic setting of the study area at the southernmost part of the Congo Craton, represen ting the continental buttress colliding with the Kalahari Craton durin g the Pan-African orogeny.