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

Manganiferous chemical sediments of Neoproterozoic age in Namibiawere subjected to high-T–low-P metamorphism during theDamara Orogeny and display unique phase assemblages. The manganeseformations are embedded in iron formations and siliciclasticcountry rocks. This sequence is petrographically subdividedinto restricted lithotypes which bear specific mineral assemblagesand compositions depending on their protolith type. In puremanganese ores the critical assemblage braunite + haematite+ jacobsite + rhodonite is frequently developed, whereas interlayeredimpure 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 derivedfrom country rock lithologies indicate peak metamorphic conditionsof 660–700C at estimated pressures of 35–45 kbar.Numerous Ba-rich pegmatitic veins restricted to the ore horizonstestify to the production of partial melts from siliciclasticstrata within the manganese formations. They are correlatedwith peak pressure conditions between 5 and 6 kbar, accompanyingthe main deformation event and pre-dating the thermal peak.An early H₂O-rich generation of fluid inclusions is interpretedas a manifestation of prograde dehydration reactions in theore horizons. This caused hydraulic fracturing of the ores and,subsequently, triggered the formation of partial melts whichintruded the fracture planes in situ. Peak metamorphism thenoccurred under strainfree conditions allowing equilibrium recrystallizationof all minerals to develop. Phase relationships of manganeseoxides and silicates modelled in the system Mn–Fe–Si–Oreveal variable chemical compositions of braunites, jacobsitesand haematites depending on their paragenesis. They indicatevery restricted oxygen reservoirs within specific strata ofthe manganese ores and eliminate a prominent mass exchange evenon a small scale. This is supported by ¹⁸O analyses of silicateassemblages which further exclude mass transfer between manganeseores and country rocks, and indicate preservation of the exchangeequilibria during cooling. The uplift path of the sequence canbe constrained using different decrepitation patterns of H₂Ofluid inclusions and a syn-to late-metamorphic CO₂-rich fluidinclusion population, which indicate high geothermal gradientsof 70C/km and more. The P–T–D evolution of thishigh-T–low-P metamorphic belt conforms with the palaeotectonicsetting of the study area at the southernmost part of the CongoCraton, representing the continental buttress colliding withthe Kalahari Craton during the Pan-African orogeny. KEY WORDS: manganiferous sediments; Damara Orogeny; Namibia; metamorphism; oxygen isotopes; fluid inclusions ^*Corresponding author. Present address Institut fr Geowissenschaften und Lithosphrenforschung, Senckenbergstrasse 3, D-35390 Giessen, Germany