『Abstract
Manganiferous chemical sediments of Neoproterozoic age in Namibia
were subjected to high-Tlow-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 abdradite-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 kbars. 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 strain-free 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.
Key Words: manganiferous sediments; Damara Orogeny; Namibia; metamorphism;
oxygen isotopes; fluid inclusions』
Introduction
Geological framework
Analytical techniques
Petrography of ore body and country rocks
Individual minerals
Braunite
Mn-Fe spinels and hausmannite
Bixbyite
Haematite
Garnets
Pyroxenes
Pyroxenoids
Amphiboles
Micas
Feldspars
Constraints of the metamorphic evolution
Phase relationships
Oxygen isotope distribution
Fluid inclusion studies
Microtextures: the timing of deformation and mineral growth
Synthesis: the P-T-D evolution
Discussion and conclusions
Acknowledgments
References