The metamorphic evolution of an ancient accretionary prism in the southern zone of the Damara Belt in Namibia
The Southern Zone of the Damara Belt in central Namibia has an apparent stratigraphic thickness that exceeds 100 km and is comprised of highly strained, metamorphosed clastic sedimentary rocks that are intercalated with slices of metamorphosed basalt and gabbro. One of the only modern geological environments in which such vast amounts of sediments can be accumulated, with intercalated mafic rocks, is in an accretionary prism above a subduction zone, where the accreted material is also subjected to high pressure – low temperature metamorphic conditions and deformation to high strains. This has been suggested as the origin of the Southern Zone. Samples collected from two localities within and immediately adjacent to the Southern Zone include representative rocks of both metapelitic and metamafic compositions. The inferred peak mineral assemblage in the metapelitic rocks consists of weak to moderately zoned garnet, staurolite and in some cases, kyanite porphyroblasts set in a fine-grained matrix of chlorite, biotite, muscovite, paragonite, epidote, ilmenite and quartz. The matrix exhibits a penetrative foliation that is defined by the alignment of the micaceous minerals. The garnet, staurolite and kyanite porphyroblasts overprint this fabric. By contrast, the metamafic rocks are very fine-grained with an inferred peak mineral assemblage of zoned amphibole, epidote, rutile, quartz, biotite and in some cases, chlorite and sphene. The amphibole and micaceous minerals define a distinct fabric. None of the minerals occur as porphyroblasts that overprint this fabric.
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