The Petrogenesis of Some Migmatites and Granites (Central Damara Orogen, Namibia): Evidence for Disequilibrium Melting, Wall-Rock Contamination and Crystal Fractionation

Abstract

The Oetmoed Granite–Migmatite Complex (OGMC), central Damara orogen, Namibia, consists of restite-rich, grt- and crd-bearing S-type granites and grt–crd–sil–Kfs-bearing metasediments, stromatic migmatites and nebulites. Both types of migmatites formed by limited in situ partial melting of metapelites under H2O-saturated conditions at ∼700°C and 5 kbar. Melanosomes of the stromatic migmatites do not resemble true residues, instead they more probably represent reaction zones between in situ melt and the metasedimentary host rock. Leucosomes of the stromatic migmatites have LREE- and HFSE-depleted disequilibrium compositions, typical of low-melt fractions generally observed in migmatite terranes. Similar δ18O values in the melanosomes and leucosomes suggest that partial melting occurred under fluid-present conditions. Nebulites are more residual than melanosomes and metasediments, indicating that separation of melt and residue must have occurred. Cordierite- and grt-bearing xenoliths in the granites do not represent residue from the site of origin of the intrusive granites; their depleted chemical composition is best explained by extensive degrees of partial melting of incorporated country rocks. Chemical variations among the grt- and crd-bearing granites are explained by fractional crystallization processes and xenolith entrainment. Major and trace element data and high δ18O values suggest that the grt- and crd-bearing granites were derived from H2O-undersaturated melting of metapelitic rocks.