The petrogenesis of some migmatites and granites (central Damara Orogen, Namibia): Evidence for disequilibrium melting, wall-rock contamination and crystal fractionation

The Oetmoed Granite-Migmatite Complex (OGMC), central Damara orogen, Namibi a; consists of restite-rich, grt- and crd-bearing S-type granites and grt-c rd-sil-Kfs-bearing metasediments, stromatic migmatites and nebulites. Both types of migmatites formed by limited in situ partial melting of metapelite s under H2O-saturated conditions at similar to 700 degrees C and 5 kbar. Me lanosomes of the stromatic migmatites do not resemble true residues, instea d they more probably represent reaction zones between in situ melt and the metasedimentary host lack. Leucosomes of the stromatic migmatites have LREE - and HFSE-depleted disequilibrium compositions, typical of low-melt fracti ons generally observed in migmatite terranes. Similar delta(18)O values in the melanosomes and leucosomes suggest that partial melting occurred under fluid-present conditions. Nebulites are mon residual than melanosomes and m etasediments, indicating that separation of melt and residue must have occu rred. Cordierite- and grt-bearing xenoliths in the granites do not represen t residue from the site of origin of the intrusive granites; their depleted chemical composition is best explained by extensive degrees of partial mel ting of incorporated county rocks. Chemical variations among the grt- and c rd-bearing granites are explained by fractional crystallization processes a nd xenolith entrainment. Major and trace element data and high delta(18)O v alues suggest that the grt- and crd-bearing granites were derived from H2O- undersaturated melting of metapelitic rocks.