Ultrahigh-temperature metamorphism and multistage evolution of garnet-orthopyroxene granulites from the Proterozoic Epupa Complex, NW Namibia

Migmatitic semipelitic granulites of the Proterozoic Epupa Complex, NW Namibia, underwent ultrahigh-temperature metamorphism as is indicated by the high alumina contents of orthopyroxene (8-11 wt % Al2O3) coexisting with garnet. Peak P-T conditions of similar to970degreesC and similar to9.5 kbar are calculated from conventional garnet-orthopyroxene geothermobarometry. Conspicuous reaction textures document a multistage retrograde uplift-cooling path: post-peak decompression initially under still ultrahigh temperatures (similar to940degreesC and similar to8 kbar) is recorded by coronas of aluminous orthopyroxene + plagioclase around garnet. Continued decompression (similar to6 kbar and similar to800degreesC) is evident from subsequently formed symplectites of cordierite + lower-alumina orthopyroxene and cordierite + lower-alumina orthopyroxene + spinel, both replacing garnet. Subsequent regrowth of garnet and biotite, mainly formed at the expense of the symplectitic phases, presumably reflects back-reactions with crystallizing melts during near-isobaric cooling to upper amphibolite-facies conditions (similar to660degreesC and similar to5 kbar). Rims of low-alumina orthopyroxene around retrograde biotite point to renewed decompression subsequent to cooling. The deduced clockwise retrograde P-T path reflects the thinning and later cooling of former thickened lower crust. Because of the limited structural and geochronological data it remains uncertain whether initial ultrahigh-temperature metamorphism was induced by a collision event or by crustal extension.