Ultrahigh-Temperature Metamorphism and Multistage Evolution of Garnet-Orthopyroxene Granulites from the Proterozoic Epupa Complex, NW Namibia

Abstract

Migmatitic semipelitic granulites of the Proterozoic Epupa Complex, NW Namibia, underwent ultrahigh-temperature metamorphism as is indicated by the high alumina contents of orthopyroxene (811 wt % Al 2 O 3 ) coexisting with garnet. Peak PT conditions of $970 C and $95 kbar are calculated from conventional garnetorthopyroxene geothermobarometry. Conspicuous reaction textures document a multistage retrograde upliftcooling path: post-peak decompression initially under still ultrahigh temperatures ($940 C and $8 kbar) is recorded by coronas of aluminous orthopyroxene plagioclase around garnet. Continued decompression ($6 kbar and $800 C) 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 ($660 C and $5 kbar). Rims of low-alumina orthopyroxene around retrograde biotite point to renewed decompression subsequent to cooling. The deduced clockwise retrograde PT 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.