Magmatic loading in the proterozoic Epupa Complex, NW Namibia, as evidenced by ultrahigh-temperature sapphirine-bearing orthopyroxene–sillimanite–quartz granulites

MgAl-rich sapphirine-bearing orthopyroxene–sillimanite–quartz granulites in the Proterozoic Epupa Complex, NW Namibia, occur as restitic schlieren in migmatitic garnet–orthopyroxene granulites. Sapphirine is present as (i) prograde inclusion in peak-metamorphic phases, (ii) peak-metamorphic porphyroblasts and, (iii) in retrograde symplectites. Reaction textures could be used to infer a detailed sequence of mineral reactions. Results from conventional geothermobarometry and P–T pseudosections point to an anticlockwise P–T path that reached ultrahigh-temperature (UHT) conditions. During near isobaric heating biotite was consumed by orthopyroxene-producing melting reactions. Inclusions of cordierite and spinel in the peak phases suggest that heating proceeded at moderate pressures of ca. 5–7 kbar. During subsequent loading orthopyroxene–sillimanite (±quartz (±garnet)) assemblages were formed at peak temperatures of ca. 1000 °C and 9.5 kbar. In addition, porphyroblastic sapphirine was formed in silica-undersaturated very MgAl-rich samples and domains. The inferred ultrahigh-temperatures are consistent with the high Al2O3 content of orthopyroxene (up to 11.9 wt%). During subsequent cooling high-Al orthopyroxene recrystallised to lower Al-orthopyroxene, which coexisted with garnet or sapphirine at ca. 970 °C. Sapphirine-bearing symplectites are interpreted to indicate ultrahigh-temperature near isothermal decompression to P < 6.5 kbar. During continued high-temperature decompression to ca. 5.5 kbar/800 °C sapphirine was replaced by spinel-bearing symplectites. Subsequent re-growth of biotite probably reflects mineral–melt interactions during late cooling.The anticlockwise P–T path and the subsidence of the granulites to crustal levels of ca. 30 km is interpreted to result from the syn-UHT magmatic accretion of anorthositic melts onto the granulite layer (i.e. magmatic loading), which led to heating of the granulites from above.