P-T-t evolution and textural evidence for decompression of Pan-African high-pressure granulites, Lurio Belt, north-eastern Mozambique

Pan-African high-pressure granulites occur as boudins and layers in the Lurio Belt in north-eastern Mozambique, eastern Africa. Mafic granulites contain the mineral assemblage garnet + clinopyroxene + plagioclase + quartz +/- magnesiohastingsite. Garnet porphyroblasts are zoned with increasing almandine and spessartine contents and decreasing grossular and pyrope contents from core (Alm(46)Prp(32)Grs(21)Sps(2)) to rim (Alm(52)Prp(26)Grs(19)Sps(3)). This pattern is interpreted as a retrograde diffusion zoning with the preserved core chemistry representing the peak metamorphic composition. Mineral reaction textures occur in the form of monomineralic and composite plagioclase +/- orthopyroxene +/- amphibole +/- biotite +/- magnetite coronas around garnet porphyroblasts. Thermobarometry indicates peak metamorphic conditions of up to 1.57 +/- 0.14 GPa and 949 +/- 92 degrees C (stage I), corresponding to crustal depths of similar to 55 km. Zircon yielded an U-Pb age of 557 +/- 16 Ma, inferred to date crystallization of zircon during peak or immediately post-peak metamorphism. Formation of plagioclase + orthopyroxene-bearing coronas surrounding garnet indicates a near-isothermal decompression of the high-pressure granulites to lower pressure granulite facies conditions (stage II). Development of plagioclase + amphibole-coronas enclosing the same garnet porphyroblasts shows subsequent cooling into amphibolite facies conditions (stage III). Symplectitic textures of the corona assemblages indicate rapid decompression. The high-pressure granulite facies metamorphism of the Lurio Belt, followed by near-isothermal decompression and subsequent cooling, is in accordance with a long-lived tectonic history accompanied by high magmatic activity in the Lurio Belt during the late Neoproterozoic-early Palaeozoic East-African-Antarctic orogeny.