CORUNDUM + QUARTZ AND MG-STAUROLITE BEARING GRANULITE FROM THE LIMPOPO BELT, SOUTHERN AFRICA: IMPLICATIONS FOR A P-T PATH

Abstract

A new occurrence of the rare corundum + quartz assemblage and magnesian staurolite has been found in a gedrite-garnet rock from the Central Zone of the Neoarchean Limpopo Belt in Zimbabwe. Poikiloblastic garnet in the sample contains numerous inclusions of corundum + quartz ± sillimanite, magnesian staurolite + sapphirine ± orthopyroxene, and sapphirine + sillimanite assemblages, as well as monophase inclusions. Corundum, often containing subhedral to rounded quartz, occurs as subhedral to euhedral inclusions in the garnet. Quartz and corundum occur in direct grain contact with no evidence of a reaction texture. The textures and Fe-Mg ratios of staurolite inclusions and the host garnet suggest a prograde dehydration reaction of St → Grt + Crn + Qtz + H2O to give the corundum + quartz assemblage. Peak conditions of 890-930 °C at 9-10 kbar are obtained from orthopyroxene + sapphirine and garnet + staurolite assemblages. A clockwise P-T path is inferred, with peak conditions being followed by retrograde conditions of 4-6 kbar and 500-570 °C. The presence of unusually magnesian staurolite (Mg / [Fe + Mg] = 0.47-0.53) and corundum + garnet assemblages provides evidence for early high-pressure metamorphism in the Central Zone, possibly close to eclogite facies. The prograde high-pressure event followed by high- to ultrahigh-temperature metamorphism and rapid uplifting of the Limpopo Belt could have occurred as a result of Neoarchean collisional orogeny involving the Zimbabwe and Kaapvaal Cratons. © 2006 Elsevier B.V. All rights reserved.