Petrology and geochronology of low-pressure mafic granulites in the Marydale Group, South Africa

Abstract Granulite- and amphibolite-facies metabasites occur within the Archaean Marydale Group (3.0 Ga) along the western edge of the mid-Proterozoic Kheis Tectonic Subprovince (1.8–1.3 Ga) of South Africa. At the northern end of the exposed Marydale Group, the metabasites are infolded with overlying quartzites from which they are separated by a low-angle fault contact. They contain two pyroxenes, hornblende and bytownite, but show widespread retrogression to coronas of almandine and hornblende. Geothermometric data for these assemblages indicate peak equilibration of the two-pyroxene assemblage at 690–760°C, and retrograde equilibration of garnet-hornblende pairs at 600–650°C. Barometric data are more uncertain though an estimate of 3–5 kbar is made from a consideration of hornblende chemistry. Using previously published data, a near-isobaric retrograde P - T path is inferred. RbSr ages of whole-rock hypersthene tonalites and mylonitized granites yield ages of 1353 ± 33 and 1355 ± 20 Ma, respectively, interpreted as the age of isotopic resetting during granulite-facies metamorphism. KAr hornblende ages of 1228 ± 61 and 1070 ± 48 Ma are recorded from fresh and sheared granulite-facies metabasites, respectively. These ages data the P - T path and show that the granulite-facies metamorphism predates the adjacent Namaqua orogeny that reset RbSr systematics at ±1210 Ma.