Abstract
Ph.D.
Preserved Archean crust dominantly recording lower temperature conditions (greenschist to amphibolites facies), the earliest widespread record of ultrahigh- temperature metamorphism occur in the Neoarchean. Considering that, collisional tectonic setting has been postulated as a possible tectonic scenario for the generation of ultrahigh-temperature metamorphism, sites where Archean cratons underwent collision can be potential sites for preservation of ultrahigh-temperature metamorphic granulites. The Limpopo Complex is a high-grade metamorphic terrain considered to have formed by collision in Neoarchean time between the Archean Kaapvaal and Zimbabwe cratons.Detailed petrographic and mineral chemical characterization of representative high Mg-Al granulites from the Southern Marginal Zone, Central Zone and the Northern Marginal Zone – forming the three subzones of the Limpopo Complex – was carried out. Evidence for the preservation of mineral assemblages considered diagnostic of ultrahigh- temperature metamorphic conditions, such as orthopyroxene+sillimanite±quartz, high-Al/(MgTs) orthopyroxene, sapphirine+quartz, spinel+quartz, corundum+quartz and antiperthite, are shown from these high Mg-Al granulites. Most of these mineral assemblages are reported for the first time from the Limpopo Complex. In addition, two unique textures are also reported – one, the discovery of corundum lamellar intergrowth with orthopyroxene from a high Mg-Al granulite from the Southern Marginal Zone, and second, the rare occurrence of sapphirine+quartz post dating orthopyroxene+sillimanite±quartz from two Mg-Al granulites from the Central Zone. Pressure-temperature calculations including representative P-T phase diagrams computed for the bulk compositions of the granulites studied clearly indicate ultrahigh- temperature conditions for all the three subzones. In contrast to two previous studies, one each for the Southern Marginal Zone (~950°C) and the Central Zone (~930°C), this study presents higher temperature estimates of ~1050 to ~1100°C for the three subzones. Together with examples of ultrahigh-temperature metamorphic conditions reported by the two previous studies, this study shows that the ultrahigh-temperature event reported here has affected the length and breadth of the three subzones of the Limpopo Complex. Further, the high-pressure conditions inferred from the early composition of orthopyroxene from the unique orthopyroxene-corundum intergrowth and the P-T phase diagrams computed for representative granulites from the three zones suggest a common high pressure event in all the three sub zones of the Limpopo Complex.