Melt- to Shear-Controlled Exhumation of Granulites in Granite–Gneiss Domes: Petrological Perspectives from Metapelite of the Neoarchean Ha-Tshanzi Structure, Central Zone, Limpopo Complex, South Africa

Abstract

Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean-Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810-830°C) exhumation from pressure of 10.0 – 10.2 kbar to 7.5-7.0 kbar during the Neoarchean event (2.68 – 2.62 Ga). The exhumation mediated by anatectic melt supports interpretation of the Ha-Tshanzi structure as a diapir-related granite-gneiss dome. During upwelling of the dome, the melt segregated into leucosome, while growth of cordierite sequestered water from the melt assisting to its crystallization at the end of the sub-isothermal decompression stage. As the rheology of the rock changed, melt-dominated deformation was transformed to solid-dominated ductile shear deformation. In contrast to earlier sub-isothermal decompression P-T path, the gentler slope ~75°/kbar of the decompression-cooling path marks the exhumation from pressures ~ 7 kbar down to pressures 5 - 4.5 kbar and cooling down to 600-550°C. Dating of zircon, monazite and rutile shows that the Neoarchean evolution of the metapelite was strongly overprinted by the Paleoproterozoic event at ca. 2.01 Ga. The results of the study highlight the significance of domical structures related to granitic diapirs in the exhumation of the Central Zone of the Limpopo Complex.