Paleoproterozoic amphibolite facies retrogression and exhumation of Archean metapelitic granulites in the Southern Marginal Zone of the Limpopo Belt, South Africa

Abstract

Granulite-facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis. The timing of retrogression is key to understanding the geodynamic circumstances of such events. This study has investigated amphibolite-facies retrogression of melt-depleted granulite-facies metapelites from the South Marginal Zone (SMZ) of the Limpopo belt. The granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at ~2.7 Ga, under conditions of ~11 kbar and 875 °C. Peak metamorphic garnet is replaced by cordierite + orthopyroxene symplectites, with cordierite subsequently partially replaced by fine-grained intergrowths of amphibolite-facies assemblages, including rutile. Zr-in-rutile thermometry yielded a temperature range of 731–846 °C for the granulite-facies rutiles and 629–690 °C for rutile in the retrograde assemblage; all rutile generations yielded a youngest U-Pb age of ~2.1 Ga. Some granulite-facies rutiles record older dates (2.55─2.25 Ga) which are non-systematic with regard to crystal size and represent non-reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite-facies metamorphism between ~2.7 and 2.2 Ga. The youngest ~2.1 Ga rutile U-Pb age represents the time of final exhumation-related cooling of the rocks following a discrete Paleoproterozoic amphibolite-facies retrograde event, Coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 °C. The lack of age-size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite-facies retrograde event which is inferred to be not much older. U-Pb dating of zircon from the same rocks confirms that the metapelites underwent granulite-facies metamorphism at 2695 ± 35 Ma. Thus, the metapelites of the SMZ record both an Archean granulite-facies event and a polymetamorphic Paleoproterozoic amphibolite-facies history that culminated in a mid-amphibolite facies peak inducing partial retrogression of the granulites. The Paleoproterozoic amphibolite-facies event is tentatively correlated with mantle melting documented between ~2.3 and 2.1 Ga to produce mafic and intermediate volcanic rocks in the Pretoria Group on the Kaapvaal Craton.