Petrology, Origin and Metamorphic History of Proterozoic-aged Granulites of the Natal Metamorphic Province, Southeastern Africa

Abstract

The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted. M 1 involved syn-D 1 high temperature/low pressure metamorphism (∼4kb and >850 oC) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (∼550 oC and ∼5kb) suggests isobaric cooling after M 1. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc-alkaline magmatic rocks. M 2 involved syn-D 2 dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D 2 tectonism post-dated all lithologies in the region, except for syn- to late-D 2 granitoid plutons, and is interpreted as a transpressional tectonothermal reworking of pre-existing (Proterozoic) crust at ∼1030Ma.