High fO2 metasomatism during Whiteschist metamorphism, Zambezi Belt, Northern Zimbabwe

Abstract

The Kadunguri Whiteschists are a group of talc- and kyanite-bearing lithologies that occur in the Chewore Inliers from the Zambezi Belt of northern Zimbabwe. They crop out on the southern margin of the Chewore Ophiolite Terrane, a Mesoproterozoic ophiolite and island arc, as a 5 km × 1.5 km, southeasterly dipping, semi-continuous block, and contain the second known natural occurence of yoderite. Major element analyses define the whiteschists within the relatively simple MFASH system. Major and trace element analyses indicate that the whiteschists originate from the metasomatic alteration of alkalic ocean-island-type metabasalts similar to those in the underlying Ophiolite Terrane. Synmetamorphic or metasomatic mineral parageneses indicate peak P-T conditions of between 13 and 15 kbar at 550-600°C, and the highly oxidizing nature of all reactions indicates the presence of a high fO2 metasomatic fluid. The peak P-T conditions require that this synmetamorphic, exotic metasomatic fluid was available at depths near 55 km. The age of high-pressure metamorphism is constrained within the Pan African tectonothermal cycle at 550-520 Ma. Tectonometamorphism in the Zambezi Belt is related to a period of extensive crustal thickening possibly related to amalgamation of Gondwanaland.