Geology of the Chewore Inliers, Zimbabwe: constraining the Mesoproterozoic to Palæozoic evolution of the Zambezi Belt

Abstract

Structural, metamorphic and geochronological studies of the Chewore Inliers of the Zambezi Belt within the Karoo age Zambezi Rift, allow recognition of a protracted multi-stage evolution, from the Mesoproterozoic to culminating in the Early. Palaeozoic Pan-African Orogeny. Tectono metamorphic events recognised in the Chewore Inliers occur throughout the Zambezi Belt and alternative models for the history of the Zambezi Belt are presented. Four terranes are recognised in the Chewore Inliers, and contacts between them are observed or inferred to be ductile thrusts, along which juxtaposition of the terranes occurred late in the Pan-African metamorphic cycle ( M 2, at 526 Ma). The oldest portion of the inliers is a metamorphosed sequence of mafic and ultramafic gneisses with an age of 1393 Ma. These constitute what is tentatively called the Ophiolite Terrane, together with closely associated high- P/moderate T schists possibly represents a suture. The other three terranes (Granulite, Zambezi and Quartzite Terranes) experienced a common history of tectonothermal events but show variable degrees of reworking during the latest tectono metamorphic event ( M 2). Concordant granitic orthogneisses were emplaced at 1087 Ma into supracrustal sequences. No Pan-African supracrustals are recognised in the Chewore Inliers, which are wholly basement gneisses and quartzites that have been reworked during successive orogenies including the Pan-African Orogeny. A high- T/low- P metamorphic event ( M 1 of possibly 1068–1071 Ma age, with a minimum age of 943 Ma, was responsible for totally recrystallizing the Granulite Terrane during south to north tectonic transport. M 1 mineral parageneses are only preserved as inclusion phases and overgrown fabrics in the other terranes. These other terranes were pervasively recrystallised at high- P/moderate T conditions accompanying a clockwise P-T path related to northeast over southwest tectonic transport and crustal over-thickening during the Pan-African metamorphic cycle ( M 2) at approximately 526 Ma. Reworking of the Granulite Terrane during M 2 was minor, leaving M 1 fabrics and mineral assemblages preserved with little recrystallization. M 2 orogenesis culminated in the juxtaposition of the terranes, rapid uplift through the thermal peak and eventual slow cooling accompanying a multitude of post-tectonic intrusions; pegmatites at 480 Ma, the Chewore Ultramafic Complex and dolerite dykes. The 830 Ma tectonothermal event involving pervasive syn-tectonic granitic orthogneisses in the south Zambezi Belt is not recognised in the Chewore Inliers, suggesting a localised, possibly extensional, regime restricted to the southern part of the Zambezi Belt at 830 Ma.