Structural evolution of the Chewore inliers, Zambezi mobile belt, Zimbabwe

Abstract

Detailed mapping of the Chewore Inliers, Zambezi valley, Zimbabwe has recognized three distinct geological terranes. The areally most significant “gneissic terrane” is dominated by amphibolite facies quartzofeldspathic gneisses with minor migmatitic, sillimanite/kyanite-garnet-biotite metapelites, kyanite-staurolite-garnet schists, calc-silicate rocks, amphibolite and quartzite. The “quartzite terrane” is comprised almost entirely of quartzite with minor kyanite-bearing metapelitic gneisses and the “granulite terrane” of anhydrous quartzofeldspathic gneisses, intermediate two-pyroxene gneisses and sillimanite-bearing metapelitic gneisses. The boundary between the quartzite and granulite terranes is a steep mylonite zone that involved oblique overthrusting of the granulites to the SW. The boundary between the quartzite and gneissic terranes is also discordant and possibly a thrust surface, although it is not exposed anywhere. The granulite terrane involved two phases of isoclinal folding (F g2 and F g3) by S to N transport and with accompanying high grade metamorphism. The granulites have well-annealed granoblastic textures. No strong planar foliation has developed, although two mineral lineations are recognized. The quartzite and gneissic terranes display a strong layer-parallel S 2-L 2 fabric that immediately post-dates the peak of the major metamorphic event (M 1). The orientations of D 2 structural elements are different in the two terranes. Furthermore, there terranes display different orientations of the major tight to isoclinal folding (D 3) and the gneissic terrane displays a second tight folding event (D 4). Both the gneissic and quartzite terranes developed conjugate crenulation cleavages (S 5) and all terranes have N-S trending open warps (F 4b and F g4). SW over NE tectonic transport during D 2 and S over N transport during D g3 cannot be correlated with any known orogenic periods in central eastern Africa. However, SE over NW transport during D 3 is tentatively correlated with the Irumide orogeny (1100–1300 Ma) and the NE over SW transport, during D 4-D 5, with the Zambezi/Mozambique orogeny (830±30 Ma). Post-D 5 events involved over-thrusting of the granulite terrane and possibly also the quartzite terrane to the SW, followed by the intrusion of the Chewore Ultramafic Complex, dolerite dykes and pegmatites. Late-stage pegmatites may have occurred at Pan-African times (450–650 Ma). The igneous and ductile structural evolution of the Chewore Inliers ceased before Karoo times (150–285 Ma). Metamorphic parageneses indicate that a total of 21 km of crust has been removed at some stage subsequent to M 1 metamorphism and prior to the deposition of the Karoo Supergroup sediments on the Chewore gneisses. A protracted period of faulting and uplift of the inliers during and after Karoo times resulted in the Chewore Inliers being a significant topographic high within the Zambezi Rift valley.