The structural-metamorphic evolution of the northern margin of the Zimbabwe Craton and the adjacent Zambezi belt in northeastern Zimbabwe

Abstract

The Archean Zimbabwe Craton in northeastern Zimbabwe is bounded along its northern margin by the east-trending Zambezi belt, which has been subdivided from south to north into the Archean Migmatitic Gneiss Terrain, overlain by Proterozoic gneiss units of the Marginal Gneiss Terrain and the Allochthonous Gneiss Terrain. The transition from the Zimbabwe Craton into the Migmatitic Gneiss Terrain occurs over a 10–15-km-wide zone across which the metamorphic grade increases from 3 to 4 Kbar and 500 °C in the northern part of the craton, to 6–7 Kbar and 700 °C in the Migmatitic Gneiss Terrain. This P-T (Pressure-Temperature) gradient was established in the late Archean and reflects exhumation of the Migmatitic Gneiss Terrain as a result of D1 sinistral transpression due to oblique collision between the Zimbabwe Craton and an unknown rock mass to the north. D1 structures are absent within the Marginal and Allochthonous Gneiss Terrains.

The craton margin and Migmatitic Gneiss Terrain were subsequently intruded and crosscut by the 2.58 Ga Great Dyke. Between 1.1 and 0.5 Ga renewed tectonism occurred along the craton margin, forming the Zambezi belt. Deformation fabrics linked to this are mainly preserved in the Marginal and Allochthonous Gneiss Terrains and include 0.85–1.05 Ga high-P granulite-facies fabrics (D2) extensively overprinted by D3 fabrics that formed ∼0.80 b.y. ago. D3 fabrics preserve mylonitic characteristics across the Marginal Gneiss Terrain, which is part of a 3–5-km-thick crustal-scale, shallowly north- to northeast-dipping shear zone that can be traced along the full length of the Zambezi belt north of the Zimbabwe Craton. This shear zone accommodated extension and exhumation of rocks in the Marginal Gneiss Terrain, the Migmatitic Gneiss Terrain, and the craton margin. A renewed burial and exhumation cycle affected the rocks of the Marginal Gneiss Terrain, the Migmatitic Gneiss Terrain, and the craton margin ca. 0.53 Ga, as units of the Allochthonous Gneiss Terrain overthrust the margin of the Zimbabwe Craton.