Pan-African structures and metamorphism in the Makuti group, north-west Zimbabwe

Abstract

The Makuti Group is composed of arkosic gneisses, amphibolites, marbles, calc-silicate rocks and minor pelitic schists and forms part of the Pan-African supracrustal sequence within the Zambezi belt in north-west Zimbabwe. The group was affected by two generations of folds (D z1 and D Z2). The D Z1 folds are isoclinal and plunge gently to the NW and SE. D z2 folds are upright and refold D z1 folds on an almost coincident axis giving rise to a type III fold interference pattern. Metamorphism within the Makuti Group is syn-D Z1 and pre-D Z2. Temperatures calculated from garnet-biotite (grt-bt) geothermometers increase to the south from 538±49°C in upper greenschist/low amphibolite facies, to 595±46°C for mid-amphibolite facies and 718±30°C for the upper amphibolite grade. Upper amphibolite grt-bt temperatures are supported by amphibole-plagioclase and garnet-hornblende (grt-hbl) temperatures. Compositional zoning is reflected by high anorthite content in plagioclase rims (An 24–30) relative to the cores (An 10–16) and by high X gro+sps cores and low X pyp+alm cores relative to the rims in garnets. Temperatures calculated using garnet and plagioclase rim compositions are 50°C higher than core temperatures. Thus growth zoning in plagioclase and garnet preserve a portion of the prograde P-T path. The metamorphic and structural data concur with the southward thrusting of the Zambezi belt.