Geochronology of the late Archaean Razi and Chilimanzi suites of granites in Zimbabwe: implications for the late Archaean tectonics of the Limpopo Belt and Zimbabwe Craton

Porphyritic granites of the Razi suite are intrusive into 2.7 - 2.6 Ga char noenderbitic gneisses of the Northern Marginal Zone of the Limpopo Belt (Zi mbabwe) and occur as elongated bodies along the North Limpopo Thrust Zone. Mineral-textural and structural relationships indicate that the Razi-type g ranites are syn- to post-tectonic, both with respect to the northeast-trend ing sub-vertical gneissic banding or schistosity of the Northern Marginal Z one and to the mylonitic foliation of the North Limpopo Thrust Zone, which is the locus of reverse movement between the Northern Marginal Zone and the Zimbabwe Craton. Conventional U-Pb zircon dating constrains the emplacemen t of the Razi granites to a period between 2.52 - 2.67 Ga. This implies tha t the Razi granites are coeval with undeformed Chilimanzi suite granitoids which intruded the Zimbabwe Craton during or subsequent to the last major p eriod of greenstone formation in the late Archaean (similar to 2.6 Ga). The Razi intrusions young from west to east over a maximum period of similar t o 150 Ma. This observation may be tied to the exhumation history of the Nor thern Marginal Zone. It seems likely that the movement on the North Limpopo Thrust Zone began in the west prior to 2.67 and continued in the east unti l at least 2.52 Ga. It is suggested that the exhumation of the Northern Mar ginal Zone was a diachronous event and most probably related to lithospheri c mantle thinning through mantle delamination progressing from west to east .