Single stage, late Archaean exhumation of granulites in the Northern Marginal Zone, Limpopo Belt, Zimbabwe, and relevance to gold mineralization at Renco mine

Abstract

Detailed field mapping and geochronological studies in the Renco mine area of the Northern Marginal Zone of the Limpopo Belt allow two distinct tectonic events to be distinguished in the late Archaean to Palaeoproterozoic. In the first event, granulite-facies metamorphism (~2720 to ~2590 Ma) was followed by the formation of pervasive steep fabrics accommodating north-northwest shortening, and a network of amphibolite-facies shear zones. Enderbitic rocks were intruded over the interval ~2739 to ~2571 Ma, and granites were intruded over a largely overlapping period of ~2654 to ~2517 Ma. The intrusion of syn-tectonic granites (~2600 to ~2500 Ma) allows the convergent tectonics to be dated as late Archaean. The second event produced deformation zones dominated by cataclasis at maximum temperatures of 400°C, and may have involved localized pseudotachylite generation. These deformation zones have maximum lengths and widths of several hundred metres and 2m respectively: they are separated along and across strike by several kilometres, and did not accommodate significant crustal displacement. They probably formed in the second event at ~2000 Ma. The granulites were therefore exhumed in the first event, following directly on granulite-facies metamorphism, implying that a substantial crustal thickness was attained in the late Archaean. Amphibolite facies gold mineralization at Renco mine probably occurred at this time, followed by Palaeoproterozoic remobilization.