Tectono-metamorphic evolution of the simatic basement of the Pietersburg greenstone belt relative to the Limpopo Orogeny: Evidence from serpentinite

Detailed studies of low-grade ultramafic rocks are integrated with regional structural field observations in the Pietersburg greenstone belt. These results show that petrographic investigations of serpentinite can reveal more about the tectonometamorphic history of greenstone belts than has previously been appreciated. The ultramafic rocks of the Pietersburg greenstone belt were affected by an early deformation event (D₁) which is reflected by the presence of a relic magnetite-fabric (S₁) in serpentinite. The S₁ fabric is crosscut by the peak metamorphic assemblage of antigorite+tremolite+talc+chlorite. Both S₁ and the peak metamorphic assemblage are deformed by a second deformation event (D₂). On a macroscale D₂ is manifested as a series of northward-verging thrust faults that affected the entire belt. Published UPb zircon data show that D₂ was active between 2.9 and 2.7 Ga prior to uplift and southward thrusting of the Limpopo Belt (LB). Major carbonatization of the simatic basement postdates peak metamorphism and occurred towards the end of D₂, coeval with the late evolution of the Limpopo Orogeny. The alteration assemblages were subsequently deformed. The sequence of events that affected the Pietersburg greenstone belt is similar to that established for the high-grade Southern Marginal Zone of the Limpopo Belt but their absolute timing is different. The results of this study demonstrate the important contribution that low-grade ultramafic rocks can make to the understanding of the tectono-metamorphic history of the Archaean cratons and terrane accretion.