Field relationships of mid- to late Archaean high-grade gneisses of igneous and sedimentary parentage in the Sand River, Central Zone of the Limpopo Belt, South Africa

Mid- to late Archaean supracrustal and granitoid gneisses exposed south of Messina in the central Limpopo Belt preserve evidence of complex polyphase deformation and high-grade metamorphism. Detailed mapping revealed a sequence of events that has been placed into a time frame using existing geochronological data. Deposition of supracrustal rocks of the Beit Bridge Complex occurred >3.24 Ga ago and was followed by mafic magmatism exemplified by the intrusion of the Messina Suite in lower crustal levels. Tonalitic to granodioritic protoliths of the Sand River Gneiss intruded 3.3 - 3.18 Ga ago and incorporated the more ancient rocks as xenoliths. An important structural event (D₁) took place at about 3.24 Ga. Strong, non-coaxial deformation imparted a characteristic lithological layering onto the rocks formed up to that time. D₁ occurred in combination with the intrusion of the leucogranitic Dorothy Gneiss precursor derived from syntectonic partial melting during high-grade metamorphic conditions (M₁). A swarm of mafic dykes intruded after D₁. These rocks were incorporated as xenoliths during subsequent emplacement, some 2.65 - 2.62 Ga ago, of granitoid protolith of the Verbaard Gneiss. A second structural episode (D₂) was responsible for folding and transposition of D₁ fabrics, the formation of a second foliation parallel to axial planes of tight to isoclinal folds and the formation of a first foliation in rocks emplaced after D₁. Intrusion of the granitic Singelele Gneiss precursor took place syntectonically with D₂. The granitic magma was derived from anatectic sources, and its intrusion occurred during a second major high-grade event (M₂) at about 2.58 - 2.55 Ga. A third deformational event (D₃) led to the formation of small-scale shear zones and small-scale asymmetric folds with northwest-southeast-oriented axial planes. Partial melt veins developed during D₃. Two sets of shear zones, slightly differing in age, were active during D₄. Partial melting at about 2.0 Ga caused the formation of irregular to elongate pods of pegmatoid rock during granulite-facies metamorphism (M₃) along the shear zones. Intrusion of a pegmatite/granite dyke swarm was followed by the intrusion of a mafic dyke swarm. Brittle deformation (D₅), probably associated with uplift of the Central Zone, finally gave rise to the development of cataclasites and pseudotachylites.