FIELD RELATIONSHIPS OF MID-ARCHEAN TO LATE-ARCHEAN HIGH-GRADE GNEISSES OF IGNEOUS AND SEDIMENTARY PARENTAGE IN THE SAND RIVER, CENTRAL ZONEOF THE LIMPOPO BELT, SOUTH-AFRICA

Mid- to late Archaean supracrustal and granitoid gneisses exposed sout h of Messina in the central Limpopo Belt preserve evidence of complex polyphase deformation and high-grade metamorphism. Detailed mapping re vealed a sequence of events that has been placed into a time frame usi ng existing geochronological data. Deposition of supracrustal rocks of the Belt Bridge Complex occurred >3.24 Ga ago and was followed by maf ic magmatism exemplified by the intrusion of the Messina Suite in lowe r crustal levels. Tonalitic to granodioritic protoliths of the Sand Ri ver Gneiss intruded 3.3 - 3.18 Ga ago and incorporated the more ancien t rocks as xenoliths. An important structural event (D-1) took place a t about 3.24 Ga. Strong, non-coaxial deformation imparted a characteri stic lithological layering onto the rocks formed up to that time. D-1 occurred in combination with the intrusion of the leucogranitic Doroth y Gneiss precursor derived from syntectonic partial melting during hig h-grade metamorphic conditions (M-1). A swarm of mafic dykes intruded after D-1. These rocks were incorporated as xenoliths during subsequen t emplacement, some 2.65 - 2.62 Ga ago, of granitoid protolith of the Verbaard Gneiss. A second structural episode (D-2) was responsible for folding and transposition of D-1 fabrics, the formation of a second f oliation parallel to axial planes of tight to isoclinal folds and the formation of a first foliation in rocks emplaced after D-1. Intrusion of the granitic Singelele Gneiss precursor took: place syntectonically with D-2. The granitic magma was derived from anatectic sources, and its intrusion occurred during a second major high-grade event (M-2) at about 2.58 - 2.55 Ga. A third deformational event (D-3) led to the fo rmation of small-scale shear zones and small-scale asymmetric folds wi th northwest-southeast-oriented axial planes. Partial melt veins devel oped during D-3. Two sets of shear zones, slightly differing in age, w ere active during D-4. Partial melting at about 2.0 Ga caused the form ation of irregular to elongate pods of pegmatoid rock during granulite -facies metamorphism (M-3) along the shear zones. Intrusion of a pegma tite/granite dyke swarm was followed by the intrusion of a mafic dyke swarm. Brittle deformation (D-5), probably associated with uplift of t he Central Zone, finally gave rise to the development of cataclasites and pseudotachylites.