LATE ARCHEAN TO EARLY PROTEROZOIC GRANITOID MAGMATISM AND HIGH-GRADE METAMORPHISM IN THE CENTRAL LIMPOPO BELT, SOUTH-AFRICA

The Central Zone of the Limpopo belt in southern Africa has previously been interpreted as a segment of Archaean crust which experienced its main deformation and metamorphism around 2.7 Ga ago. We report new si ngle zircon U/Pb and Pb/Pb ages for granitoid gneisses, supracrustal r ocks and anatectically derived granitic melt patches in the area aroun d Messina, South Africa. The Sand River Gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2.6 and 3.2 Ga. The Singelele gneiss, a heterogeneous granodioritic to quartz monzonitic rock. has protolith ages between 2.55 and 2.58 Ga. Since bo th the Sand River and Singelele gneisses experienced polyphase high-st i ain ductile deformation this must have occurred later than 2.55 Ga a go. Granulite-facies pelitic gneisses of the Belt Bridge Complex conta in abundant spherical, multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism. These zircons provide ages with a mean al 2026.5 +/- 6.3 Ma which is interpreted as reflecting a high P-T event (>10 kbar, 825 +/- 25 degrees C). Granitic melt patche s in the metapelites as well as in the Sand River Gneiss and anatectic granites are probably related to rapid near-isothermal decompression to below 3-5 kbar and 600-750 degrees C. These rucks contain new magma tic zircons which yielded a mean age of 2005.6 +/- 4.4 Ma and probably reflect a crustal melting event resulting from rehydration of the gra nulitic assemblage. Our zircon data support previous suggestions for o nly one single granulite-facies event in the Central Zone, and we sugg est that this event occurred c. 2027 Ma ago. Since most of the deforma tion seen in the gneisses of the Messina area must have occurred later than 2.55 Ga ago, it is likely that the 'Limpopo Orogeny', at least i n the Central Zone, is not an Archaean event but took place in the ear ly Proterozoic.