Single zircon ages for felsic to intermediate rocks from the Pietersburg and Giyani greenstone belts and bordering granitoid orthogneisses, northern Kaapvaal Craton, South Africa

Previous models for the temporal evolution of greenstone belts and surround ing granitoid gneisses in the northern Kaapvaal Craton can be revised on th e basis of new single zircon ages, obtained by conventional U-Pb dating and Pb-Pb evaporation. In the Pietersburg greenstone belt, zircons from a meta quartz porphyry of the Ysterberg Formation yielded an age of 2949.7 +/- 0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/-18 Ma. These data show felsic volcanism in t his belt to have been coeval with felsic volcanism in the Murchison belt fa rther east, and the date of similar to 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani g reenstone belt has a zircon age of 3203.3 +/- 0.2 Ma, while a younger and c ross-cutting feldspar porphyry has an emplacement age of 2874.1 +/- 0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giy ani greenstones. Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in com position from tonalite to granite and texturally from well-layered to homog eneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constitu ted a basement to the Giyani greenstone sequence, while the younger gneisse s are intrusive into the older gneiss assemblage and/or into the greenstone s. The Giyani and Pietersburg belts probably define two separate crustal en tities that were originally close together but were later displaced by stri ke-slip movement. (C) 2000 Elsevier Science Limited. All rights reserved.