PALAEOARCHAEAN SEA-FLOOR ALTERATION AND METAMORPHISM OF PILLOW BASALTS FROM THE NONDWENI GREENSTONE BELT, SOUTHEASTERN KAAPVAAL CRATON, SOUTH AFRICA

Abstract

Pillow basalts from the Witkop Formation of the Nondweni granitoid-greenstone terrain preserve evidence of Palaeoarchaean sea-floor alteration and high-grade metamorphism. Post-depositional sea-floor alteration preferentially affected volcanic glass and led to a marked contrast in compositions and mineral assemblages of the cores and rims of the pillows. The pillow rims have elevated Al 2 O 3 , TiO 2 , Fe 2 O 3 , Zr, Nb, Nd, Th concentrations and lower SiO 2 , Na 2 O, Sr contents, relative to pillow cores. Igneous clinopyroxene and plagioclase is preserved exclusively in pillow cores, whereas the pillow rims consist of a metamorphic mineral assemblage of garnet-hornblende-plagioclase-chlorite-quartz. The whole rock δ 18 O values as high as 9.6‰ suggest that hydrothermal alteration of the pillow basalts initially occurred under low-temperature conditions. δ 18 O values as low as 2.8‰ may indicate water/rock interaction at a later stage under high-temperature conditions during metamorphism. Following sea-floor alteration, the pillow basalts were deformed and metamorphosed under amphibolite facies conditions that led to formation of hornblende-defined foliation and syntectonic garnet growth in pillow rims. Garnet-hornblende-plagioclase-quartz geothermo-barometry from pillow rims reveal peak metamorphic conditions of ~6.5 kbar, 600 to 650°C. P-T pseudosection analysis also confirms that contrasting mineral assemblages in pillow cores and rims developed during metamorphism principally due to the difference in major element concentrations in these domains. P-T pseudosection analysis also predicts an isobaric cooling of the basalts along an anticlockwise P-T path. The P-T estimates indicate burial to a depth of ~17 to 20 km post-tectonic to hydrothermal alterations. The occurrence of high-grade mineral assemblages in the basalts was observed only in the vicinity of the intrusive Mvunyana granodiorite. The presence of migmatitic gneiss with steeply dipping foliations at their contact indicate that metamorphism of basalts was concomitant with intrusion of the granodiorite. Hence the time-frame of metamorphism of the Witkop Formation pillow basalts can be constrained from the crystallization age of the Mvunyana granodiorite, which is ~3.29 Ga. These observations along with the high geothermal gradient (30°C/km) recorded during peak metamorphism, and the post-peak near-isobaric cooling along an anticlockwise P-T path, are consistent with a magmatic arc setting.