The Petragenesis of some Archaean Volcanic Rocks from Southern Africa

Abstract

Major, rare earth and transition elements, have been determined on a selection of volcanic rocks from greenstone belts in Rhodesia (∼2.6 by) and South Africa (∼3.4 by). In Rhodesia two distinct series can be recognized: a komatite-tholeiite series which occurs early in the greenstone belt evolution and apparently grades into a second, calc-alkaline, series at higher structural levels. Peridotitic komatites reflect higher degrees of partial melting than any Phanerozoic rocks so far observed and are therefore used to place limits on the composition of their source. At lower MgO contents they merge into low K tholeites which have significantly higher CeN/YbN and Rb/Sr ratios (at any MgO content) than those observed in modern ocean floor and island are environments. The calc-alkaline series is characterized by andesites which exhibit a marked heavy REE depletion, but similar light REE and transition metal contents to the more evolved tholeitic rock types. The continuum of compositions from komatiites to tholeiites and calc-alkaline andesites suggests that the bulk of greenstone belt volcanics could have been derived by differential partial melting, and polybaric fractionation of an essentially homogeneous peridotite source. Late stage dacitic lavas and intrusions probably reflect melting of a more eclogitic source region. The distinctive trace element geochemistry of Archaean volcanics, particularly the high Ni and low Yb values of the calc-alkaline rocks precludes direct comparison with modern island are associations. Rather the large decrease in liquidus temperatures (500 °C) with increasing structural height within greenstone belts, coupled with the fact that most of the volcanics could have been derived from an essentially homogeneous source, may suggest that greenstone belts developed in a rifting environment. It appears unlikely that the tholeiite/calc-alkaline association observed in the Archaean may be taken as an indication of subduction at that time.