Sedimentology, geochemistry and palaeogeographic implications of volcanic rocks in the Upper Archaean Campbell Group, western Kaapvaal craton, South Africa

Abstract

Tuffs and lava interbedded in Campbell Group carbonates and shales are investigated for their sedimentological and geochemical properties. The most proximal tuffs occur at the southwestern margin of the Kaapvaal craton, in Griqualand West. They were deposited in a shallow-marine to tidal carbonate environment from hydroclastic eruptions. The tuffs along this margin are subdivided into two main facies types: (1) graded fallout-lapilli to ash tuffs, containing accretionary lapilli; and (2) current-reworked, laminated and rippled lapilli to ash tuffs. Calculation of water depth from settling velocities of tuff particles indicates deposition in depth of a maximum of some 30 m. The tuff beds generally thin out towards the north and northwest, but distal tuff layers are widespread in the Campbell Group and allow for some lateral correlation within regionally narrow limits. Single zircon age dates suggest that correlation over the entire basin is unreliable. Two geochemical groups of tuffs are tentatively distinguished. One group includes andesites and basalts and tends to concentrate in higher stratigraphic levels than the other, which consists exclusively of basalts. Volcanism was of a tholeiitic, within-plate character, situated in a rifted cratonic setting that failed to develop an oceanic basin. Geochemical similarity to some Karoo volcanic rocks is striking. A cratonic rift basin model for the Griqualand West depository in which carbonate sedimentation in shallow environments prevailed is discussed. The sedimentation of carbonates was periodically interrupted by hydroclastic eruptions along a rifting graben or half graben system.