The volcano-sedimentary succession of the Archean Sodium Group, Ventersdorp Supergroup, South Africa : volcanology, sedimentology and geochemistry

Abstract

The volcano-sedimentary succession of the Sodium Group, in the south-western corner of the Kaapvaal craton, rests unconformably on basement granites and is usually correlated with the middle Ventersdorp Supergroup. This Neoarchean (∼2700 Ma) large igneous province (LIP) has been interpreted as deposited in a tectonic escape basin scenario, following the collision of the Zimbabwe and Kaapvaal cratons, or alternatively, in a mantle plume induced, cratonic rift basin. The basaltic-komatiitic to andesitic-dacitic volcanic sequence and the accommodated sedimentary deposits developed in a graben to half-graben setting within a cratonic, failed-rift system. The Sodium Group sedimentary facies and geochemistry of volcanic rocks correlate well with the Ventersdorp Supergroup. The major and trace element and REE geochemistry correspond with previously published results (Crow and Condie, 1988; Grobler et al., 1989); and the Sr and Nd isotopy confirms crustal contamination of the Sodium Group magmatic rocks. A preliminary weighted mean 207Pb/206Pb zircon age of 2739 ± 39 Ma for a tuff in the upper Ongers River Formation is reported for the lower part of the Sodium Group. The Ongers River, T’Kuip and Omdraaivlei Formations reflect sedimentation and volcanism in a rift induced graben setting with fast sedimentation from horst flanks and high burial rates in the graben valleys, and subsequently, a less pronounced topography, post-graben environment. Accompanying explosive phreatomagmatic and effusive, mafic to acidic, bimodal volcanism, with abundant pyroclastics, delivered ample material for graben fill deposits. Repeated horst reactivation has episodically exposed the basement to renewed erosion. Sedimentation along graben flanks comprised alluvial fans, mass flow deposits, braided river and related modes of deposition. In distal, shallow lakes, volcaniclastic suspension and current sedimentation, low-density turbidites and stromatolitic reefs were accumulated.