VOLCANIC AND VOLCANICLASTIC ROCKS OF THE MESOARCHAEAN PONGOLA SUPERGROUP IN SOUTH AFRICA AND SWAZILAND: DISTRIBUTION, PHYSICAL CHARACTERISTICS, STRATIGRAPHY AND CORRELATIONS

Abstract

The Mesoarchaean Pongola Supergroup is an exceptionally well-preserved succession of volcanic and sedimentary rocks that extends for 270 km close to the eastern margin of the Kaapvaal Craton in South Africa and Swaziland. It represents a volcano-sedimentary continental or epicontinental deposit that is one of the most extensive coherent Archaean terrains in the world, and the oldest of this extent. Its characteristics are unique amongst supracrustal terrains of this age and are unlike those of other Archaean greenstone belts. Its unique nature is that it marks the transition in southern African crustal development from preceding early greenstone belts, such as observed in the Barberton and Nondweni greenstone belts, to late Archaean basin formation on stable continental crust. The volcano-sedimentary succession of the Pongola Supergroup is divided into the lower, predominantly volcanic, Nsuze Group, which contains subsidiary but nonetheless important sedimentary formations, and the upper, dominantly sedimentary, Mozaan Group, which also contains several volcanic successions of highly variable forms and thicknesses. A basement high separates the Pongola Supergroup into a Northern Domain, which is relatively undeformed, and the Southern Domain, which is strongly deformed and folded. The focus of this paper is on the occurrence, field relations (with detailed maps), description and correlation of the volcanic rocks in both the Nsuze and Mozaan Groups in all areas of the Pongola Supergroup, focusing on 11 key areas in South Africa and Swaziland. The compositional range from basalt to rhyolite is remarkable, and this paper focuses on the distribution of different magma types and styles of eruption throughout the belt, including the first record of ultramafic compositions in the Pongola Supergroup. Ultramafic pyroclastic rocks in the Southern Domain contain highly vesicular fragments that may explain the absence of lava flows of the same composition. Volcanic styles transgress the entire length of the belt, with subaerial sheet flows and minor pillow development in the southern Nkandla region through to a possible continental marginal setting with both subaqueous and subaerial volcanism in the south of the Northern Domain and, finally, into a bimodal suite of felsic and mafic to intermediate volcanic rocks in the northernmost regions. Felsic ignimbrite volcanic rocks with abundant lithophysae are present in the central region. A 770 m-thick section of volcanic rocks that includes andesite and dacite compositions as well as fragmental and volcaniclastic units, including an ultramafic volcaniclastic deposit, is observed in a borehole section of the Tobolsk Formation of the Mozaan Group, that is completely overlain by Karoo sediments in the east. Cyclical episodes of volcanism, many of which were of an explosive nature, were interspersed with periods of rapidly deposited coarse-grained sediments in alluvial fans and braided rivers close to the continental margin. Carbonate formation, together with the development of stromatolites, took place during periods of volcanic quiescence, or reduced volcanic activity, in highly protected surrounds and in intertidal zones. Synchronous volcanism, sedimentation and deep erosion gave rise to the marked lateral variations in both subaerial and subaqueous deposits and marked lithologic variability. The largely volcaniclastic (with a high proportion of ultramafic component) Ndikwe Formation in the Southern Domain is enigmatic with contrasting suggested settings of an early phase of Pongola formation, or a later stage nappe structure, with a possible Mozaan Group time equivalence. No single tectonic setting, on modern day analogues, can explain all features of the Pongola volcano-sedimentary belt. Current evidence for the Nsuze Group is most consistent with a continental arc migrating into a continental rifted environment to the north. The overlying Mozaan Group is inferred to have formed in a cratonic basin following thermal relaxation after the preceding cycles of voluminous volcanism.