Structure, Morphology and Chemical Compositions of a Succession of High-Ti and Low-Ti Komatiites and Komatiitic Basalts in the Nondweni Greenstone Belt, South Africa

The 3.35 - 3.45 Ga Nondweni Greenstone Belt is located in the southern part of the Kaapvaal Craton, South Africa, and occurs as well-preserved, but poorly exposed inliers, within the mid-Archaean Pongola Sequence and the Mesozoic Karoo cover. The stratigraphic lowest part of the succession (Mogongolozi Formation; 7000 m thick) consists of massive and spinifex textured komatiitic basalt flows and lava lakes, subvolcanic intrusions of pyroxenite and diorite and minor chert bands. The stratigraphically intermediate Toggekry Formation (2000 m thick) is a locally developed sequence of rhyolite flows, quartz - sericite schists, metaquartzites and graphitic schists, and represents a felsic volcanic centre. The uppermost Witkop Formation (7500 m thick) is dominated by komatiite basalt, basalt and komatiite with minor graphitic cherts, calc-silicates, clastic sediments and stromatolites. Tectonic thickening and duplication of stratigraphy took place as an early deformation stage and most rocks are now steeply dipping.

A succession of continuous outcrop has been discovered in the Witkop Formation comprising more than 50 flow units over a stratigraphic interval of 300 m. The compositions range from komatiite (24% MgO) through a continuum of komatiitic basalt (to as low as 7% MgO). Both massive and spinifex textured flow units are present. The continuous outcrop allows the contacts of flows to be examined and individual flow units to mapped over a limited distance. Truncation, termination, thickness variation and interaction of flows are recorded. A major feature of the spinifex textured komatiites are exceptionally large (up to 5 m in height) downward - fanning cone structures of either pyroxene as sheafs of needles, or olivine as plates. One type can merge into the next within the same flow unit. The cone directions irrefutably point to way-up. Massive or brecciated flows separate the spinifex textured units and they can also grade into spinifex textured forms clearly indicating a lateral control on morphology. Dykes feeding flows are observed to cut through stratigraphic units.

The succession studied can be divided chemically into a stratigraphically lower high-Ti section and an upper low-Ti section for the same range in MgO with alternating flows at the interface comprising one or the other type.

Although such compositional differences have been observed in other komatiite successions it is the first time these are recorded in juxtaposition. The low-Ti suite also has higher Al2O3 and is enriched in Y and HREE. On-going studies (Shirey, written communication) also show that the two komatiite suites have distinct Re-Os isotopic signatures. The identical form of the exposures rules out later effects such as alteration and metamorphism as being responsible for the two compositional suites. Origins relating to crustal contamination and/or enriched mantle sources are considered.