SPINIFEX-KOMATIITE LAVA FLOWS, 3.5 GA KOMATI FORMATION, SOUTH AFRICA

Spinifex komatiites from the 3.5 Ga Komati Formation have lenticular and sheet-like shapes, similar to subaerial pahoehoe lava flows. However, these highly fluid ultramafic lavas lack a ropy upper surface and significant vesiculation, have interiors spinifex zones with megacrystic blades of skeletal olivine, and erupted in a submarine lava-plain environment. Detailed outcrop maps reveal that flow-lobe emplacement inverted the topography, a feature characteristic of inflation. Flow fronts were less than 0.25 m thick as indicated by the thinnest lobes and pointed lateral margins with concave upper surfaces that mark a gradient of increasing inflation toward the interior of the lobes. Some spinifex flows have dikes that extend from their bases into the underlying flow. Despite their intrusive appearance, the dikes passively filled open fissures, some of which may have formed as inflation clefts. Flow-top breccias of angular glassy fragments formed in situ by cracking the brittle, outer, glassy flow top in response to viscous upwelling and stretching that accommodated inflation. Spinifex zones record the growth of the upper crust above the cumulate zone, and textural discontinuities record the influx of lava during crustal growth. Since 70% of the komatiites are massive flows choked with phenocrysts, a critical factor in the unhindered growth of large olivine blades in the upper spinifex zones is the ‘open space’ for crystal growth. Variable ratios of cumulate to total flow thickness record variable phenocryst loads and upstream accumulation of olivine. As a result, spinifex textures developed under the right combination of three inter-related factors: 1) olivine-saturated magma chemistry, 2) thermal setting, and 3) the mode of emplacement. This paper examines the morphology of spinifex komatiites -- the shape and distribution of flows, dike-filled fissures, glassy flow tops and breccias, and textural zoning -- and concludes that these features are best understood within the context of an inflationary mode of lava flow emplacement.