The 3.5 Ga Komati Formation, Barberton Greenstone Belt, South Africa, Part I: New maps and magmatic architecture

Abstract

The 3.48 Ga Komati Formation, famous for its spinifex komatiites, was mapped (1:5000 scale) to provide an objective base for reconstructing the magmatic and tectonic architecture. All outcrops were drafted on digital orthophotographs using GIS software. The best outcrops were mapped to determine the textural zoning and shapes of chill-bound cooling units and to distinguish volcanic, intrusive, and tectonic contacts. The Komati and overlying Hooggenoeg Formations compose a 6-km-thick section of submarine lava flows and intrusions. The section is overlain by c. 3.45 Ga dacitic volcanic rocks, and the Komati Shear Zone marks the base. The Komati Formation is divided into upper and lower members. The Lower Komati, 1.3 km thick, consists of layers of olivine komatiite (48%) and komatiitic basalt (52%). The Upper Komati, 1.8 km thick, consists predominantly of pillow lavas of komatiitic basalt (84%). Komatiites of the Lower Komati consist of massive (61%), spinifex (37%), and vesicular (2%) cooling units. Massive units are interpreted to be undifferentiated sheet flows (≤50 m thick by 11 km in extent). Spinifex komatiites are concentrated in five distinct horizons that generally overlie thicker massive sheets. Spinifex cooling units occur as sheet flows (≤8 m by >1 km) and thinner lenses that are analogous to pahoehoe flow lobes. Vesicular komatiite occurs as a sheet (~ 11 m by >1.8 km) and a channel (55 m by <0.3 km) with 20-25% vesicles, locally interlayered in zones with olivine spinifex. Interlayered with pillows in the Upper Komati, sheet flows of komatiitic basalt (5-15 m thick) have pyroxene-spinifex zones. Similar textures occur in locally crosscutting sills in the Lower Komati. Dykes and sills of komatiite, komatiitic basalt, diabase, tonalite, and wehrlite intruded the Komati Formation. Komatiitic dykes occur within cooling units and may be related to inflation of volcanic flows. Wehrlite dykes, compositionally similar to the komatiites, are younger intrusions, possibly related to an overlying volcanic sequence. Dykes and sills of komatiitic basalt may relate to volcanic rocks in the Upper Komati or Upper Hooggenoeg Formation. Volcanic flows occur at the structural base of the Komati Formation with no evidence for sheeted dykes.