ABSTRACT

Recent pioneering work on active and modern mafic volcanism has provided a comprehensive understanding of the processes of emplacement of regionally extensive mafic pahoehoe flow fields on shallow slopes in Hawaii and in continental flood basalt terrain. Important components of the emplacement model are persistent long-lived lava flow and its predominance over lava supply rate as the controlling factor influencing aerial dimensions of flow fields; the inflation of sheet flow lobes under composite viscous and brittle crusts; endogenous growth; and the development of preferred lava pathways or tubes which serve to focus lava flow and provide thermally-efficient delivery of lava to lobe-by-lobe advancing flow fronts. The resulting volcanic architecture is that of parent lava pathways flanked by a broad, complex, stratigraphic sequence of episodically-emplaced, variably-inflated lateral lobes and subsidiary flows fed by their own distributary pathways.

The thermal regimes required to develop recurring lithological field relationships, facies variations, and internal structures of flow units which we have identified in komatiite sequences are consistent with those required by the volcanic architecture of their basaltic analogues. Likewise, inflation, endogenous growth, and pathway delivery, are processes that can account for the aerial extent, thickness, and lithological profiles documented for komatiite flows and component flow lobes in the Archaean greenstone belts of Western Australia.

We have therefore applied the dynamic inflationary model to the evolution of komatiite flow fields and developed a broad volcanic facies classification that is commensurate with this model and results from detailed field-based studies. Flood Flow Facies: (1) Unconstrained sheet flow forming extensive regional sheet-like layered olivine adcumulate bodies and layered ultramafic-gabbro sequences, flanked by olivine orthocumulates and spinifex-textured, flow lobes; (2) Constrained lava flow in large erosional pathways forming large trough-shaped bodies of predominantly layered adcumulate up to 1 km thick and 2 km wide, commonly grading upwards into pyroxenite and gabbro, and flanking breakouts of lava producing extensive olivine orthocumulate and rarer inflated spinifex-textured, flow lobes. Compound Flow Facies: Long-lived lava flow (? distal) in laterally-confined smaller lava pathways (up to 10 km long and 200m wide) filled with a variety of layered olivine ortho-mesocumulate, flanked by episodically-emplaced lava breakouts manifest as inflated spinifex-textured flow lobes.