Invasive komatiite lava flows, Kambalda, Western Australia

Komatiite-sedimentary substrate relations are difficult to assess in polyphase-deformed Archean terrains, as contacts are commonly modified or overprinted by shear zones and/or metasomatic mineral assemblages. A detailed assessment of komatiite contact relations at Kambalda, Western Australia, indicates that preserved primary contacts are very rare (< 1%).

The majority of preserved primary komatiite lava-sediment contacts do not show evidence of erosion, marginal autoclasis and/or peperite formation. The presence of planar contacts, lobate morphology, and preservation of delicate in situ hyaloclasts in underlying flow top breccias suggests these komatiitic lava flows were passively emplaced. Exceptions are rare komatiitic shallow intrusions in the Victor area of Kambalda, which have irregular and peperitic margins, with apophyses into the surrounding felsic volcaniclastics and carbonaceous metapelites. The relationship of the intrusions to adjacent demonstrable lava flows, suggest the shallow intrusions are formed by burrowing of komatiite lava flows into unconsolidated sediments.

These observations are explained by the much higher density of komatiite lava (up to 2780 kg/m3) compared to unconsolidated water-saturated felsic volcaniclastics and carbonaceous mudstones (1500-2000 kg/m3). The higher density and resultant negative buoyancy caused komatiites to load, scour, and burrow into the unconsolidated sediments. This process led to invasive flows, similar to those recorded in modern basalt lavas and flood basalts.