The physical volcanology of Archaean komatiite sequences from Forrestania, Southern Cross Province, Western Australia

Abstract

The Archaean supracrustal sequence at Forrestania contains at least five komatiitic belts which can be traced along strike for distances in excess of 30 km. Lithologies range from olivine cumulates, which crystallised at the interface between substrate and flowing lava, to spinifex-textured rocks, which underwent rapid crystallisation within ponded lava lobes. Characteristic associations of different rock types are interpreted in terms of variations in eruption rate, which presumably reflect proximity to the vent or to a major lava river. Thick sequences of olivine adcumulate to mesocumulate crystallised in a dynamic environment, where lava flow was continuous and eruptive velocities were high. Sheeted bodies of olivine cumulate formed from voluminous, turbulent flow of lava over relatively refractory substrates, such as basalt and oxide-facies banded iron-formation. Ribbon-shaped olivine-cumulate bodies formed in an equally dynamic environment within major lava channels, where the substrate was less refractory and consisted of quartz-rich sedimentary rocks which were susceptible to thermal erosion. Distributary lava channels in more distal settings are represented by thinner ribbon-like accumulations of olivine mesocumulate to orthocumulate. Unusual poikilitic pyroxene-bearing rocks are present at the base of some lava channels and appear to represent the crystallisation product of a komatiitic lava which has been contaminated by cherty sedimentary rocks. Pyroxene- and plagioclase-bearing cumulates are present at the top of other channel sequences and reflect late-stage ponding and in situ differentiation of komatiitic lava within the channel. The channel-facies rocks are flanked by sequences comprising multiple, thin, spinifex-textured flow-units, which formed from the episodic overflow of lava from the distributary channels. Similar rocks also occur stratigraphically above many of the thick olivine-cumulate bodies and reflect episodic emplacement of thin cooling units during the waning stages of the eruptive cycle.