Archaean komatiitic and tholeiitic volcanics at Kambalda, Western Australia

Claoué-Long, Jonathan Charles (1986) Archaean komatiitic and tholeiitic volcanics at Kambalda, Western Australia. University of Southampton, Doctoral Thesis.

Abstract

Archaean metamorphosed mafic and ultramafic greenstone volcanics at Kambalda, W. Australia, divide into five stratigraphic and lithological groups: (a) lower footwall tholeiitic basalts with ∼8% MgO and spherulitic pyroxene textures; (b) upper footwall tholeiitic basalts with ∼8% MgO and subophitic textures; ∼30%-20% MgO olivine spinifex textured komatiite lavas; (c) moderate to low MgO variolitic pillow basalts with felsic cores; (d) moderate to low MgO olivine - and pyroxene-phyric basalts. The lavas were erupted subaqueously, probably in deep water. The footwall basalts and komatiites share chondritic mantle ratios of Al/Ca/Ti/Zr/Y/REE, and geochemical trends suggest that the low-MgO basalts are minimum density cotectic lavas derived via olivine loss from komatiite parent magma. Spherulitic basal textures result from eruption as superheated liquids, subophitic textures from eruption at subliquidus temperatures. The parent magma of the footwall basalts was subsequently erupted as the Kambalda komatiite lavas, and the derivative nature of the footwall basalts implies the existence below Kambalda of an olivine cumulative body. High-Mg basalts overlying the komatiites are LREE-enriched, and cannot be related to LREE-depleted komatiite compositions by crystal fractionation. They either represent independent melts of enriched source compositions, or crustal contamination of komatiite magma. Felsic cores to variolitic pillows of high-Mg basalt probably result from metasomatic alteration, and do not represent immiscible igneous liquids. Geochemical trends thus directly link the high-Mg komatiite with the low-Mg tholeiite series. Pyroxene spinifex textured high-Mg basalts are not intermediate between the two and are not simply related to komatiites. Application of the term komatiitic to these basalts is therefore misleading. Conflicting evidence supports an eruption age of either ∼3.2Ga, in which case the mantle was chemically but not isotopically differenitiated, or ∼2.7Ga in which case either the mantle had gross Nd isotopic heterogeneities or LREE-enriched magmas are crustally contaminated. (D74037/87)

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