Archaean calc-alkaline volcanism in the Pilbara Block, Western Australia

Abstract

Archaean calc-alkaline volcanics occur in two distinct settings in the Pilbara Block. Within the ca. 3500 Ma old Warrawoona Group, sequences of intermediate to acid volcanics are interlayered with an extensive sequence of komatiites and tholeiitic basalts. In the western Pilbara, the Whim Creek Group, a younger sequence of intermediate to acid volcanics is associated with terrigenous sediments and minor mafic lavas. Both suites have typical calc-alkaline mineralogy and chemical compositions. The major and trace element chemistry of the most mafic lavas (basalts and basaltic andesites) is consistent with derivation by partial melting of the mantle and a spectrum of fractional crystallization trends (involving the observed phenocryst phases) appears to relate more evolved members to parent magmas. The fractionation processes involved in the evolution of the calc-alkaline suites operated at higher fluid pressures than those involved in the development of associated komatiitic and tholeiitic lava sequences. The setting of volcanism in the Pilbara has a number of broad similarities (and some differences) to modern volcanically active continental margins in which growth of new continental crust appears to be dominated by calc-alkaline plutonism with calc-alkaline volcanism playing a subordinate role. However, more detailed information is required before direct comparisons with modern environments can be made with confidence or rejected in favour of non-uniformitarian models for Archaean crustal evolution.