Modern-style subduction processes in the Mesoarchaean: Geochemical evidence from the 3.12 Ga Whundo intra-oceanic arc

The ca. 3.12 Ga Whundo Group forms an exotic terrane on the western margin of the old (≥3.4 Ga) nucleus of the Pilbara Craton in northwestern Australia. It is exceptional amongst Mesoarchaean, or older, volcanic sequences in that it preserves a range of geological and geochemical features that together provide unambiguous evidence of modern-style subduction processes. The group includes boninites, interlayered tholeiitic and calc-alkaline volcanics, Nb-enriched basalts and adakites. Low Th/La (0.07-0.14) and Ce/Yb (<40) and a lack of evidence for felsic basement point to an intra-oceanic arc setting. Contamination of the tholeiitic magmas by Pilbara crust cannot generate both the lower Nb and higher Th and La concentrations in the associated calc-alkaline basalts and andesites. Distinct mantle sources are required and numerous hybrid magmas result from mixing of these sources or of primitive magmas. Large ion lithophile element (LILE) variations in some of the calc-alkaline rocks appear unaffected by post-magmatic alteration. In these unaltered rocks, decreasing La/Sm and La/Yb correlate with decreasing LILE, Cr, Ni and Mg# and with increasing Nb, Zr and Yb, providing evidence for flux melting of an Archaean mantle wedge. High Ba/La ratios reflect fluid-mediated source metasomatism, but a systematic decrease in Ba/La suggests an increasing slab-melt component up the calc-alkaline stratigraphic pile, culminating in the eruption of adakitic rocks and Nb-enriched basalts.