Physical volcanology and geochemistry of a Late Archaean volcanic arc: Kurnalpi and Gindalbie Terranes, Eastern Goldfields Superterrane, Western Australia

Late Archaean (2715-2704 Ma) calc-alkaline volcanic complexes in the Kurnalpi Terrane are identical in facies and geochemistry to modern intra-arc volcanic complexes. Preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra-oceanic arc-related volcanoes. The least-altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite, and are the product of magmas similar in composition to those of modern arcs. Intermediate magmas were most likely derived from large ion lithophile element (LILE) enriched mafic magmas by fractional crystallisation (with or without assimilation) processes. The Kurnalpi Terrane thus provides an example of a Late Archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis. TTD-suite volcanism derived from slab melting was either absent or played a subordinate role. The adjacent Gindalbie Terrane contains younger (∼2692-2680 Ma) bimodal (basalt-rhyolite) volcanic complexes, with associated quartz-rich sedimentary rocks, and calc-alkaline intermediate-silicic volcanic complexes. Volcanosedimentary basement to the Gindalbie volcanic sequences is deeply eroded equivalents of the calc-alkaline volcanic complexes, their associated sedimentary rocks and younger basaltic sequences found in the Kurnalpi Terrane. Gindalbie basalt-rhyolite successions locally contain volcanic-hosted massive Cu-Zn mineral deposits. Rhyolites derived by low-pressure fractional crystallisation, or melting, of andesites are more abundant than in the Kurnalpi volcanic complexes. Facies associations and the local occurrence of distinctive incompatible element-enriched, mildly peralkaline (A-type) rhyolites are consistent with rifting of a mature arc system. The two terranes most likely represent volcanic evolution of the same Late Archaean volcanic arc but are different tectonic fragments that were welded together by ∼2660 Ma.