Abstract

This invited keynote address was presented at the Sixth International Hutton Symposium, Origin of granites and related rocks, held at the University of Stellenbosch, South Africa, on 2-6 July, 2007. Felsic magmatism in Australia ranges in age from Paleoarchean to Mesozoic. Extensive episodes occurred in the Paleo-, Meso-, and Neoarchean, Paleo- and Mesoproterozoic, and Silurian-Devonian, and Carboniferous-Permian. Like the Archean elsewhere, the Archean Pilbara and Yilgarn Cratons in Western Australia are dominated by sodic felsic granites of the tonalite-trondhjemite-granodiorite (TTG) suite. This earliest magmatism reflects melting of a basaltic protolith, mostly at high pressures, either within subducted slabs or thickened crust. Sodic granites also form a persistent component of Proterozoic and Palaeozoic magmatism in Australia although in no specific period are they found in the same abundance as for the Archean. The other dominant magmatic series in the Australian Archean are potassic granites. These mostly postdate and largely reflect crustal reworking of the sodic granites. An important sub-class are those with high-silica and often moderately to strongly differentiated compositions. These first appear in the Mesoarchean, but are most extensive in the Neoarchean of the Yilgarn Craton (>100,000 square kilometres within a 25 million year period). Similar high-silica rocks occur in both the Proterozoic and Palaeozoic. Tectonic environments for these rocks are not well understood. One feature of these rocks is elevated thorium (Th) and uranium (U) contents. A wide variety of other (intermediate-) felsic magmatic rocks are present in the Archean to Palaeozoic geological provinces of Australia. These include: high-Mg diorite as well as components of the basalt-andesite-dacite-rhyolite series (locally with boninite-like rocks), which provide strong evidence for modern-style arc-related processes as far back as the Mesoarchean; peralkaline rocks such as syenites (as old as Mesoarchean); and high temperature, Fe- and HFSE-rich rocks, i.e., A-types (as old as Paleoarchean). Perhaps the most significant secular change in Australia is for S-type magmatism, which is rare in the Archean, minor in the Proterozoic, and common in the Palaeozoic. The Australian record shows that, in general, none of the felsic magmatic series are confined to or excluded from any particular time, but rather it is the relative proportions of such rocks which vary with time. Although similar chemistries could reflect similar responses to different processes, the simplest interpretation is that they reflect similar processes. A corollary is that a similar range of tectonic processes has operated from the Paleoarchean to now, but the dominance of particular processes has changed. This conclusion is balanced against observed subtle changes in chemistry which may indicate secular changes in tectonic processes, e.g., slab melting (TTGs) to slab dehydration (calc-alkaline rocks).
Google map showing geographic bounding box with values North bound -9.0 East bound 156.0 West bound 110.0 South bound -44.0
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2007

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geochemistry
Earth Sciences

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2008-12-04

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