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1 Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia (e-mail: r.smithies{at}dme.wa.gov.au)
2 Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia (e-mail: david.champion{at}agso.gov.au)
Numerous felsic alkaline intrusions were emplaced in the granite-greenstone terrain of the Eastern Goldfields, Yilgarn Craton, between c. 2650 and 2630 Ma. Intrusion coincided with the waning stages of voluminous regional granitic magmatism. The alkaline rocks are localized along and adjacent to major crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suites; viz. the Mount Monger, Emu, Claypan and Ninnis suites. The rocks have features in common with A-type granites, including high total alkalis, an anhydrous primary mineralogy, an apparent anorogenic setting, and enrichment in some HFSE, although they also have low concentrations of Nb and Y. They show a range of features that, together, point to a crustal source; e.g. the alkaline rocks are not associated with voluminous mafic magmatism, they have high LILE concentrations, they carry zircon xenocrysts of similar age to whole-rock Sm–Nd model ages, they show evidence for regional source heterogeneity, suites have constant and distinct La/Y and Nb/Y ratios and the rocks show many compositional similarities to the slightly older, voluminous, crust-derived granites. Extreme enrichments in Sr and Ba in the Mount Monger Suite, however, suggests that their source may have been metasomatized before or during melting. Intrusion of the alkaline rocks heralds a poorly recognized third tectonothermal event in the evolution of the eastern part of the Yilgarn Craton, and was broadly synchronous with the intrusion of voluminous felsic fractionated granites, crustal extension and widespread Au-mineralization. The Late Archaean geology of the Eastern Goldfields shows many features in common with Phanerozoic collisional belts thought to be profoundly influenced by processes of lithospheric delamination or convective thinning. This newly recognized third tectonothermal event in the Eastern Goldfields, may relate to the delamination or convective thinning of a dense, garnet-rich, lower crust, that resulted from extraction of earlier voluminous granite magma, and it is possible that such processes have had a greater influence on Late Archaean crustal evolution than previously recognized.
Key Words: Yilgarn Craton • Late Archaean • alkaline rocks • gold mineralization • tectonics
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