Late Archaean felsic alkaline igneous rocks in the Eastern Goldfields, Yilgarn Craton, Western Australia: a result of lower crustal delamination?

Numerous felsic alkaline intrusions were emplaced in the granite-greenstone terrain of the Eastern Goldfields, Yilgarn Craton, between c. 2650 and 263 0 Ma. Intrusion coincided with the waning stages of voluminous regional gra nitic magmatism. The alkaline rocks are localized along and adjacent to maj or crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suite s; pi;. the Mount Monger, Emu, Claypan and Ninnis suites. The rocks have fe atures in common with A-type granites, including high total alkalis, an anh ydrous primary mineralogy, an apparent anorogenic setting, and enrichment i n some HFSE, although they also have low concentrations of Nb and Y. They s how a range of features that, together, point to a crustal source; e.g. the alkaline rocks are not associated with voluminous mafic magmatism, they ha ve high LILE concentrations, they carry zircon xenocrysts of similar age to whole-rock Sm-Nd model ages, they show evidence for regional source hetero geneity, suites have constant and distinct La/Y and Nb/Y ratios and the roc ks show many compositional similarities to the slightly older, voluminous, crust-derived granites. Extreme enrichments in Sr and Ba in the Mount Monge r Suite, however, suggests that their source may have been metasomatized be fore or during melting. Intrusion of the alkaline rocks heralds a poorly re cognized third tectonothermal event in the evolution of the eastern part of the Yilgarn Craton, and was broadly synchronous with the intrusion of volu minous felsic fractionated granites, crustal extension and widespread Au-mi neralization. The Late Archaean geology of the: Eastern Goldfields shows ma ny features in common with Phanerozoic collisional belts thought to be prof oundly influenced by processes of lithospheric delamination or convective t hinning. This newly recognized third tectonothermal event in the Eastern Go ldfields, may relate to the delamination or convective thinning of a dense, garnet-rich, lower crust, that resulted from extraction of earlier volumin ous granite magma, and it is possible that such processes have had a greate r influence on Late Archaean crustal evolution than previously recognized.