Archaean felsic volcanism in the northeastern Yilgarn Block, Western Australia

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Abstract
Felsic volcanic rocks in the northeastern Yilgarn Block form two major groups with distinctive petrochemical characteristics and tectonic settings. Discrete calc‐alkaline volcanic centres composed predominantly of andesite were formed during the development of a major volcano‐sedimentary basin and were erupted contemporaneously with tholeiitic basalt. Parental calc‐alkaline melts were derived by shallow, hydrous partial melting of LIL‐enriched mantle; Y‐ and HREE‐depleted, Sr‐enriched dacitic volcanic rocks and dacite to rhyolite undepleted in Y and HREE and depleted in Sr associated with andesite in some of the volcanic centres were derived from an intermediate parent by crystal fractionation involving variable proportions of the modal minerals. Calc‐alkaline volcanism is believed to have been triggered by “hot spot’ activity rather than subduction. Rhyolite enriched in Zr, Nb, Y and HREE was deposited in a linear trough in a tectonic zone which disrupted the western margin of the volcano‐sedimentary basin. The rhyolite contains no associated andesite and is considered unlikely to have formed by the fractionation of an intermediate parent. A crustal source, tapped during the initial phase of a period of increased crustal extension, is considered most likely. Both varieties of felsic volcanic rock are spatially associated with distinctive, and probably genetically related, plutonic suites. The calc‐alkaline volcanic rocks are intruded by contemporaneous, cogenetic hornblende‐bearing granodiorite porphyry and are spatially associated with numerous younger plutons of hornblende‐bearing granodiorite to monzogranite of probable calc‐alkaline affinity. Rhyolitic volcanic rocks are believed to be cogenetic with high‐level plutons of syenogranite to alkali feldspar granite. The contrasting nature of the two felsic volcanic/plutonic suites is a direct result of the particular tectonic environment of eruption.