Piggy-back supervolcanoes-long-lived, voluminous, juvenile rhyolite volcanism in mesoproterozoic central Australia

Abstract

The Talbot Sub-basin is one of several bimodal volcanic depositional centres of the Mesoproterozoic Bentley Basin in central Australia. It is dominated by rocks of rhyolitic composition and includes ignimbrites, some forming large to super-eruption size deposits. Ferroan, incompatible trace element enriched, A-type compositions, anhydrous mineralogy and clear evidence for local rheomorphism indicate high eruption temperatures, with apparent zircon-saturation temperatures suggesting crystallization at >900°C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions (εNd(1070) +1·24 to –0·96) and La/Nb ratios (1·2–2·1) as the basalts (εNd(1070) +2·1 to –1·1: La/Nb 1·2–2·3) and are compositionally far removed from all older basement and country-rock components (average εNd(1070) = –4, La/Nb = 10). The rhyolites and basalts are cogenetic through a process probably involving both fractional crystallization of mafic magmas and partial melting of recently crystallized mafic rock in a lower crustal intraplate, extraction of dacitic magmas to a voluminous upper crustal chamber system, and separation of rhyolite by processes involving rejuvenation and cannibalization of earlier chamber material. More than 230 000 km3 of parental basalt is required to form the >22 000 km3 of preserved juvenile rhyolite in the Talbot Sub-basin alone, which represents one of the most voluminous known felsic juvenile additions to intracontinental crust.Zircon U–Pb age components are complex and distinct from those of basement and country rock and contain antecrystic components reflecting dissolution–regrowth processes during periodic rejuvenation of earlier-emplaced chamber material without any significant interaction with country rock. The overall duration of magmatism was >30 Myr but can be divided into between two and four separate intervals, each probably of a few hundred thousand years’ duration and each probably reflecting one of the distinct lithostratigraphic groups defined in the sub-basin. Neither the composition nor style of felsic and mafic volcanism changes in any significant way from one volcanic event to the next and the range of zircon U–Pb ages indicates that each period utilized and cannibalized the same magma chamber. This volcanism forms a component of the 1090–1040 Ma Giles Event in central Australia, associated with magma-dominated extension at the nexus of the cratonic elements of Proterozoic Australia. This event cannot be reasonably reconciled with any putative plume activity but rather reflects the >200 Myr legacy of enhanced crustal geotherms that followed the final cratonic amalgamation of central Australia.