Piggy-back Supervolcanoes—Long-Lived, Voluminous, Juvenile Rhyolite Volcanism in Mesoproterozoic Central Australia

• Published in: Journal of petrology 2015-04, Vol.56 (4), p.735-763
• Main Authors: Smithies, R H, Howard, H M, Kirkland, C L, Korhonen, F J, Medlin, C C, Maier, W D, de Gromard, R Quentin, Wingate, M T D
• Format: Article
• Language: English
• Subjects: Australia; Basalt; Basements; Chambers; Magma; Rhyolite; Rocks; Volcanic activity
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egv015

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 degree C. Comagmatic basalt is of mantle origin with minor Proterozoic basement contamination. The rhyolites cover the same range of Nd isotope compositions ( epsilon sub(Nd(1070)) +1.24 to -0.96) and La/Nb ratios (1.2-2.1) as the basalts ( epsilon sub(Nd(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 epsilon sub(Nd(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 230000km super(3) of parental basalt is required to form the >22000km super(3) 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 >30Myr 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 1090-1040Ma Giles Event in central Australia, associated with magma-dom