Determining the controls on rheomorphism of a Mesoproterozoic lava-like ignimbrite in the subaqueously emplaced, intra-caldera, high-silica Kathleen Ignimbrite, west Musgrave Province, central Australia

摘要: The 1071±5 Ma Kathleen Ignimbrite eruption sequence of the Mesoproterozoic west Musgrave Province of central Australia is a rare example of a thick intra-caldera rheomorphic ignimbrite emplaced in a shallow-water environment. These ignimbrites is an evolved, highsilica, A-type, metaluminous (to slightly peraluminous) rhyolite that is significantly enriched in the rare earth elements (REE; up 794 ppm) and F (≤ 0.8 wt%) compared to crustal values. This study focuses on the rheomorphic nature of the deposit and assesses the factors that controlled the extremely high-grade welding conditions attained during and after emplacement. Despite the ignimbrite’s subaqueous emplacement, the< 50 m thick basal facies contains folded and deformed lava-like flow banding and laminations, with a basal contact with an underlying shallow-water turbidite succession characterised by pervasive hyaloclastite and peperite development.This study uses petrological and textural observations, whole-rock geochemistry and geothermometry data to determine the viscosity of the deposit during eruption. The results indicate that the ignimbrite was erupted at lower temperatures and lower viscosities than similar rheomorphic ignimbrites, potentially related to the elevated fluorine concentrations present in the magma and possibly the presence of water. However, the modelling undertaken during this study indicates that removing all of the F and H2O from the magma that formed the Kathleen Ignimbrite would not have prevented rheomorphism. The data presented here also indicate that the Kathleen Ignimbrite would have behaved rheomorphically at higher strain …