Abstract
Abstract. Rhyolite and dacite lavas of the Mesoproterozoic upper Gawler Range Volcanics (GRV) (>30 000 km3 preserved), South Australia, represent the remnants of one of the most voluminous felsic magmatic events preserved on Earth. Geophysical interpretation suggests eruption from a central cluster of feeder vents which supplied large-scale lobate flows >100 km in length. Pigeonite inversion thermometers indicate eruption temperatures of 950–1100 °C. The lavas are A-type in composition (e.g. high Ga/Al ratios) and characterised by elevated primary halogen concentrations (~1600 ppm fluorine, ~400 ppm chlorine). These depolymerised the magma such that temperature-composition-volatile non-Arrhenian melt viscosity modelling suggests they had viscosities of <3.5 log η (Pa s). These physicochemical properties have led to the emplacement of a Large Rhyolite Province, which has affinities in emplacement style to Large Basaltic Provinces. The low viscosity of these felsic magmas has produced a unique igneous system on a scale which is either not present or poorly preserved elsewhere on the planet. The Gawler Range Volcanic Province represents the erupted portion of the felsic end member of the family of voluminous, rapidly emplaced terrestrial magmatic provinces.
Highlights
The viscosity of silicate magmas is a critical control on the rate and scale of planetary differentiation, material and heat transfer, and crustal growth (Reese et al, 1998; Zaranek and Parmentier, 2004)
The calculated viscosities of the upper Gawler Range Volcanics range between 3.4–5.0 log η (Pa s): a range closer to those viscosities exhibited by basalts (
The low viscosity magma erupted from a central cluster of feeder vents from which emanated large-scale lobate flows, representing a viable emplacement mechanism to produce a flood rhyolite province with volumetrically insignificant basalts and andesites
Summary
The viscosity of silicate magmas is a critical control on the rate and scale of planetary differentiation, material and heat transfer, and crustal growth (Reese et al, 1998; Zaranek and Parmentier, 2004). The viscosity of natural silicate melts ranges over more than 10 orders of magnitude (Cashman et al, 1998; Romano et al, 2001) and represents a complex interplay between temperature and compositional factors These include silica content, crystallite content, alkali/Al ratio, water content, halogen concentration and oxygen fugacity (Dingwell et al, 1985, 1988, 1993; Dingwell and Virgo, 1988; Giordano et al, 2004b; Mysen and Virgo, 1989; Mysen, 1990; Webster and De Vivo, 2002; Webster and Rebbert, 1998). Giordano et al, 2008) which incorporate a large number of variables for felsic compositions allows quantitative calculation of magma intensive parameters, and more rigorous testing of geological and geophysical observations In this contribution we apply a combination of aeromagnetic and physicochemical analysis to the Mesoproterozoic (1592 ± 2 Ma; Fanning et al, 1988) upper Gawler Range Volcanics, South Australia. We demonstrate that a large felsic magmatic system can behave like a mafic large igneous province given an appropriate combination of viscositylowering factors, notably eruption temperature and halogen content
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