The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A >675 km3, compositionally zoned lava of Mesoproterozoic age

Abstract

The Eucarro Rhyolite is remarkable in being far more voluminous (>675 km3) and extensive than most felsic lavas (<1 km3). Its dimensions are comparable to the largest known volcanic units (felsic ignimbrites and flood basalts). It is one of several voluminous felsic units within the Mesoprot erozoic (ca. 1592 Ma), Gawler Range Volcanics, an intracontinental, intraplate volcanic province in South Australia and is typical of the voluminous felsic magmatism associated with Laurentia (cf. Belt Supergroup, Midcontinent United States). The lava is 300 m thick, with groundmass textures and lithofacies characteristics consistent with cooling and degassing as a single unit. There are gradational compositional and textural variations throughout its highly elongate distribution (225 km east-west, 74 wt% SiO2). Pyroxene is the dominant ferromagnesian phase, but at the base in the west, primary amphibole and olivine are present. Granitic and mafic clasts are ubiquitous but most abundant (5 vol%) in western and eastern parts of the unit. The main Eucarro magma was compositionally zoned, and several smaller batches of compositionally distinct magma were tapped during the eruption. Concentrations of granitic clasts coincide with changes in bulk composition and may record the rupture of wall rock between nearby but separate magmas. The distribution, subtle lateral variations in composition and texture, and local presence of compositional flow banding along the northern margins suggest that the Eucarro Rhyolite was derived from several source vents located along an east-trending fissure(?) system. Although very widespread, outflow of the Eucarro lava was probably no more than a few tens of kilometers from source vents.