Mingled silicic lavas in the Mesoproterozoic Gawler Range Volcanics, South Australia

Abstract

Interlayered and mingled plagioclase rhyolite, vesicular rhyolite and quartz rhyolite lithofacies occur in a unit of the Mesoproterozoic Gawler Range Volcanics in the southern Gawler Ranges, South Australia. The three rhyolite lithofacies have distinctive phenocryst assemblages, groundmass textures and compositions. Mingling styles include isolated lenses or blobs of one lithofacies surrounded by another lithofacies, mm- to m-scale compositional flow banding, and swirled and contorted combinations of lithofacies. Areas where the plagioclase rhyolite, vesicular rhyolite or quartz rhyolite lithofacies are volumetrically dominant have been mapped. In all sections, plagioclase rhyolite occurs at the base and is succeeded by variably mingled plagioclase rhyolite, vesicular rhyolite, and quartz rhyolite. Relatively homogeneous quartz rhyolite dominates the upper parts. The compositionally heterogeneous interval is ∼100–200 m thick and extends ∼25 km along strike. It is the topmost part of a far more extensive (∼180 km strike length) and thicker (∼350 m) plagioclase rhyolite unit (Eucarro Dacite) that is relatively homogeneous. All three rhyolite types are evenly porphyritic and either massive or flow banded. The compositional flow banding and mingling textures imply that the entire unit was emplaced as lava, and also that the eruption style was fundamentally effusive or very weakly explosive (fountaining). Geochemical data suggest that three rhyolitic magmas existed in a magma chamber that was compositionally heterogeneous. After a large volume of plagioclase rhyolite magma had been withdrawn, quartz rhyolite and/or vesicular rhyolite were entrained into the flow. Mingling occurred during laminar flow in the conduit and continued during extrusion, resulting in compositional flow banding and more irregular combinations.