Mingled silicic lavas in the Mesoproterozoic Gawler Range Volcanics, SouthAustralia

Interlayered and mingled plagioclase rhyolite, vesicular rhyolite and quart z rhyolite lithofacies occur in a unit of the Mesoproterozoic Gawler Range Volcanics in the southern Gawler Ranges, South Australia. The three rhyolit e lithofacies have distinctive phenocryst assemblages, groundmass textures and compositions. Mingling styles include isolated lenses or blobs of one l ithofacies 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 an volumetrically dominant have been mapped. In all sections, plagioclase rhyolite occurs at the base and is succeeded by variably mingle d plagioclase rhyolite, vesicular rhyolite, and quartz rhyolite. Relatively homogeneous quartz rhyolite dominates the upper parts. The compositionally heterogeneous interval is similar to 100-200 m thick and extends similar t o 25 km along strike. It is the topmost part of a far more extensive (simil ar to 180 km strike length) and thicker (similar to 350 m) plagioclase rhyo lite unit (Eucarro Dacite) that is relatively homogeneous. All three rhyoli te types are evenly porphyritic and either massive or flow banded. The comp ositional flow banding and mingling textures imply that the entire unit was emplaced as lava, and also that the eruption style was fundamentally effus ive or very weakly explosive (fountaining). Geochemical data suggest that three rhyolitic magmas existed in a magma cha mber that was compositionally heterogeneous. After a large volume of plagio clase rhyolite magma had been withdrawn, quartz rhyolite and/or vesicular r hyolite were entrained into the flow.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. (C) 2000 Elsevier Science B.V . All rights reserved.