Extreme effusive eruptions: Palaeoflow data on an extensive felsic lava in the Mesoproterozoic Gawler Range Volcanics

Abstract

The Gawler Range Volcanics are the eroded remnants of a Mesoproterozoic dominantly felsic large igneous province. The Eucarro Rhyolite is one of three voluminous (> 675 km 3) units in the upper part of the succession. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS) and petrofabric analysis in order to determine the source location. The AMS foliations (Kmax/Kint plane) consistently strike northwest and most dip steeply. At sites in the western and central parts of the unit, there is a change from steep southwesterly dips near the base, to vertical or northeasterly dips in the middle and near the top. In the same area, Kmax axes plunge to the south or southwest in the lower parts, and to the north in upper parts, of the unit. At sites in the eastern part of the unit, the pattern is reversed: AMS foliations dip northerly near the base and southerly near the top, and Kmax axes plunge northerly near the base and southerly near the top. Such up-section reversals in dip or plunge directions of AMS parameters resemble the patterns shown by other lavas. A dominance of very steep dips and an up-section reversal in dip direction are also expected for flow bands in felsic lavas. Furthermore, the AMS pattern implies a northeasterly flow direction for the western and central parts of the unit, and a southerly flow direction for the eastern part. The opposing flow directions may indicate that the western and central parts of the unit versus the eastern part belong to different flow lobes, or that they came from separate sources. Lineations defined by preferred alignment of elongate phenocryst intersections on gently dipping surfaces show a less consistent pattern and, alone, could not be used to define palaeoflow directions. This study confirms the existence of felsic lavas that have dimensions comparable with the largest known mafic lavas.