New constraints on source location and emplacement mechanisms of extensive felsic units in the Gawler Range Volcanics, South Australia

Abstract

Eucarro Rhyolite is one of three voluminous (>675 km3 ) units in the upper part of the Gawler Range Volcanics. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS). The unit has AMS characteristics similar to those of lavas. AMS foliations (Kmax/Kint plane) consistently strike northwest and are very steeply dipping. Seven sections show a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top. Furthermore, the AMS pattern implies outflow from a source located to the southwest of present outcrops. In the remaining section, the AMS pattern implies outflow from the northeast. The opposing flow directions may be delineating the boundary between adjacent lava flow lobes. This study suggests that felsic lavas can have dimensions comparable with those of mafic lavas.