Textures formed during emplacement and cooling of a Palaeoproterozoic, small-volume rhyolitic sill

Abstract

The 52-m-thick rhyolitic sill at Onedin, in northwestern Australia, intruded wet, unconsolidated sediment of the Koongie Park Formation during the Palaeoproterozoic. The sill is composed of five main zones. A thin (1-2 m thick) basal peperitic contact zone with sparse large (61 cm) amygdales and abundant spherulites, grades up over 1 m into the lower zone. The lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric quartz and feldspar and has less than 5% amygdales. In the overlying middle zone (20m thick), amygdales increase in abundance from 5% near the base to 20% near the top. This zone is dominated by spherulitic rhyolite. The 18-m-thick upper zone is complex, with alternating, 10-cm-thick layers containing abundant spherulites, lithophysae, amygdales and perlite. Amygdales make up to 40% of the rock. A pumiceous, 2-3-cm-thick layer occurs at the upper contact. Fluidal and blocky peperite marks the 1-2-m-thick upper contact zone. The five zones formed in response to different cooling rates within the sill. The lower zone experienced the slowest cooling rate, whereas the upper and basal contact zones, with abundant glass, cooled rapidly. There is an overall upward increase in the amygdale abundance. In many cases, early-formed bubbles became nucleation sites for lithophysae. Spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded. Irregularly shaped bubbles formed relatively late in crystalline zones. Textural zones in the rhyolitic sill at Onedin are most similar to those in large-volume rhyolitic lavas, even though it is a small-volume intrusion. This similarity may be related to efficient heat retention beneath covering sediments for the sill, or beneath a thick crust for large-volume lavas. The presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite, and the lack of autobreccia and sub-vertical flow banding near the top, distinguish this rhyolite as a sill rather than a lava.