Rhyodacite

This felsic volcanic rock is from the Port Stephens area north of Newcastle , Australia. It forms the currently exposed southern extremity of a Carboniferous volcanic arc that is part of the New England Orogen. Mineralogically, it is a rhyodacite, with roughly equal proportions of plagioclase and alkali feldspar, along with quartz and minor altered biotite. All of these phenocrysts are embedded in a brown-coloured volcanic ash groundmass, which looks massive in hand specimen. In thin section, the groundmass displays a prominent eutaxitic texture that reveals the rock is an ignimbrite. The ash is dominated by partly devitrified glass shards and prominent spherulites. A very different looking [rhyodacite vitrophyre](https://skfb.ly/opy9B) from a nearby locality was erupted as a flow - the two have almost identical composition.