Spherulites, quench fractures and relict perlite in a Late Devonian rhyolite dyke, Queensland, Australia

A Late Devonian rhyolite dyke displays perlitic and other fracture sets, as well as textures generated by crystallisation of the glass. The dyke is less than a metre wide and has sharp contacts with ignimbrite. Although originally glassy, no glass is preserved. Aligned magnetite (after pyroxene?) microlites and trains of small (0.5–1 mm) spherical spherulites crystallised early, at temperatures above the glass transition temperature and before formation of the fracture sets. Long, subplanar fractures oriented perpendicular to the dyke walls extend almost the full dyke width and end by merging with adjacent long fractures. Short, subplanar cross fractures are perpendicular to and terminate at the long fractures. Well-defined perlitic fractures are present within the volumes of rock, generally < 10 × 5 × 5 mm, defined by the long and cross fractures. The geometry of the fracture sets suggests that the long fractures formed first, followed by the cross fractures and finally the perlitic fractures. The long and cross fractures are interpreted to be first- and second-order quench fractures, respectively. The perlitic fractures formed in the closely fractured glass, probably in response to strain inherited from rapid cooling contraction and volume changes associated with low-temperature hydration. Formerly glassy domains now consist of K-feldspar crystals radiating outward from the fractures (similar to axiolitic spherulites) and enclosing areas of polygonal quartz and oligoclase. This assemblage is the result of devitrification of the perlitised glass after initial cooling, most likely promoted by hydration, interaction with groundwater and elevated temperature. Fractures have been accentuated by concentrations of iron oxide deposited during surface weathering.