SPHERULITES, QUENCH FRACTURES AND RELICT PERLITE IN A LATE DEVONIAN RHYOLITE DYKE, QUEENSLAND, AUSTRALIA

A Late Devonian rhyolite dyke displays perlitic and other fracture set s, 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 transi tion temperature and before formation of the fracture sets. Long, subp lanar 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 withi n the volumes of rock, generally < 10 x 5 x 5 mm, defined by the long and cross fractures, The geometry of the fracture sets suggests that t he long fractures formed first, followed by the cross fractures and fi nally the perlitic fractures. The long and cross fractures are interpr eted to be first- and second-order quench fractures, respectively. ?he perlitic fractures formed in the closely fractured glass, probably in response to strain inherited from rapid cooling contraction and volum e changes associated with low-temperature hydration. Formerly glassy d omains now consist of K-feldspar crystals radiating outward from the f ractures (similar to axiolitic spherulites) and enclosing areas of pol ygonal quartz and oligoclase. This assemblage is the result of devitri fication of the perlitised glass after initial cooling, most likely pr omoted by hydration, interaction with groundwater and elevated tempera ture, Fractures have been accentuated by concentrations of iron oxide deposited during surface weathering.