A Miocene basanite peperitic dyke at Stanley, northwestern Tasmania, Australia

A Miocene basanite dyke at Stanley, northwestern Tasmania, Australia, displays well preserved peperite texture. The dyke is 2 m wide and has intruded basaltic breccia ("host sediment"). One contact of the dyke is fluidally shaped, and amoeboid apophyses 10-25 cm long extend into the host sediment, whereas the other contact is characterized by blocky peperite texture comprising tabular to wedge-shaped clasts up to 30 cm across separated by host sediment. The clasts have internal spherical fractures and some show splinter texture. Vesicles are common in the clasts, and those intersected by clast margins have been filled with sediment. The interior of the dyke comprises close-packed blocky peperite consisting of tabular, wedge-shaped and polyhedral clasts tens of centimetres across separated by host sediment. These clasts show well developed jigsaw-fit texture.

The textures and structures in the basanite dyke are inferred to have formed in two stages: an earlier, hotter, apophysis-forming stage and a later, cooler, angular clast-forming stage, both of which occurred during the intrusion of magma into wet, poorly consolidated sediment in a shallow marine environment. During the apophysis-forming stage, the magma had relatively low viscosity and progressively displaced wet sediment. The wet sediment around the dyke was partly fluidized by vaporization of pore water. The angular clast-forming stage reflects a change in the rheological behaviour of the magma from ductile to brittle, most likely in response to decreasing temperature. The chilled parts of the dyke were subject to stress arising from cooling contraction and also from continued, pulsatory movement of hotter, still ductile magma in the interior of the dyke, resulting in brittle fragmentation. Brittle fragmentation was accompanied by movement of host sediment into the newly created open spaces, forming blocky peperite. Spherical fractures, splinter texture and sediment-filled vesicles formed during the angular clast-forming stage. Because the host sediment is texturally identical either side of the dyke, increasing magma viscosity as temperature decreased, combined with pulsatory intrusion, were evidently important in the production of the blocky peperite.