Early Devonian subaqueous andesitic volcanism in the New England Fold Belt, eastern Australia

Altered andesites interstratified with are-related sedimentary rocks of the Tamworth Group, New England Fold Belt represent submarine intrusions and extrusions of an Early Devonian volcanic chain. Two emplacement histories are represented: unnamed andesites emplaced as a series of discontinuous intrusions in poorly consolidated, water-saturated sediments of the Bog Hole Formation; and the Copes Creek Andesite emplaced as a submarine volcanic centre in volcaniclastic breccias of the Drik Drik Formation. Andesites in both units are extensively brecciated due to the quenching and fragmentation of magma that came into contact with water. These 'hyaloclastite' breccias are locally characterised by in situ fragmentation textures; angular, splintery fragments; and convex-concave edges of clasts. The andesite intrusions in the Bog Hole Formation form lens-shaped bodies that range from thin (< 180 m), elongate masses that are highly brecciated to thicker bodies (> 300 m) that exhibit only minor brecciation. These variations reflect changing conditions of the host sediment during intrusion and are similar to Neogene submarine volcanic facies described from Central Japan. The remnant volcanic centre in the Drik Drik Formation is represented by a narrow lens of discontinuous, distinctly brecciated masses. The centre formed largely by intrusion into wet sediments, magma locally extruding onto the sea floor and producing abundant hyaloclastic debris. Following large-scale erosion, the volcanic centre was overlain by successive mass-flow units.