Eruption Styles and Setting of Late Palaeozoic Silicic Volcanic Centres, New England Orogen, Northeastern New South Wales

Abstract

During the late Palaeozoic history of the New England Orogen, silicic volcanic centres were active at many localities in northeastern New South Wales. Their existence is indicated by ignimbrites, lavas and volcanogenic epiclastic sequences in the Tamworth Belt and the Tablelands Complex. Field data from well-exposed Late Carboniferous, Early Permian and Late Permian records of this volcanism are presented herein, with emphasis on reconstruction of the styles and settings of eruptions. The Currabubula Formation (Late Carboniferous) and the Boggabri Volcanics (Early Permian) are exposed close to the western edge of the northern part of the Tamworth Belt. The Coombadjha Volcanic Complex (Late Permian) is situated in the Tablelands Complex, adjacent to the Demon Fault System. ... It is concluded that this high-aspect ratio ignimbrite was deposited from poorly fluidised, slow-moving pyroclastic flows which were sensitive to topographic control. Models for ignimbrite eruptions suggest that such pyroclastic flows would be generated by relatively low, continuously collapsing eruption fountains. This style of eruption is compatible with other features of the geology of the crystal-rich ignimbrite. In particular, there is no evidence of a plinian phase prior to the formation of pyroclastic flows, and there are no coarse grained lag breccias which might indicate the collapse of a high eruption column. Sustained production of pyroclastic flows followed a much smaller magnitude eruption of compositionally similar ignimbrite and porphyritic lava. The former has a high proportion of lithic fragments, possibly collected during excavation of conduits which were later exploited by the main eruption. ... The Late Permian volcanics of the Coombadjha Volcanic Complex are high K₂0 calc-alkaline dacites and rhyolites. More basic eruptives have not been found in the Complex. Elsewhere in the Late Permian volcanic sequences of New England, there is also a dearth of intermediate and basic calc-alkaline extrusives. In view of the predominance of silicic compositions, the volcanic centres were most probably underlain by continental crust.