The Whitsunday Volcanic Province, Central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province

Contrary to general belief, not all large igneous provinces (LIPs) are char acterised by rocks of basaltic composition. Silicic-dominated LIPs, such as the Whitsunday Volcanic Province of NE Australia, are being increasingly r ecognised in the rock record. These silicic LIPs are consistent in being: ( 1) volumetrically dominated by ignimbrite; (2) active over prolonged period s (40-50 m.y.), based on available age data; and (3) spatially and temporal ly associated with plate break-up. This silicic dominated LIP, related to t he break-up of eastern continental Gondwana, is also significant for being the source of > 1.4 x 10(6) km(3) of coeval volcanogenic sediment preserved in adjacent sedimentary basins of eastern Australia. The Whitsunday Volcanic Province is volumetrically dominated by medium- to high-grade, dacitic to rhyolitic lithic ignimbrites. Individual ignimbrite units are commonly between 10 and 100 m thick, and the ignimbrite-dominated sequences exceed 1 km in thickness. Coarse lithic lag breccias containing clasts up to 6 m diameter are associated with the ignimbrites in proximal s ections. Pyroclastic surge and fallout deposits, subordinate basaltic to rh yolitic lavas, phreatomagmatic deposits, and locally significant thicknesse s of coarse-grained volcanogenic conglomerate and sandstone are interbedded with the ignimbrites. The volcanic sequences are intruded by gabbro/dolerite to rhyolite dykes (u p to 50 m in width), sills and comagmatic granite. Dyke orientations are pr imarily from NW to NNE. The volcanic sequences are characterised by the int erstratification of proximal/near-vent lithofacies such as rhyolite domes a nd lavas, and basaltic agglomerate, with medial to distal facies of ignimbr ite. The burial of these near-vent lithofacies by ignimbrites, coupled with the paucity of mass wastage products such as debris-flow deposits indicate s a low-relief depositional environment. Furthermore, the volcanic successi on records a temporal change in: (1) eruptive styles; (2) the nature of sou rce vents; and (3) erupted compositions. An early explosive dacitic pyrocla stic phase was succeeded by a later mixed pyroclastic-effusive phase produc ing an essentially bimodal suite of lavas and rhyolitic ignimbrite. From th e nature and distribution of volcanic lithofacies. the volcanic sequences a re interpreted to record the evolution of a multiple vent, low-relief volca nic region, dominated by several large caldera centres. (C) 2000 Elsevier S cience B.V. All rights reserved.