The Whitsunday Volcanics occupy most of the offshore islands from the Whitsunday Group, southeastwards through the Cumberland and Northumberland Groups, possibly extending to the mainland adjacent to Shoalwater Bay, a distance of 370 km. Some of the islands of this group are granitic, but preliminary Rb-Sr data indicate these to represent Permo-Triassic basement._x000D_
Cretaceous volcanism is interpreted to have occurred during initial rifting prior to the continental breakup of northeastern Australia and subsequent development of the Tasman Basin._x000D_
The volcanics comprise mainly lithic ignimbrites, with local development of inter- bedded surge and bedded tuff, accretionary lapilli tuffs, lithic concentration zones, and lag deposits. Isolated fluidual rhyolite and dacite domes occur. The deposits are considered subaerially emplaced. A Rb-Sr isochron age for volcanism of 110 Ma is determined from new data._x000D_
The total thickness of the volcanics is unknown, but a minimum thickness of 4-5 km on Whitsunday Is. is suggested. Eruptive centres are inferred to be distributed throughout the volcanic zone._x000D_
The volcanics are cut by numerous, dominantly near N-S, dykes ranging from dolerite through to rhyolite in composition._x000D_
Chemically, the volcanics and dykes exhibit a continuous range of compositions from basaltic andesite, through andesite, dacite, to rhyolite, with minor more alkalic compositions. Phenocryst mineralogy in the silicic rocks is mainly plagioclase, augite, and Fe-Ti oxides, with less frequent quartz and hornblende occurrence. Sr and Nd isoptic compositions for volcanics and dykes are close to mantle values. These data are tentatively interpreted in terms of a lower crustal magma arc source separated from the mantle only a geologically short time prior to Cretaceous magmatism.