The Early Cretaceous volcanics exposed in the southern Molle Group comprise mafic to silicic pyroclastics and lavas of the Proserpine Volcanics, at the eastern edge of the Airlie Block, northeast Queensland. Early Cretaceous volcanism was erupted into rift basins, grabens or troughs, signifying rifting of the east Australian plate margin.
Geological mapping has revealed that basaltic lavas are predominant in the volcanic sequence, showing greater similarities to, and are now considered part of the Proserpine Volcanics. Previously unrecognised tuff cone deposits, volcaniclastic sediments, a debris-flow deposit, scoria deposits, accretionary lapilli and basaltic ash tuffs have also been mapped. The gabbro sill intruding the volcanics is redefined as a 136 m thick lopolithic intrusion.
The volcanic sequence progresses from predominantly explosive eruptive deposits at the base, to mainly effusive volcanism towards the top of the sequence. Silicic ignimbrites are interbedded with basaltic lavas at the top of the sequence, and are crystal-rich (25 to 40 %), containing phenocrystal hornblende and quartz. Phreatomagmatic activity is evident within the project area, forming tuff cone (surge) deposits at the northern end of South Molle Island. Volcaniclastic sediments are interbedded with the surge deposits, possibly representing lacustrine-fill. The volcanic sequence is intruded by thick dolerite dykes (up to 50 m), with the gabbro lopolith representing the most primitive sample, geochemically, analysed from the Whitsunday and Proserpine Volcanics.
The volcanics display a continuous range in chemistry, from doleritesjgabbro through to high-silica rhyolite, with geochemical signatures similar to modern destructiveplate margin volcanics. The spectrum of chemical compositions are interpreted to reflect magma mixing and hybridisation of a within-plate tholeiite source, represented by the gabbro lopolith, and low-silica rhyolites. The Spion Kop Rhyolite represents a fractionated low-silica rhyolite melt.
The southern Molle Group is strongly affected by the tectonic development of the Whitsunday area. Tilting of the volcanics to 50° occurred penecontemporaneously with volcanism, reflecting half graben formation ( Basin and Range-style). Strata are vertical at the western .side of South Molle, reflecting strike-slip faulting of Late Cretaceous to Early Tertiary age.