PETROLOGY OF THE RABAUL-CALDERA AREA, PAPUA-NEW-GUINEA

The volcanic rocks erupted in the Rabaul area mostly belong to a calc- alkaline association here designated the ''main series'' They range fr om high-alumina basalt (minimum SiO2=48.4%) through medium-K basaltic andesite, to high-K andesite, dacite and rhyodacite (maximum SiO2=69.3 %). Rare sodic rhyolites (SiO2 approximate to 73-75%) are mineralogica lly and geochemically distinct from the main-series volcanics. The rhy olites contain quartz and hornblende, whereas the main-series volcanic s have a quartz-free, anhydrous mineralogy, even in the most siliceous rhyodacites. Amphibole and biotite which occur rarely in dacite are d euteric. Major and trace elements in the main-series rocks show cohere nt, continuous variation trends against SiO2. Rhyolite analyses consis tently depart from these trends, particularly for the incompatible ele ments K, Rb, Ba, Zr, Y and REE. Least-squares mass-balance calculation s for major elements and Rayleigh fractionation computations for trace elements show that the main-series magma compositions can be modelled stepwise as products of a pure crystal fractionation process. Rhyolit e cannot be derived from main-series magma by crystal fractionation. G eochemical scatter in the main series may be caused by co-genetic magm a mixing, as seen in the Malaguna Pyroclastics and Latlat Pyroclastics units. Hybrid mixing between main-series magma and rhyolite is rare, but may account for some anomalous medium-K dacites such as the Kulau Ignimbrite. Derivation of parental high-alumina basalt by partial melt ing in the mantle wedge is assumed, while rhyolite, which has an inapp ropriate geochemistry for a partial melt from subducted lithosphere is thought to be a product of partial melting of high-alumina basalt und erplating the crust. Volcanic hazard is related to magma composition, with dacite (SiO2>65%) and rhyolite capable of producing ignimbrite be ing the most dangerous. Changes in the style of Rabaul volcanicity thr oughout the life of the caldera are not well documented, but progressi ve weakening of the faulted substructure may now be allowing more prim itive basalts and less-fractionated intermediate magmas to reach the s urface more freely.