Petrology of the Rabaul Caldera area, Papua New Guinea

Abstract

The volcanic rocks erupted in the Rabaul area mostly belong to a calc-alkaline association here designated the “main series”. They range from high-alumina basalt (minimum SiO 2 = 48.4%) through medium-K basaltic andesite, to high-K andesite, dacite and rhyodacite (maximum SiO 2 = 69.3%). Rare sodic rhyolites (SiO 2 ≈ 73–75%) are mineralogically and geochemically distinct from the main-series volcanics. The rhyolites contain quartz and hornblende, whereas the main-series volcanics have a quartzfree, anhydrous mineralogy, even in the most siliceous rhyodacites. Amphibole and biotite which occur rarely in dacite are deuteric. Major and trace elements in the main-series rocks show coherent, continuous variation trends against SiO 2. Rhyolite analyses consistently depart from these trends, particularly for the incompatible elements K, Rb, Ba, Zr, Y and REE. Leastsquares mass-balance calculations 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. Rhyolite cannot be derived from main-series magma by crystal fractionation. Geochemical scatter in the main series may be caused by co-genetic magma 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 melting in the mantle wedge is assumed, while rhyolite, which has an inappropriate geochemistry for a partial melt from subducted lithosphere is thought to be a product of partial melting of high-alumina basalt underplating the crust. Volcanic hazard is related to magma composition, with dacite (SiO 2 > 65%) and rhyolite capable of producing ignimbrite being the most dangerous. Changes in the style of Rabaul volcanicity throughout the life of the caldera are not well documented, but progressive weakening of the faulted substructure may now be allowing more primitive basalts and less-fractionated intermediate magmas to reach the surface more freely.