The Talasea Peninsula is situated in the New Guinea island arc complex which represents the tectonically active margin of the Australian continent. The peninsula is composed of a chain of Quaternary composite volcanoes and a caldera, with lavas ranging in composition from basalt to rhyolite, although andesite is the dominant variety. The caldera is roughly elliptical in shape, 10 × 13 km, with a volume of collapse of about 75 km3. Located within the caldera are two fault-bounded blocks which appear to be remnants of the ancestral mountain. Post-collapse volcanic activity is found at Mt. Makalia, an andesitic volcano which has developed in the central part of the caldera. It last erupted about 80 years ago. The other volcanoes of the peninsula are largely andesitic composite cones, some of which almost certainly have been active in the last few hundred years. A large area of the peninsula is composed of acid extrusions with an average thickness of 100 m.

The basalts, andesites, and dacites are often highly porphyntic, containing phenocrysts of plagioclase and diopsidic augite, with olivine in the basic members and hypersthene and a titanomagnetite in the intermediate varieties. The rhyolite and rhyodacite lavas are weakly porphyritic and are characterized by phenocrysts of pyroxene, amphibole, biotite, and quartz. The groundmass of the basic lavas is often mostly glass, but when partly crystalline, it contains plagioclase, hypersthene, and/or pigeonite (usually the latter only), augite, iron-titanium oxides, and accessory tndymite or cristobalite.

The Talasea basic lavas are quartz-normative, low in TiO2, and show moderate absolute iron-enrichment. The series has high K/Rb ratios and is low in K2O in comparison to other New Guinea lava series. Numerical evaluation of crystal fractionation of a possible basaltic parent shows that this process could account for all but the acid lavas of the Talasea series; it is only the dominance of andesite that suggests that the series was generated at a depth where andesite is the low melting composition.

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