Amphibole-Bearing Inclusions from Boisa Island, Papua New Guinea: Evaluation of the Role of Fractional Crystallization in an Andesitic Volcano

Abstract

Boisa Island is the tip of a Quaternary island-arc volcano. It consists of an older cone made up of porphyritic, amphibole-free, mafic rocks, and two younger cumulodomes of hornblende-bearing high-silica andesite. One of the cumulodomes contains coarse-grained igneous inclusions consisting of amphibole, plagioclase, clinopyroxene, orthopyroxene, olivine, and spinel. Some of these minerals in the andesites themselves have both cognate and xenocrystal components. Both the anhydrous phenocryst assemblage in the basalts and the amphibple-bearing assemblage of the inclusions provide low residuals in least-squares linear-mixing calculations, suggesting that fractionation of either assemblage in the basalts could have given rise to the andesites. However, this is not supported by the results of Rayleigh-fractionation calculations for several incompatible elements. Four possible interpretations are: (1) the Rayleigh-fractionation results are inappropriate and misleading; (2) the andesites were formed by crystal fractionation of a mafic parental magma not represented on Boisa Island; (3) mixing of a fractionated parental magma with an incompatible-element depleted mafic magma (also not represented on Boisa); (4) mixing of mafic crystals represented by the Boisa inclusions with a magma richer in $$SiO_{2}$$ than the most felsic rocks on Boisa. This last interpretation is supported by petrographic evidence, but the origin and nature of the postulated felsic magma remain unclear.