Undersaturated lavas from Ambittle Island, Papua New Guinea

Abstract

Ambittle is part of a gently arcuate chain of Quaternary volcanic islands east of the Bismarck Archipelago. Lavas include basanities, tephrites, ankaramitic lavas and trachyte. Phenocrysts of olivine, clinopyroxene and plagioclase are found in bauonite along with hauyne. Olivine and less commonly hauyne are absent from tephrite while amphibole and biotite are common. Nodules in the lavas are composed of phenocryst phases and are thought to represent crystal clots or cumulates. Low Ti, high Al, low Ni and Cr and high Sr, Rb mark these lavas as being very different from basanites of intra-continental regions. Suggested parallels are shoshonitic lavas from the Aeolian Islands and feldspathoidal lavas from Indonesia. Although the structure in the region of Ambittle is not well known it seems likely that the magmas were linked to tectonic processes along a subducting plate margin. The presence of basalts of transitional affinity in other islands of the chain suggests a petrochemical link with the undersaturated types. Derivation of undersaturated lavas form partial melts formed in the mantle only seems possible if ol + cpx are fractionated at moderate pressures in order to enrich residual liquids in Al. If the undersaturated lavas are related to a parental transitional tholeiite magma then fractionation processes alone are insufficient to explain their composition and processes of volatile differentiation may have occurred.