Remnants of ancient Australia in Vanuatu: implications for crustal evolution in island arcs and tectonic development of the southwest Pacific

Abstract

We report new geochemical and geochronological data from igneous rocks of the little studied western belt of the Vanuatu intraoceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place the time of formation of these rocks in the late Eocene to Miocene; therefore, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general matches the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not been found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intraoceanic arc provides an alternative source for the crustal trace element and isotopic signature of island arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intraoceanic arcs.