TECTONIC AND VOLCANIC INTERPLAYS IN EARTH’S LARGEST AND ONLY KNOWN ACTIVE BONINITE VOLCANO PROVINCE

Boninite (high Mg, low Ti andesite) volcanism has long been associated with the early stages of oceanic subduction zone magmatism on Earth. Work by ourselves and others indicates that the NE Lau Basin hosts a large (roughly 20x20 km) province of young boninite volcanoes in the rear arc region, near the plate boundary at the northern extreme of the basin, as well as older (up to 2 Ma) boninite occurrences in the nearby forearc. The youngest boninite volcanoes are the Mata seamounts, which include recently active West Mata Seamount and 8 other elongate volcanoes that are aligned sub-parallel to other ridges and tectonic lineaments in the area; these do not align with the regional relative or absolute plate motion vectors. Lack of sediment cover and generally youthful morphologies provide extensive outcrops for geological, structural, and petrological studies, from which we infer that some or all of these volcanic structures may be monogenetic. Bathymetric mapping and sampling by dredge and ROV on four research expeditions since 2009 by ourselves and colleagues indicates a surprising diversity of boninite and closely related high-Mg andesite lithologies in the Mata volcano group, having major element compositions that are similar to other boninite localities in the western Pacific (for example, the Izu-Bonin-Mariana system), but that display significant trace element and isotopic variability. These observations indicate a common process for boninite petrogenesis, with a range or source compositions and melting conditions. The arrangement of volcanoes within a basin near the plate boundary, the number, size and spacing of volcanoes, and the presence of fault block structures all suggest a tectonic control on the locus of boninite volcanism in the area. These do not appear to require subduction initiation per se, but probably reflect analogous phenomena that occur during subduction boundary renewal due to migration and frequent rearrangement of the boundaries along microplates in this fastest-opening back arc basin on Earth. Rapid migration between different volcanic centers appears to facilitate the widespread eruption of boninite magmas in the area. The implications of these observations for the relationship of boninite volcanism and subduction initiation in general will be discussed.