Mantle dynamics, magma and crust formation in an oceanic island arc - back-arc system: a case study from the Tonga arc and Valu Fa Ridge

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Subduction zones are important regions of the Earth where crustal material is recycled into the mantle and where crust is formed that may be a precursor for continents. Oceanic subduction systems often consist of volcanoes at the island arc front as well as in extensional back-arc rifts and thus provide insights into magma formation and mantle wedge dynamics. Here we suggest studying the petrology and geochemistry of abundant volcanism in an about 250 km long section of the southern Tonga island arc and the 40 km distant Valu Fa back-arc ridge as well as of numerous off-axis volcanoes between the two structures. The abundance of young volcanoes that have been sampled makes this region an ideal natural laboratory for the study of magmatic processes at subduction zones. We propose the analysis of a large sample set of some 200 back-arc and 300 arc and off-axis samples using modern geochemical methods to generate a data set of major and trace elements as well as Sr-Nd-Pb-Hf-O-Th-U-Ra isotopes on glasses, whole rocks and minerals. On the basis of these data we will be able to define 1) the mantle and melt flow in this region, 2) the melting, mantle depletion and mixing processes between arc front and back-arc magmas, and 3) the variation of magma evolution and crust formation in the different settings.