Boninite Volcanism in the Back-Arc: Testing Tectonic and Magmatic Controls at the NE Lau Basin Mata Seamounts
Abstract
The North East Lau Basin is an actively spreading back-arc basin that has erupted a variety of igneous products. These include a series of young seamounts in the northeast of the basin called the Matas, which lay between the transform boundary marking the northern edge of the basin, the Tofua arc to the east, and the northeast Lau spreading center to the west. Sampling so far indicates that all are constructed of boninite pillow lavas except for one seamount which is composed of meimechite; nearly all are high Mg orthopyroxene-clinopyroxene-olivine porphorytic, with somewhat differentiated glass compositions relative to the whole rocks. The Mata volcanoes are geologically young (an eruption was observed at West Mata Volcano in May 2009 (Resing et al., Nat. Geosci, 4, 2011) and several of the northern Mata group have active hydrothermal systems), providing unique opportunities for insights into boninite formation. The Mata seamounts are in a unique setting, very close to the arc, but within the backarc region. The small, possibly monogenetic cones have formed in a small extensional basin along en-echelon fractures trending obliquely away from the subduction trench (Rubin and Embley 2012, AGU meeting). This pattern is in contrast to the arc-parallel lineated northeast Lau spreading center that intersects the E-W arm of the trench within ~25 km west of the Mata basin. We test two possible models for formation of the Matas using geochemical signatures within the lavas. In the first model, volcanism is sourced in part by fluids derived from the actively subducting slab, causing geochemical signatures related to the nearby active arc that decrease with distance from the arc. This model predicts that subduction related geochemical signatures would be greatest in the South where the volcanoes are closer to the downgoing slab. In an alternate model, volcanism is fed by extension-driven upwelling of back-arc mantle, which was enhanced by subduction-derived fluids at some time in the geological past. The proximity of the extensional zone to the arc taps a portion of the mantle wedge between the magmatic arc and the backarc spreading center that does not normally produce eruptive products. In this case, extension may have opened conduits for this material to reach the surface. Thus, the subduction-related geochemical signature would not necessarily be a function of the distance to the subducted slab, and no spatial gradient in subduction-related trace element signatures would be predicted. To test the plausibility of the flux melting and decompression melting models, we will present a comparison of geochemical signatures in Mata lavas sampled from throughout the volcano group on four research expeditions in the past three years. Preliminary XRF and ICPMS trace element data suggests that subduction related signatures, such as elevated Ba/La and Ba/Nb, become stronger towards the south of the chain, closer to the arc. Ongoing work will enlarge the data set and refine these trends to better constrain these tectono-magmatic models for boninite petrogenesis.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T51D2605G
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3001 MARINE GEOLOGY AND GEOPHYSICS / Back-arc basin processes;
- 8413 VOLCANOLOGY / Subduction zone processes