Intra-oceanic subduction initiation recorded by the metamorphic sole of the New Caledonia ophiolite: petrological, structural and age constraints

<p>Subduction initiation is commonly identified as a major enigma in plate tectonics. Attention to subduction initiation is growing in the community, as is our understanding of the sequences of geologic events that precede and postdate this critical stage of the Wilson cycle. Nevertheless, the direct triggers of subduction initiation and their regional to global consequences remain uncertain. The New Caledonia ophiolite has formed in a supra-subduction zone setting in the vicinity of an active spreading centre. The metamorphic sole, which represents the ancient subduction interface, is locally preserved beneath the ophiolite. Unravelling its tectono-metamorphic record is essential in order to determine the timing of subduction initiation and the tectonic processes operating at the plate interface during the early stages of subduction.  We have sampled and studied amphibole-bearing rocks of the metamorphic sole that crop out in three newly found and three previously known localities that are scattered across the island (160 km * 50 km in size). The amphibolites form laterally discontinuous meter-size lenses that crop out within or beneath the serpentinite sole at the base of the ophiolite nappe.  Preliminary U-Pb zircon ID-TIMS geochronology yields a crystallization age of 56±1 Ma in agreement, but with a narrower timespan compared to previously published data (Cluzel et al., 2012).  We use whole-rock geochemistry, mineral chemistry and thermodynamic modelling to constrain the Pressure-Temperature-time history of the amphibolites.  Microstructural data such as dominant deformation mechanisms, crystallographic preferred orientations, grain size distributions determined by EBSD allow to constrain the deformation processes and rheological behavior of the amphibolites during subduction infancy.</p><p> </p><p>Cluzel, D., Jourdan, F., Meffre, S., Maurizot, P., and Lesimple, S., 2012. The metamorphic sole of New Caledonia ophiolite: 40Ar/39Ar, U-Pb, and geochemical evidence for subduction inception at a spreading ridge. Tectonics, VOL. 31, TC3016, doi:10.1029/2011TC003085.</p>