Insights into the Tectonic Affinities and Fluid Histories of Ultramafic Rocks from the Massif du Sud, Central Chain, and HP/LT Terranes, New Caledonia

The southwest Pacific island of New Caledonia contains the second-largest obducted ophiolite exposure in the world. The large volume of exposed ultramafic rocks and their alteration products (e.g., laterite soils) have been well-studied, particularly for their economic value. Serpentinites of unknown origin, however, exist in separate tectonic domains elsewhere on the island: in numerous meter-to-kilometer-scale shear zones in the northeastern HP/LT complex and interleaved among Central Chain basement terranes in the interior of the island. The provenance of these serpentinites and their relationship to the ultramafic rocks of the Massif du Sud ophiolite has remained enigmatic. The HP/LT complex in New Caledonia is regarded as a preserved paleo-subduction interface, and the occurrence of serpentinites in this terrane has been interpreted to represent incorporation of upper-plate material in the subduction zone (Fitzherbert et al., 2004). Nevertheless, stable isotope evidence and re-interpretation of the tectonic structure of the HP/LT terrane point to a possible subducted plate origin (Spandler et al., 2008; Vitale-Brovarone et al., 2018). Similarly, in the Central Chain, two origins for the serpentinites have been proposed (Maurizot et al., 1985).

We present a cross-terrane textural and geochemical analysis of serpentinites on New Caledonia, discussing the incorporation of these ultramafic lithologies during subduction and exhumation of the HP/LT terrane and during the accretion of the basement terranes. These results add to our understanding of the role of serpentinites as sources and media for fluid and volatile migration, and as rheologically-important components in subduction shear zones.