Temperatures and cooling rates recorded by the New Caledonia ophiolite: implications for cooling mechanisms in young forearc sequences.

To unveil how forearc lithosphere cools and re-equilibrates, we carried out a comprehensive geothermometric investigation of the New Caledonia ophiolite, which represents a rare example of proto-arc section generated during subduction infancy. A large dataset, including more than eighty samples (peridotites and mafic-ultramafic intrusives), was considered. Closure temperatures calculated for the lherzolites using slow (TREE-Y) and fast diffusing (TCa-in-Opx, TBKN, TCa-in-Ol, TOl-Sp) geothermometers provide some of the highest values ever documented for ophiolitic peridotites, akin to modern sub-oceanic mantle. Cooling rates deduced from TREE-Y and TBKN yield values of ≈ 10-3 °C/y, similar to those obtained with TCa-in-Ol. These features are consistent with a post-melting history of emplacement, possibly along a transform fault, and thermal re-equilibration via conduction. Cpx-free harzburgites register a high-T evolution, followed by quenching and obduction. The relatively high TCa-in-Ol, TOl-Sp and cooling rates computed from TCa-in-Ol (≈ 10-3 °C/y) are atypical for this geodynamic setting, mirroring the development of an ephemeral subduction system, uplift and emplacement of the Peridotite Nappe. Temperature profiles across the crust-mantle transect point to high closure temperatures, with limited variations with depth. These results are indicative of injection and crystallization of non-cogenetic magma batches in the forearc lithosphere, followed by thermal re-equilibration at rates of ≈ 10-4-10-3 °C/y. Our study shows that the thermal conditions recorded by forearc sequences are intimately related to specific areal processes and previous lithospheric evolution. Thus, detailed sampling and exhaustive knowledge of the geological background are critical to unravel the cooling mechanisms in this geodynamic setting.