Beyond zircon fingerprinting: Zircon and TiO2 polymorphs constrain genealogy and evolution of the New Caledonian ophiolite

Abstract

Ophiolitic mantle rocks play a crucial role in understanding deep geochemical cycling and the transfer of components associated with slab-mantle interactions. Geochemical and geochronological and geochemical analysis of zircon and TiO2 polymorphs (e.g., rutile) present within mantle rocks of the Eocene New Caledonian ophiolite reveal discrete magmatic and xenocrystic populations amongst harzburgite from Me Maoya and chromitite from Tiébaghi massifs. Most mineral grains examined have a xenocrystic origin and are concentrated particularly in the harzburgite. Source tracking involved the use of UPb dating and trace element analysis. They are inferred to have been recycled from the northern end of the Norfolk Ridge that formed the leading edge of a thin continental slab of Gondwana affinity which had previously rifted from eastern Australia together with other elements of Zealandia. The slab was drawn into an intra-oceanic subduction zone beneath the proto-Loyalty arc immediately before the emplacement of the forearc ophiolite today represented by the New Caledonian Peridotite Nappe. Geochemical and geochronological data of these xenocrystic grains support a scenario in which these minerals were recycled into the mantle wedge at moderate mantle depths (above 60–80 km depth) within the subduction channel. In contrast, Eocene UPb ages of zircon and rutile grains, mostly found within the Tiébaghi chromitite, imply their formation through magmatic processes. These minerals' distinctive geochemical characteristics (e.g., HFSE enrichment in rutile) likely indicate an association with late-stage percolation of relatively enriched fluids and melt. Overlapping ages of the Eocene zircon and rutile grains, ArAr pargasite ages, and Zircon UPb ages of cross-cutting dikes in the Tiébaghi chromitite provide evidence that it cooled at ca. 47–46 Ma. This study demonstrates that mineral geochemistry of mantle rocks, when combined with data from geochronology, mineralogy, and petrology, is critical to enhance our comprehension of forearc mantle wedge evolution. It also highlights the potential for studying recycled solid mineral phases, tracing their thermal evolution, and characterizing potential subducted source terranes.