A REVIEW OF MELT, FLUID AND SOLID-PHASE TRANSFERS IN THE NEW CALEDONIA FOREARC OPHIOLITE

Abstract

The Eocene Peridotite Nappe of New Caledonia is a natural laboratory for forearc ophiolite processes and their impact on mantle rocks. The ophiolite includes highly refractory harzburgite-dunite association (up to 2500 m thick) with minor lherzolites, overlain by mafic and ultramafic cumulates forming a proto-arc lower crust. The mafic-ultramafic association was obducted during the latest stages of Eocene subduction (̴ 34 Ma). Harzburgites, which constitute most of the ophiolite, are highly residual and preserve evidence for hydrous melting superimposed on an earlier anhydrous melting event. Their broad isotopic variability further reflects the evolution of an ultra-depleted fore-arc mantle, variably overprinted by fluid and melt inputs during Eocene subduction. Combined mineralogical, geochemical, and Sr–Nd–Pb isotope data indicate melting accompanied by syn- to post-melting metasomatism driven by slab-derived fluids and melts. Lherzolites, restricted to the northern massifs, may represent the remnants of a pre-Eocene marginal basin located to the east of New Caledonia/Norfolk Ridge. These moderately depleted rocks show evidence for anhydrous melting and post-melting re-enrichment. The ophiolite is crosscut by a diverse suite of pre-obduction dikes (55–50 Ma), generated mainly from adakitic slab melts that variably interacted with mantle-wedge peridotites, producing secondary melts, which differentiated into boninite-series felsic dikes and rare clinoenstatite boninites. Boninite-like and highly depleted tholeiitic melts interacted with harzburgites, producing a thick dunite transition zone and associated cumulate lenses composed of wehrlites, websterites, and gabbronorites. The occurrence of diverse xenocrysts (rutile, zircon, low ¹³C diamond and moissanite) in unserpentinized peridotites and chromite pods documents fluid-driven solid-phase transfer from subducted sediments into the mantle wedge peridotites. At lower temperatures, slab-derived hydrous fluids circulation led to the serpentinization of the mantle wedge peridotites, up to 100% near the base of the ophiolite. The fractures in already serpentinized peridotites were locally infiltrated by Ca(-Sr)-rich hydrous fluids derived from the leaching of felsic dikes and reactivated in a transcurrent regime. Syntectonic Ca metasomatism resulted in tremolite formation, then antigorite+tremolite and finally antigorite-bearing crack seals with the exhaustion of Ca-rich fluids. At the end of the episode of forearc magmatism (50-47 Ma), island-arc tholeiitic dikes associated with slab steepening and mantle upwelling intruded the already cooled forearc peridotites. Subsequent evolution relates to relates to post-obduction supergene evolution (33 Ma–present), while serpentinization in deep parts of the ophiolite is still moderately active and marked by hydrogen seepage in hyperalkaline springs. Overall, the New Caledonia ophiolite records a polyphase evolution of the forearc mantle, documenting successive melting events and pervasive metasomatic overprint driven by fluid, melt and solid phase transfers from the slab to the mantle wedge and within the mantle wedge itself.