A reappraisal of the Poya Terrane (New Caledonia): accreted late Cretaceous-Paleocene marginal basin upper crust, passive margin sediments, and early eocene E-MORB sill complex

The Poya Terrane of New Caledonia is a composite lithotectonic unit made of (i) Campanian-Paleocene enriched mid-ocean ridge basalt (E-MORB) and BABB-type (back-arc basin basalt) basalts and abyssal argillite (Poya Terrane basalts) and (ii) Coniacian-Santonian sandstone, turbidites, and abyssal argillite (Kone Facies) intruded by early Eocene E-MORB sills. Remapping reveals that the Kone Facies is more extensive than previously thought. Field data, petrography, and U-Pb geochronology of detrital zircons show that Kone Facies sediments have the same provenance as coeval autochthonous sediments (Formation a Charbon), albeit with more abundant contemporaneous zircons. They accumulated on the eastern continental slope of the Norfolk Ridge and eventually mixed with abyssal argillite. Temporally, sill emplacement is related to subduction inception at circa 56 Ma, thus suggesting a possible genetic link. We postulate that either (i) E-MORB intrusion was related to oblique extension and thinning of the down going plate or, alternatively, (ii) the “enriched” (off axis?) partial melt zone of the ancient ridge swept the lower plate continentward, generating E-MORB dikes in the upper marginal basin crust, and sills in passive margin sediments before it became extinct. Thereafter, sliced marginal basin upper crust, passive margin sediments, and associated dolerite sills were obliquely accreted to the fore-arc region, and in the NE part of the terrane, subducted and recrystallized into the blueschist facies. The Poya Terrane was eventually thrust onto the Norfolk Ridge when the latter reached the subduction zone and debris from the thrust sheet fed middle to late Eocene syntectonic basins. At the same time, mafic portions of the Poya Terrane were subducted at depth where they recrystallized into the eclogite facies, mixed with serpentinite to form the Pouebo mélange, and, finally, were exhumed in the fore-arc region. Finally, late Oligocene faulting and hydrothermal events overprinted the NE part of the terrane in probable connection with postobduction granitoid emplacement.