Geochemistry and tectonic significance of basalts in the Poya Terrane, New Caledonia

Abstract

The Norfolk-New Caledonia Ridge represents a continental slice which drifted away from Australia during the Late Cretaceous breakup of the eastern Gondwana margin. The presence of widespread basaltic rocks beneath the main ophiolite nappe of New Caledonia has been long known but the origin and the age of the Poya Terrane basalts (PTB herein) remained controversial. Recent palaeontologically determined ages date the PTB as Late Cretaceous (Campanian). New geochemical data show that two main discrete groups constitute the PTB: a MORB-like tholeiitic suite, and a more alkaline intra-plate basaltic suite distinguished mainly on immobile HFSE and REE elements. Furthermore, low ϵNd and high ThNb relative to MORB, and weak negative Nb anomalies, reflect limited assimilation of continental crust by these otherwise MORB-like tholeiites. Inter-PTB sedimentary rocks all have a pelagic or hemi-pelagic origin; detrital material originated from the nearby Norfolk-New Caledonia ridge basement. The PTB form a parautochthonous sheet below the main harzburgitic nappe constituting the New Caledonian ophiolite. They are genetically unrelated to the ophiolite, and are interpreted to be 70–85-Ma-old rift tholeiites formed during of the easternmost continental part of Mesozoic Gondwana, and opening the East New Caledonia Basin. The Norfolk-New Caledonia Ridge formed the western passive margin of this new oceanic basin, but the rifted-off eastern block is less easily identified. It may form part of the basement of the Western Belt of the New Hebrides island arc (Vanuatu). The cessation of rifting of the eastern Australian margin around 56 Ma was followed by an eastward-directed subduction which produced boninitic melts and its associated refractory harzburgitic mantle, in the forearc of the primitive Loyalty-d'Entrecasteaux arc. Following the major Pacific plate motion reorganization around 42 Ma, collision of the Norfolk-New Caledonia Ridge with the forearc region of the intra-oceanic Loyalty-d'Entrecasteaux arc around 40 Ma led first to westward thrusting of the PTB as a slice picked up from the upper crustal section of the colliding Norfolk Ridge. Subsequent collisional tectonism led to detachment of the main New Caledonian harzburgitic nappe from its forearc location in the Loyalty arc, and westward emplacement of this nappe over the PTB nearby allochthon. The presence of parautochthonous sheets of basalts unrelated to immediately overlying forearc-derived, boninite-bearing harzburgitic ophiolites is briefly discussed in the light of two other examples in arc-continent collision settings.