GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF BASALTS IN THE POYA TERRANE, NEW-CALEDONIA

The Norfolk-New Caledonia Ridge represents a continental slice which d rifted away from Australia during the Late Cretaceous breakup of the e astern Gondwana margin. The presence of widespread basaltic rocks bene ath the main ophiolite nappe of New Caledonia has been long known but the origin and the age of the Poya Terrane basalts (PTB herein) remain ed controversial. Recent palaeontologically determined ages date the P TB as Late Cretaceous (Campanian). New geochemical data show that two main discrete groups constitute the PTB: a MORE-like tholeiitic suite, and a more alkaline intra-plate basaltic suite distinguished mainly o n immobile HFSE and REE elements, Furthermore, low epsilon(Nd) and hig h Th/Nb relative to MORE, and weak negative Nb anomalies, reflect limi ted assimilation of continental crust by these otherwise MORE-like tho leiites. Inter-PTB sedimentary rocks all have a pelagic or hemi-pelagi c origin; detrital material originated from the nearby Norfolk-New Cal edonia ridge basement. The PTB form a parautochthonous sheet below the main harzburgitic nappe constituting the New Caledonian ophiolite. Th ey are genetically unrelated to the ophiolite, and are interpreted to be 70-85-Ma-old rift tholeiites formed during of the easternmost conti nental part of Mesozoic Gondwana, and opening the East New Caledonia B asin. The Norfolk-New Caledonia Ridge formed the western passive margi n 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 Bel t of the New Hebrides island arc (Vanuatu). The cessation of rifting o f the eastern Australian margin around 56 Ma was followed by an eastwa rd-directed subduction which produced boninitic melts and its associat ed refractory harzburgitic mantle, in the forearc of the primitive Loy alty-d'Entrecasteaux arc. Following the major Pacific plate motion reo rganization 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 Rid ge. Subsequent collisional tectonism led to detachment of the main New Caledonian harzbugitic nappe from its forearc location in the Loyalty arc, and westward emplacement of this nappe over the PTB nearby alloc hthon. 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. (C) 1998 Elsevier Science B.V. All r ights reserved.