Geochemistry and tectonic setting of Matakaoa Volcanics (East Coast Allochthon, New Zealand); supra-subduction zone affinity, regional correlations and origin.

Abstract : In north-eastern New Zealand, the Late Cretaceous to Eocene submarine volcanic rocks of the East Coast Allochthon referred to as Matakaoa Volcanics formed a dominantly mafic melange with minor abyssal sediments during the subduction of "oceanic" crust. Accreted and melanged pillow lavas and dolerites display the geochemical and isotopic features of tholeiites with some diversity referred to as midocean ridge basalts, island-arc tholeiites and back-arc basin basalts. The association of supra-subduction tholeiitic magmas, abyssal sediments and polymetallic volcanogenic sulphide deposits is typical of many back-arc basins; therefore, a correlation with the alkaline and transitional basalts of the Hikurangi-Manihiki- Ontong Java Plateau may be ruled out. These petrological features and the fossil ages as well, are closely similar to those of the Tangihua Complex (Northland Allochthon) and allow Matakaoa and Tangihua mafic volcanic rocks, now separated by the modern Havre Trough, to be correlated within one single Late Cretaceous to Eocene 2 basin. The consideration of possible younger ages (Oligocene) is also discussed. The occurrence of Late Cretaceous-Eocene "oceanic" crust with supra-subduction zone affinities to the east of the Norfolk Ridge poses the problem of the intervening southwest-dipping subduction, the missing Late Cretaceous-Eocene arc and their relationship with Southeast Gondwana marginal break-up.