U–Pb ZIRCON DATING OF POST-OBDUCTION VOLCANIC-ARC GRANITOIDS AND A GRANULITE-FACIES XENOLITH FROM NEW CALEDONIA. INFERENCE ON SOUTHWEST PACIFIC GEODYNAMIC MODELS

Abstract

In New Caledonia, the occurrence of one of the World's largest and best-exposed subduction/ obduction complex is a key point for the understanding of the geodynamic evolution of the whole Southwest Pacific region. Within the ophiolite, pre-and postobduction granitoids intrude the ultramafic allochthon and provide new time constraints for the understanding of obduction processes. At 27.4 Ma, a new East-dipping subduction generated the active margin magmatism along the western coast of the island (Saint-Louis massif). At 24.3 Ma, the eastward shift of the magma activity and slightly different geochemical features (Koum-Borindi massif) was either related to the older slab break-off; or alternatively, due to the eastward migration of the mantle wedge following the collision of the eastern margin of the Low Howe rise. Finally, the occurrence of a granulite-facies xenolith in the Koum-Borindi massif with comparable 24.5 Ma U–Pb zircon age and isotopic features (initial eNd = 5.1) suggests that these evolved magmas were generated within the lithospheric mantle beneath a continental crust of normal thickness. Geochronological evidence for continuous convergence during the Oligocene infers an East-dipping Eocene-Oligocene subduction/obduction system to have existed in the Southwest Pacific from the d'Entrecasteaux zone to the North Island of New Zealand.