Discovery of Early Cretaceous rocks in New Caledonia: New geochemical and U-Pb zircon age constraints on the transition from subduction to marginal breakup in the Southwest Pacific

New U-Pb dating of detrital zircon and geochemical features of Permian-Mesozoic arc-derived volcanic rocks and volcaniclastic turbidites (graywackes), when compared with those of the volcanic rocks associated with unconformable Late Cretaceous shallow-water sediments, reveal that subduction in New Caledonia, once thought to be extinct in the Late Jurassic (ca. 150 Ma), was still active at least from ca. 130 to 95 Ma. The accumulation of volcanic arcderived sediments during the late Early Cretaceous suggests that, as in New Zealand, active-margin activity went on for a short time in spite of the assumed subduction jamming by the Hikurangi Plateau at ca. 100 Ma. Meanwhile, the rift-related magmatic activity that preceded the marginal breakup migrated eastward from ca. 130 Ma (130-95 Ma) in eastern Australia, to 110 Ma (110-82 Ma) in New Zealand, and, finally, to ca. 89 Ma (89-83 Ma) in New Caledonia and generated large volumes of silicic magma. In contrast, marginal basins opened synchronously at ca. 83 Ma when the stretched continental crust finally broke out. In general, intraplate and volcanic arc signatures coexisted in Cretaceous syn-rift magmas. Therefore, the Australian marginal breakup appears to be the final effect of continuous southward unzipping of Gondwana that interfered with the subduction-modified mantle wedge of the Mesozoic active margin. The occurrence of lateral flow of the upper asthenospheric mantle due to the rapidly eastwardmigrating Australian plate margin possibly prevented the formation of a volcanic arc at the eastern end of the system. © 2010 by The University of Chicago. All rights reserved.