Geochemistry and age of the Nouméa Basin lavas, New Caledonia: Evidence for Cretaceous subduction beneath the eastern Gondwana margin
Résumé
The Nouméa Basin in New Caledonia is perhaps the best preserved sequence of in-situ Late Cretaceous marine sediments and volcanic rocks in the western Pacific region. Previous tectonic interpretations suggest that the basin formed during a period of large-scale extension between New Caledonia and Antarctica during the break-up of the eastern Gondwana margin. However, new geochemical analyses have identified continental arc signatures in the lavas, suggesting a well-developed Late Cretaceous volcanic arc system active in the New Caledonia sector of the eastern Gondwana margin, possibly extending as far south as New Zealand. There are two distinct suites of lavas in the Nouméa Basin. The older lavas are predominately mafic, low to high-K, and have a calc-alkaline fractionation trend. Chondrite normalised trace element plots show patterns that are light rare earth element (LREE) enriched, and mid-ocean ridge basalt (MORB) normalised trace element plots show enrichment of most incompatible trace elements with discernable negative Nb, Ta and Ti anomalies. Trace element ratios identify a continental arc signature in these lavas which were generated from an N-MORB-like source. Overlying the mafic lavas is a sequence of younger voluminous siliceous, generally subalkaline lavas (+/−88 Ma). These lavas are LREE enriched with slight positive Nb-Ta anomalies and negative Eu and Ti anomalies. The geochemical data indicates these lavas have within plate characteristics with minor continental affinities and an enriched source. We propose that the older mafic lavas were generated during large scale subduction under the eastern Gondwana margin during the Late Cretaceous. Whereas the younger lavas may have been generated during extension; caused by slab roll-back of the subduction system along the Southwest Pacific plate boundary. The presence of fragments of a detached slab in this process would result in lavas chemically similar to those found in the Nouméa Basin, with minor continental characteristics, and generated from an enriched mantle source. What is of fundamental importance is the evidence that the arc system extended from New Caledonia southwards to New Zealand and was likely contemporaneous