Elemental, Hf–Nd isotopic and geochronological constraints on an island arc sequence associated with the Cretaceous Caribbean plateau: Bonaire, Dutch Antilles

On the Caribbean margins, Upper Cretaceous oceanic plateaux fragments are juxtaposed with island arc fragments of a similar age; until this study, the relationship between them was unknown. This work represents the first detailed study of one such island arc sequence, the Bonaire Washikemba Formation (BWF). These rocks display typical arc-like trace element and Hf–Nd isotopic characteristics (negative Nb and Ta anomalies, εHf of +12 to +14, εNd of +6.5 to +8). They show no indication for the involvement of oceanic plateau material in their source. This is confirmed by binary mixing hyperbolae, which indicate that the Hf–Nd isotopic composition can be modelled by mixing of <1% of Pacific pelagic sediment with average Pacific MORB. 87Sr/86Sr data for apatite mineral separates show significantly lower isotopic ratios than the whole rock, signifying that whole-rock Sr isotope ratios have been affected by alteration processes. New 40Ar–39Ar ages indicate that the Formation is at least 96±4 Ma, which is older than the main phase of the Caribbean plateau (88–91 Ma). Together, the data presented in this study suggest that the Bonaire Washikemba Formation is part of an intra-oceanic arc unrelated to the Carribean plateau, but the Carribean plateau is probably indirectly responsible for the transport and ultimate preservation of this arc sequence.