Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador Marc Mamberti a,b , Henriette Lapierre b, * , Delphine Bosch c , Etienne Jaillard b,d , Raynald Ethien e , Jean Hernandez a , Mireille Polve ´ f a IMP, Universite ´ de Lausanne, 1015 Lausanne, Switzerland b UMR 5025-CNRS, LGCA, PB 53, 38041 Grenoble Cedex 9, France c UMR 5568-CNRS, Tectonophysique, U.M. II, 34095 Montpellier, France d IRD, CSS1, 209-213 rue Lafayette, 75840 Paris Cedex 10, France e UMR 6524, Universite ´ J. Monnet, 42023 St. Etienne Cedex 02, France f UMR 5563-CNRS, Univ. P. Sabatier, 38 rue des 36 Ponts, 31400 Toulouse, France Received 27 August 2001; accepted 9 September 2002 Abstract The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous ( c 123 Ma) oceanic plateau accreted around 85 –80 Ma (San Juan unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high eNd i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Gala ´pagos HIMU component; their eNd i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower eNd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan – Multitud Unit by higher Pb ratios and lower eNd i . The Ecuadorian and Gorgona 88 –86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92 – 86 Ma Mg-rich basalts of the Caribbean – Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (c 68–65 Ma). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Oceanic plateaus; Colombian– Caribbean; Western Ecuador; Trace element and radiogenic isotopic geochemistry; High-Mg melts 1. Introduction The margins of the Caribbean plate and the western margin of northern South America include accreted oceanic sequences (Gansser, 1973), most of which 0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0024-4937(02)00218-9 * Corresponding author. Tel.: +33-4-7663-5906; fax: +33-4- 7651-4058. E-mail address: henriette.lapierre@ujf-grenoble.fr (H. Lapierre). www.elsevier.com/locate/lithos Lithos 66 (2003) 173 – 199