TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 45 (No. 1), pp. 57-78, 2011

Petrography and geochemistry of accreted oceanic fragments below the Western Cordillera of Ecuador

Andrea Amórtegui,1 Etienne Jaillard,1 Henriette Lapierre,1  Jean-Emmanuel Martelat,1 Delphine Bosch2 and Francois Bussy3

1Université de Grenoble (UJF), LGCA, UMR 5025, Maison des Géosciences, BP 53, 38041 Grenoble Cedex 09, France
2Géosciences Montpellier, UMR-CNRS 5243, Institut des Sciences de la Terre, de l'Eau et de l'Espace de Montpellier, Place Eugène Bataillon, 34095, France
3University of Lausanne, Institute of Mineralogy and Geochemistry, BSFH-2, CH-1015 Lausanne, France

(Received October 26, 2009; Accepted June 10, 2010)

Abstract: The Western Cordillera of Ecuador consists of Cretaceous crustal fragments of oceanic plateaux and superimposed insular arcs, which were accreted to the northwestern South American margin during the Late Cretaceous and Paleocene. Slices of high-grade metabasites, ultramafic rocks, gabbros and basalts, unmetamorphosed radiolarian cherts and scarce garnet-bearing metasediments were randomly exhumed along Miocene to Recent transcurrent faults crosscutting the Western Cordillera. The basalts show geochemical characteristics of oceanic plateau basalts (flat REE patterns, La/Nb = 0.85). The gabbros differ from the basalts in having lower REE levels, positive Eu anomalies, and negative Nb and Ta anomalies; they are interpreted as resulting from arc magmatism. The amphibolites and banded amphibolites have major and trace element chemistry similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or to cumulate gabbros. The granulite shares with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and εNdi values (+7.6). Continent-derived metasediments are depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly. Foliated lherzolites, melagabbronorites and pyroxenites consist of serpentinized olivine + cpx + opx ± Ca-plagioclase. Lherzolites, melagabbronorites and pyroxenites are LREE depleted with positive Eu anomalies, while the harzburgite displays a U-shaped REE pattern. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chondritic and primitive mantle values). The ultramafic rocks represent fragments of depleted mantle, deformed cpx-rich cumulate, and continental lithospheric mantle or mantle contaminated by subduction-fluid. Except the scarce quartz-rich metasediments, all these rocks likely represent remnants of accreted oceanic crustal fragments and associated depleted mantle. Since these samples were randomly sampled at depth by the fault, we propose that the Western Cordillera and its crustal root are mainly of oceanic nature.
Key words: Ecuador, Western Cordillera, accreted oceanic terranes, exhumation

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