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 epsilon(Ndi) values (+7.6). Continent-derived metasediments are
depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly.
Foliated Iherzolites, 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 chonctritic 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.