Sections through an oceanic plateau are preserved in tectonic slices in
the Western Cordillera of Ecuador (South America). The San Juan section
is a sequence of mafic-ultramafic cumulates. To establish that these
plutonic rocks formed in an oceanic plateau setting, we have developed
criteria that discriminate intrusions of oceanic plateaus from those of
other tectonic settings. The mineralogy and crystallization sequence of
the cumulates are similar to those of intra-plate magmas. Clinopyroxene
predominates throughout, and orthopyroxene is only a minor component.
Rocks of intermediate composition are absent, and hornblende is
restricted to the uppermost massive gabbros within the sequence. The
ultramafic cumulates are very depleted in light rare-earth elements
(LREE), whereas the gabbros have flat or slightly enriched LREE
patterns. The composition of the basaltic liquid in equilibrium with the
peridotite, calculated using olivine compositions and REE contents of
clinopyroxene, contains between 16% and 8% MgO and has a flat REE
pattern. This melt is geochemically similar to other accreted oceanic
plateau basalts, isotropic gabbros, and differentiated sills in western
Ecuador. The Ecuadorian intrusive and extrusive rocks have a narrow
range of epsilonNd(i) (+8 to +5) and have a rather large range of Pb
isotopic ratios. Pb isotope systematics of the San Juan plutonic rocks
and mineral separates lie along a mixing line between the depleted
mantle (DMM) and the enriched-plume end members. This suggests that the
Ecuadorian plutonic rocks generated from the mixing of two mantle
sources, a depleted mid-oceanic ridge basalt (MORB) source and an
enriched one. The latter is characterized by high (Pb-207/Pb-204)(i)
ratios and could reflect a contamination by recycled either lower
continental crust or oceanic pelagic sediments and (or) altered oceanic
crust (enriched mantle type I, EMI). These data suggest that the San
Juan sequence represents the plutonic components of an Early Cretaceous
oceanic plateau, which accreted in the Late Cretaceous to the Ecuadorian
margin.