The eastern part of the Western Cordillera of Ecuador includes fragments
of an Early Cretaceous ( approximate to 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 epsilonNd(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 Galapagos HIMU component; their epsilonNd(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 epsilonNd; 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 epsilonNd(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
CCOR 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 (approximate to 68-65 Ma). (C) 2002
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