Cretaceous Basaltic Terranes in Western Colombia: Elemental, Chronologicaland Sr-Nd Isotopic Constraints on Petrogenesis

Abstract

Accreted terranes comprising Mid to Late Cretaceous picrites, basalts and dolerites occur in three north–south trending belts in western Colombia, in the Central Cordillera, Western Cordillera and along the Pacific coast. The geochemistry of these rocks is consistent with an oceanic plateau (plume-related) origin, and they most probably formed in the Pacific as part of the Caribbean oceanic plateau. These igneous rocks display small but significant inter-cordillera variations, being younger and more depleted in incompatible trace element ratios (and with more positive εNd values) to the west. The igneous rocks of the Pacific coast (Serranía de Baudó) are dated at 73–78 Ma (40Ar/39Ar), and those of the Western Cordillera at ∼90 Ma, whereas the volcanics of the Central Cordillera are believed to be older than 100 Ma. Most of the igneous rocks are basaltic, and it is suggested that they have fractionated from picritic primary magmas, generated by partial melting within a hot mantle plume. Variable and positive εNd values reveal that the plume must have been heterogeneous, originating from a mantle source with a long-term history of depletion. Partial melt modelling suggests that the composition of the basalts requires at least some input from a mantle source region containing garnet and that the extent of partial melting required to reproduce the composition of the erupted basalts is of the order of ∼20%. Mixing of melts from different depths, either in the mantle melting column or during fractionation in lithospheric magma chambers, can explain the relative homogeneity of basaltic lavas erupted to form this (and other) oceanic plateaux. The Caribbean–Colombian oceanic plateau may have formed at an oceanic spreading centre, and valuable comparisons can be made between Iceland and the Caribbean–Colombian plateau.