Volume 38: January - December 1997

Issue 6: June 1997

Abstract

Cretaceous basaltic terranes in western Columbia: elemental, chronological and Sr-Nd isotopic constraints on petrogenesis

AC. Kerr, GF. Marriner², J. Tarney¹, A. Nivia³, AD. Saunders¹, MF. Thirlwall² and CW. Sinton
¹Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK and ²Department of Geology, Royal Holloway University of London, Egham, London TW20 0EX, UK³Ingeominas-Regional Pacifico, AA 9724, Cali, Colombia⁴College of Oceanography, Oregon State University, Corvallis, OR 97331, USACorresponding authorPresent address: Graduate School of Oceanography, Rhode Island University, Narragansett, RI 02882, USA
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 [epsilon]Nd values) to the west. The igneous rocks of the Pacific coast (Serrania de Baudo) 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 [epsilon]Nd 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.

Keywords: basalt; Colombia; geochemistry; mantle plume; oceanic plateau

Pages: 677 - 702

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