THE PETROGENESIS OF GORGONA KOMATIITES, PICRITES AND BASALTS - NEW FIELD, PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

Gorgona Island, Colombia is remarkable not only because it contains th e only Phanerozoic komatiites, but also because it has mafic to ultram afic lavas with a wide range of compositions, from moderately enriched to extremely depleted (relative to Bulk Earth), The komatiite flows a re, in many respects similar to Archaean komatiites; they formed from MgO-rich (18%) liquids and have upper spinifex zones and lower cumulat e zones. The cumulate zones of Archaean komatiites contain many solid grains, in contrast more than 90% of the olivine in the Gorgona cumula tes is highly skeletal. This combined with the fact that the Gorgona c umulate zones are thinner than those in Archaean komatiites, suggests that the komatiite magma became strongly superheated en route to the s urface. The komatiites have trace element contents intermediate betwee n those of the basalts and the ultramafic tuffs. Some basalts have iso tope compositions indicative of long-term enrichment in incompatible e lements, whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion. It is apparent that the plume source region of the Gorgona magmas was markedly heterogeneous, with at least two source components contributing to the observed vari ation in composition. This heterogeneity may have resulted from the in corporation of different components into the plume source, or it may b e the result of complex melting and melt extraction processes during t he ascent of a heterogeneous plume, Despite earlier suggestions that t here may have been a significant age gap between depleted komatiite an d basalt flows and the enriched basalts, new Ar-40-Ar-39 dating of bas alts and gabbros are more consistent with all being generated at 87 Ma during formation of the Caribbean/Colombian plateau, possibly at the Galapagos hotspot.