Geochemical study of ultramafic volcanic and plutonic rocks from Gorgona Island, Colombia: The plumbing system of an oceanic plateau

The only known post-Archacan komatiites are found on Gorgona, a small islan d off the Colombian coast that forms part of the Caribbean oceanic plateau. Mafic and ultramafic intrusions air located in the interior of the island. To establish the relationship between intrusive and extrusive phases of ul tramafic magmatism, and to help understand how an oceanic plateau is constr ucted, we undertook the first petrological and geochemical study of the int rusive rocks. Rare earth clement patterns in gabbros range from almost flat to moderately depleted; in dunites and wehrlites, the depletion is more pr onounced. These patterns fall midway in the range measured, in Gorgona volc anics, whose compositions indicate two distinct to extremely depleted. Nd i sotope compositions indicate two distinct mantle sources, one highly deplet ed, the other less depleted. MgO contents of parental liquids are estimated from olivine compositions at 20-25% in ultramafic lavas, and 12-13% in the intrusives. Petrographic observations and similarities in trace-element co ntents indicate that the two magma types are comagmatic, related through ol ivine fractionation. Modelling of major and trace elements indicates that t he primary ultramafic magmas formed by advanced critical melting at high pr essure in a rising mantle plume. The plumbing system that fed the Gorgona p lateau was complex, being characterized by a series of magma chambers at di fferent crustal levels. Mantle-derived ultramafic liquids either travelled directly to the surface to erupt as komatiite flows, or were trapped in mag ma chambers where they differentiated into basaltic liquid and mafic to ult ramafic cumulates. Gorgona gabbros and peridotites formed in shallow-level examples of these intrusions.