Geochemical Study of Ultramafic Volcanic and Plutonic Rocks from Gorgona Island, Colombia: the Plumbing System of an Oceanic Plateau

The only known post-Archaean komatiites are found on Gorgona,a small island off the Colombian coast that forms part of theCaribbean oceanic plateau. Mafic and ultramafic intrusions arelocated in the interior of the island. To establish the relationshipbetween intrusive and extrusive phases of ultramafic magmatism,and to help understand how an oceanic plateau is constructed,we undertook the first petrological and geochemical study ofthe intrusive rocks. Rare earth element patterns in gabbrosrange from almost flat to moderately depleted; in dunites andwehrlites, the depletion is more pronounced. These patternsfall midway in the range measured in Gorgona volcanics, whosecompositions vary from slightly enriched to extremely depleted.Nd isotope compositions indicate two distinct mantle sources,one highly depleted, the other less depleted. MgO contents ofparental liquids are estimated from olivine compositions at20–25 % in ultramafic lavas, and 12–13% in the intrusives.Petrographic observations and similarities in trace-elementcontents indicate that the two magma types are comagmatic, relatedthrough olivine fractionation. Modelling of major and traceelements indicates that the primary ultramafic magmas formedby advanced critical melting at high pressure in a rising mantleplume. The plumbing system that fed the Gorgona plateau wascomplex, being characterized by a series of magma chambers atdifferent crustal levels. Mantle-derived ultramafic liquidseither travelled directly to the surface to erupt as komatiiteflows, or were trapped in magma chambers where they differentiatedinto basaltic liquid and mafic to ultramafic cumulates. Gorgonagabbros and peridotites formed in shallow-level examples ofthese intrusions. KEY WORDS: Gorgona Island, Colombia; komatiite; mantle melting; oceanic plateau; melt transport