ABSTRACT

Gorgona island is the only place in the world where young ultramafic (pyroxenitic) komatiitic lava flows are known to occur. The island, which appears to be a small southern remnant of the Colombian Coastal Cordillera, is built up of serpentinised peridotites, gabbros, basalts and associated Palaeogene oceanic sediments. Spinifex texured rocks occur within the basaltic and doleritic complex, closely associated with pillow basalts of tholeiitic composition. During rapid cooling the order of crystallisation was: olivine (Fo_90-91), high Al-calcic pyroxene and Cr-spinel as the major quenched phases; then olivine (Fo_86-82). low Al-calcic pyroxene, plagioclase (An_75-81) and Ti-magnetite as quenched products in the interstitial groundmass. Bulk chemistry: MgO ∼ 16wt.% (Mg/Mg+Fe²⁺ = .74), Cr ∼ 1250ppm, Ni ∼ 700ppm, Sc ∼ 33ppm, Ba ∼ 5 ppm, Zr ∼ 30 ppm, Y ∼ 15 ppm; flat REE pattern (5 to 8 x chondrites) with slight depletion of LREE. The associated basalts and dolerites yield similar pattern, although slightly enriched in LREE. Model calculations incorporating major and trace elements show that fractionation of 20-25% olivine + 0.5% Cr-spinel from the komatiitic liquid could produce olivine tholeiites similar to those of Palaeogene Nazca plate. The unique structural setting of the Gorgona rocks probably results from a rapid uplift of a central part of the Nazca ridge including immature magma chambers.