HR: 0800h
AN: U51A-08    [Abstracts]
TI: New Ages for Gorgona Island, Colombia: Implications for Previous Petrogenetic and Tectonic Models
AU: * Serrano Duran, L
EM: lserrano@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AU: Lopez Martinez, M
EM: marlopez@cicese.mx
AF: Depto. de Geologia, CICESE, Km 104 carr. Tijuana Ensenada, Ensenada, BC , Mexico
AU: Ferrari, L
EM: luca@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AB: The Gorgona Island, located 50 km to the west of the Colombian Pacific coast, is the only known site with Phanerozoic komatiites in the world besides a key element in several reconstruction of the interaction between the Caribbean and the South America Plate. The Gorgona komatiites are part of an igneous complex that also includes picritic basalts and breccias, gabbros and peridotites (dunites and wherlites), and is covered by deformed mid-Eocene and younger underformed marine sediments. Datings of the igneous rocks were only performed on basalts and include an 86 Ma K-Ar age, an 88.9 ± 1.2 Ma weighted mean of four Ar-Ar ages and an 89.2 ± 5.2 Ma Re-Os isochron age from basalts. Gorgona rocks are affected by reverse faulting with a general eastward vergence. The island is the only subaerially exposed part of a NE elongated sliver accreted in a dextral transpressional regime to the South America continental margin between the Late Eocene and the Early Miocene. Petrologic studies found large spread in radiogenic isotopes and incompatible trace element ratios in Gorgona ultramafic rocks, which have been interpreted as requiring at least two different sources of: 1) a depleted mantle responsible for the generation of the komatiites and most basalts, and 2) an enriched mantle responsible for some rarer enriched basalts and picrites. Despite the large compositional and isotopic heterogeneity the most common interpretation is that the Gorgona ultramafic rocks are the product of a single mantle plume, although it has recently proposed that this would be a separate plume from that generating the bulk of the Caribbean plateau at ~90 Ma. Our new study focused on the geochronology of the Gorgona igneous suite as we consider that this tectonically and petrologically complex island is unlike to have such a narrow age range. We attempted to date eight samples of komatiites, basalts and gabbros by Ar-Ar laser step heating. For four of these samples we successfully obtain reliable plateau and/or isochron ages. Only one basaltic sample, located in the western coast, yielded an age comparable with those previously reported in the literature. For two basalts intercalated with komatiites and a gabbro exposed in the north-eastern coast of the island we obtained younger ages, similar to those reported for some mafic and ultramafic rocks along the Pacific coast of Colombia. The two sets of ages for the ultramafic suite of Gorgona also correspond to different petrologic types. The depleted rocks in the eastern coast are younger than the enriched basalts and picrites located in the southern and western part of the island with ages around 90 Ma, suggesting a more complex tectonic evolution with the accretion of at least two different blocks. This eventually questions the "single plume" model for the formation of the Gorgona Island plateau.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 8139 Obduction tectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 9360 South America
SC: Union [U]
MN: 2007 Joint Assembly