Abstract

Detailed structural and petrological investigations were carried out on the Santa Elena-Nicoya ophiolitic complex in Costa Rica, which is represented by a classic association of mantle tectonites (Santa Elena Peninsula), a cumulitic gabbroic-plagiogranite sequence, and basaltic volcanics and sills (Matapalo and Esperanza Units of the Nicoya Peninsula), with discontinuous radiolarian covers. Despite biostratigraphically different ages (Late Jurassic for Matapalo and Mid-Late Cretaceous for Esperanza) in both units, the major and trace element data, Sr-Nd isotope systematics, and mineral phase crystallization order show identical tholeiitic fractionation trends, where the marked Fe-Ti enrichment of the volcanics is reflected in the gabbroic-Fe gabbroic-plagiogranite intrusives which, in turn, represent the cumulitic counterparts of an essentially analogous magmatic system. Parental basaltic magmas for the Nicoya complex share geochemical features with both Pacific MORB and oceanic plateaus, as indicated by the characteristic flat REE patterns, Zr/Y and Zr/Nb ratios, and Sr-Nd isotopic data ( 87 Sr/ 86 Sr: 0.70295-0.70323; 144 Nd/ 143 Nd: 0.51309-0.51275). On the other hand, basalts and gabbros from Santa Elena display typical N-MORB affinity, as shown by their LREE-depleted patterns. These data, together with the unusual abundance of Fe-Ti differentiated rocks, suggest that the Santa Elena-Nicoya ophiolites may have been generated at oceanic spreading centres in a mantle plume region, analogous to the present Galapagos ridge/hot spot system. The present fore-arc structural setting of the deformed Costa Rica ophiolites is the result of several tectogenetic events recorded by three main ductile deformation phases (from Late Cretaceous to Tertiary): the oldest d1 related to W-E convergent tectonics, followed by d2 related to N-S compression, and the youngest d3 related to NE-SW convergence. The oceanic lithosphere represented by the Costa Rica ophiolites may have been generated at the westernmost termination of the spreading system, leading to the formation of the Caribbean plateau, and later involved in complicated intra-oceanic convergence and rotation processes. These resulted in the progressive building and insertion of the Chorotega block (present Costa Rica) between the Maya-Chortis and Choco-South America continental blocks.

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