Petrological and structural significance of the Santa Elena-Nicoya ophiolitic complex in Costa Rica and geodynamic implications

Detailed structural and petrological investigations were carried out on the Santa Elena-Nicoya ophiolitic complex in Costa Pica, which is represented by a classic association of mantle tectonites (Santa Elena Peninsula), a cu mulitic gabbroic-plagiogranite sequence, and basaltic volcanics and sills ( Matapalo and Esperanza Units of the Nicoya Peninsula), with discontinuous r adiolarian covers. Despite biostratigraphically different ages (Late Jurass ic for Matapalo and Mid-Late Cretaceous for Esperanza) in both units, the m ajor and trace element data, Sr-Nd isotope systematics, and mineral phase c rystallization order show identical tholeiitic fractionation trends, where the marked Fe-Ti enrichment of the volcanics is reflected in the gabbroic-F e gabbroic-plagiogranite: intrusives which, in turn, represent the cumuliti c counterparts of an essentially analogous magmatic system. Parental basalt ic magmas for the Nicoya complex share geochemical features with both Pacif ic MORE and oceanic plateaus, as indicated by the characteristic flat REE p atterns, Zr/Y and Zr/Nb ratios, and Sr-Nd isotopic data (Sr-87/Sr-86: 0.702 95-0.70323; Nd-144/Nd-143: 0.51309-0.51275). On the other hand, basalts and gabbros from Santa Elena display typical N-MORB affinity, as shown by thei r LREE-depleted patterns. These data, together with the unusual abundance o f Fe-Ti differentiated rocks, suggest that the Santa Elena-Nicoya ophiolite s may have been generated at oceanic spreading centres in a mantle plume re gion, analogous to the present Galapagos ridge/hot spot system. The present fore-are structural setting of the deformed Costa Rica ophiolit es is the result of several tectogenetic events recorded by three main duct ile deformation phases (from Late Cretaceous to Tertiary): the oldest dl re lated to W-E convergent tectonics, followed by d2 related to N-S compressio n, and the youngest d3 related to NE-SW convergence. The oceanic lithospher e represented by the Costa Rica ophiolites may have been generated at the w esternmost termination of the spreading system, leading to the formation of the Caribbean plateau, and later involved in complicated intra-oceanic con vergence 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.