The presence of a hotspot near a spreading center can impose large geochemical, morphological, and tectonic anomalies on lithosphere formed at mid-ocean ridges. Differences in the geochemistry of hotspot and spreading center magmas result from differences in source depth and composition. Deconvolving the causes of these geochemical anomalies in areas subject to plume-ridge interaction can help elucidate processes such as mantle source mixing, depth of melting, and tectonic and magmatic processes responsible for the surface expression of plume-ridge interaction. The proximity of the Galápagos hotspot to the Cocos-Nazca spreading center (CNS) makes this region a good natural laboratory for studies of plume-ridge interaction (Fig. F1), as the volume of published results shows (e.g., White et al., 1993; Werner et al., 1999; Hoernle et al., 2000; Harpp and White, 2001; Detrick et al., 2002; Cushman et al., 2004; Harpp et al., 2005). The products of plume-ridge interaction on the Cocos plate are subducted at the Middle American Trench (MAT) and therefore may influence the type of alteration of the subducted oceanic plate as well as the chemical composition of the mantle sources of the volcanic arc lavas (Feigenson et al., 2004).
During Legs 170 and 205 of the Ocean Drilling Program (ODP), the Cocos plate was drilled across the MAT offshore of the Nicoya peninsula, Costa Rica (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). Shipboard major and selected trace element analyses of igneous rocks gathered during Legs 170 and 205 suggested a Galápagos-like geochemistry rather than the expected mid-ocean-ridge basalt (MORB)–like basement above which they lie. This observation was striking, given that the location of these samples is farther north than any other igneous rock overprinted by the Galápagos hotspot. A >180-m-thick mafic igneous complex cored during these legs allows investigation of plume-ridge interaction with the expectation that geochemical variations both within these samples and in relation to regional volcanic products may reflect variations of plume-ridge interaction.
This paper uses new major and trace element analysis and isotopic data to investigate the processes of source mixing, partial melting, and fractional crystallization responsible for the generation of Leg 170 and 205 samples. We subsequently discuss the petrogenesis of the igneous complex in the context of regional tectonics and seafloor volcanism in the evolving plume-ridge system. The role of low-level off-axis volcanism and localized fluid flow within the igneous complex in modifying the shallow crustal thermal structure, in the light of recent work by Fisher et al. (2003), is discussed elsewhere (Dreyer et al., 2005).