The microcrystalline to medium-grained plagioclase-clinopyroxene igneous units cored during Legs 170 and 205 are low- to medium-K subalkaline tholeiites; trace element systematics distinguish two geochemical groups that are also stratigraphically separated as Subunits 4A and 4B. Models of mixing between regional geochemical end-members indicate that these sills are 50%–70% enriched compared to depleted MORB-source mantle. Models suggest that small differences in degrees of partial melting (2.5%–3.5% vs. 5%–7%) can explain the distinct geochemical groups. Geochemical variability in the lower Subunit 4B is consistent with modest amounts of fractional crystallization and/or crystal accumulation.
Within the context of the complex regional tectonic and volcanic history of the Cocos plate, the igneous complex likely represents melts of upper mantle that was previously close to, but not above, the Galápagos hotspot. Fractures, ridge jumps, and abandoned spreading centers on the southern Cocos plate could be related to proximity of the plume (Barckhausen et al., 2001). These lithospheric weaknesses may have facilitated distal reach of material overprinted by the plume. This material was likely melted subsequent to the time of enrichment at intermediate depths less than those of garnet stability (i.e., <60 km). A change in tectonic stresses associated with subduction of the irregular lithosphere of the southern Cocos plate may have facilitated local decompression melting and emplacement.