Steep REE patterns and enriched Pb isotopes in southern Central American arc magmas: Evidence for forearc subduction erosion?

Abstract

The appearance of adakitic magmas with steep rare earth element (REE) patterns in southern Costa Rica and Panama at ∼4 Ma coincides with the collision of the Cocos Ridge and the inception of slab shallowing along the margin. Distinctly higher 206Pb/204Pb and 208Pb/204Pb ratios in these adakitic lavas than in older Miocene lavas suggest that components enriched in radiogenic Pb also entered the mantle magma source at ∼4 Ma. Published Pb‐isotopic data for Central American arc lavas show that a similar radiogenic component is not present in lavas farther north and that maxima in post‐Miocene 206Pb/204Pb and 208Pb/204Pb ratios occur in central Costa Rica and western Panama. Cretaceous and early Tertiary ophiolites in the forearc, whose origins have been linked to the Galápagos hot spot, show a similar spatial pattern in Pb isotopic ratios. The incorporation of ophiolitic forearc crust into the mantle wedge by forearc subduction erosion can explain the along‐arc spatial and temporal pattern of Pb‐isotopic ratios in southern Central American arc lavas. Partial melting of crust removed from the base of the forearc and subjected to high‐pressure metamorphism in the subduction channel provides an explanation for the steep adakitic REE patterns in some Costa Rican and Panamanian arc lavas.