Widespread arc tholeiitic and calc-alkaline volcanism characterized the geologic evolution of western Panama and southeastern Costa Rica in middle to late Miocene and Quaternary time. Volcanism was and is clearly associated with subduction of Nazca lithosphere. Geochemical data suggest that formation of mantle-wedge-derived melts and their differentiated products predominated during Miocene time, whereas slab-derived melts were generated in the Quaternary. This difference in magma chemistry is attributed to a change in the thermal regime of the Nazca lithosphere, which may be related to emplacement of the Galapagos plume.

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