Silicic ignimbrites within the Costa Rican volcanic front: evidence for the formation of continental crust

The origin of silicic magmas (greater than 65% silica) in areas without old, evolved continental crust has long been an enigma. In oceanic arcs, silicic volcanic deposits are not common, which has generally been attributed to subduction-related magmas rising, without stalling or fractionating, through a relatively thin, mafic oceanic crust. The volcanic arc in Costa Rica is built on a thick (∼40 km) oceanic plateau, and extensive silicic magmatism, dominated by silicic ignimbrites, has occurred in the region in the absence of old, evolved continental crust. The silicic deposits display the common large ion lithophile element (LILE) enrichment and high field strength element (HFSE) depletion observed in magmas generated by subduction processes, which are not characteristic of melts generated from oceanic plateaus. Key trace element ratios in the ignimbrites vary with position along the volcanic front and this variation is similar to that of the volcanic front lavas. Ignimbrites contain unique cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit. We propose that the Caribbean oceanic plateau in Costa Rica is being converted to continental crust, in large part, by the emplacement of silicic magmas in this area. The origin of these silicic magmas is due to reprocessing, juvenile, mantle derived, subduction related magmas that have ponded in the crust.