MANTLE RE-FERTILIZATION BY ARC-HOTSPOT INTERACTION IN SOUTHERN CENTRAL AMERICA

Most Central American magmas have a typical arc geochemical signature, however magmas in southern Central America (central Costa Rica and Panama) have isotopic and trace element compositions with an OIB affinity, similar to the Galapagos-OIB lavas (e.g., Ba/La<40, La/Yb>10, 206Pb/204Pb>18.8). Our new data for Costa Rica suggest that this signature, unusual for a convergent margin, has a relatively recent origin (Late Miocene ~6 Ma). We also show that there was a transition from typical arc magmas (analogous to the modern Nicaraguan volcanic front) to OIB-like magmas similar to the Galapagos hotspot. The geographic distribution of the Galapagos signature in recent lavas from southern Central America is present landward from the subduction of the Galapagos hotspot tracks (the Seamount Province and the Cocos/Coiba Ridge) at the Middle American Trench. The higher Pb isotopic ratios, relatively lower Sr and Nd isotopic ratios and enriched incompatible-element signature of central Costa Rican magmas can be explained by arc-hotspot interaction. The isotopic ratios of central Costa Rican lavas require the subducting Seamount Province (Northern Galapagos Domain) component, whereas the isotopic ratios of the adakites and alkaline basalts from southern Costa Rica and Panama are in the geochemical range of the subducting Cocos/Coiba Ridge (Central Galapagos Domain). Geological and geochemical evidence collectively indicate that the relatively recent Galapagos-OIB signature in southern Central America represents a geochemical signal from subducting Galapagos hotspot tracks, which started to collide with the margin ~8 Ma ago. The Galapagos hotspot contribution decreases systematically along the volcanic front from central Costa Rica to NW Nicaragua. Geochemical and isotopic modeling suggests that the geochemical variations along the volcanic front (e.g. La/Yb, Ba/La, etc.) reflect the extent and type of metasomatic processes caused by the subducting input not just the degree of partial melting. The higher volcanic volumes in Costa Rica are possibly related to a more fertile metasomatized mantle (enriched veins from melts of the subducting Galapagos hotspot tracks).