GEOCHEMISTRY OF LAVAS FROM SUMACO VOLCANO; A STEP TOWARD RESOLVING THE SLAB-MELTING DEBATE IN ECUADOR (Invited Presentation)
The rear-arc of Ecuador is defined largely by three volcanoes; El Reventador, Pan de Azucar and Sumaco. El Reventador produces calc-alkaline lavas, whereas to the south Sumaco Volcano produces phonolitic lavas that are more alkaline than any lavas produced in a rear-arc setting. Trace element and isotope characteristics of the rear-arc volcanoes are consistent with a decrease in water content from north to south. For example, Sumaco rocks have the highest 230Th excesses of 3-15 %, compared to 2-10% at Reventador and 4 % at Pan de Azucar. Lavas from Sumaco Volcano also have the highest reported Sr concentrations (4000 ppm) in the NVZ, reflecting an addition of a component with at least 300 ppm to the mantle wedge. The high Sr concentration is correlated with high 143Nd/144Nd values that are consistent with an enriched mantle source component similar to Galapagos OIB and potentially due to decompression melting of upwelling, enriched mantle.
Decompression melting is not typically associated with arc settings, however the signature of the Galapagos plume is apparent in Central American lavas and decompression melting certainly plays a role in the formation of rear-arc basins. The geophysical data in Ecuador show that beneath Sumaco Volcano the slab dips steeply to the north due to the flat-slab section of the Nazca plate beneath Peru. One interpretation is that the slab is thinned and has formed a slab window that potentially allows hot mantle to well up through narrow openings in the slab, triggering decompression melting and the formation of alkaline lavas at Sumaco.