GEOCHEMISTRY OF LAVAS FROM SUMACO VOLCANO; A STEP TOWARD RESOLVING THE SLAB-MELTING DEBATE IN ECUADOR (Invited Presentation)

The Northern Volcanic Zone (NVZ) of Ecuador has been the focus of slab melting debates for over a decade due to several factors, including ambiguous geochemical signatures and lack of seismic data that allow for various interpretations and petrogenetic models. A recent study of unique rear-arc phonolitic rocks from Sumaco Volcano adds an important piece of data to the slab melting discussion. Combined with geophysical data and models of slab-surface topography, these data are the basis of a petrogenetic model that includes dehydration melting, decompression melting and a possible role for melting of the subducting slab. Further refinement of this model may help to resolve the long-lived debate regarding the role of slab melting in the NVZ of Ecuador.

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.