SHALLOW-LEVEL DIFFERENTIATION PROCESSES RECORDED IN MINERALOGY OF PHONOLITE-SERIES LAVAS AT SUMACO VOLCANO, ECUADOR

Sumaco Volcano is located in the rear-arc of Ecuador and produces phonolite-series alkaline lavas. The most mafic lavas are picrobasalts that contain titanaugite as the primary mineral phase, whereas the most evolved lavas are tephra-phonolites and contain the assemblage titanaugite + plagioclase + haüyne. These mineral assemblages, together with geochemistry indicate fractionation over a range of pressures from 5-25 kbar (14- 75 km) and at a consistently high temperature of 1000-1100C. Minerals record multiple cycles of recharge and mixing accompanied by an increase in fO₂ and sulfur concentration during differentiation. Results from numerical modelling are consistent with 75-80 % crystallization. The unusual geochemistry and mineralogy observed in Sumaco lavas are not present in volcanoes to the west (Antisana) and north (Pan de Azucar, El Reventador), indicating that the magma source is confined to a narrow region beneath this volcano. The trace element data are consistent with a component of dehydration melting however the phonolite-series signature is more consistent with localized, shallow decompression melting than regional melting of deep (160 km) rear-arc mantle. The high temperature and sulfate-saturated conditions at shallow depths suggest that magma ascends rapidly, and the Sumaco lavas reflect high temperature, low viscosity magmas that rise sufficiently fast such that SO₄ does not completely degas from the system but instead stabilizes the sulfate mineral hauyne. The narrow melt column of high temperature magma rises from lower crust depths to an upper magma reservoir that is relatively shallow and where ~30 % of crystallization takes place. The low-viscosity lavas would likely produce extensive lava flows which is consistent with observations of lavas around Sumaco Volcano.