SABALAN VOLCANO, NORTHWEST IRAN: GEOCHEMISTRY AND U-PB ZIRCON GEOCHRONOLOGY

Sabalan Volcano (NW Iran) is an isolated voluminous (4821 m elevation; >800 km³) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (55-66% SiO₂) with high-K calc-alkaline and adakitic signatures. The lavas and pyroclastic deposits erupted in two major cycles which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. Volcanic debris-flow and debris-avalanche deposits in the pre-caldera sequence indicate episodes of sector collapse during the Paleo-Sabalan stage. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. In the Neo-Sabalan stage, three dacitic domes extruded which form the summits of Sabalan (Soltan, Heram and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic. Paleo-Sabalan rocks are trachyandesites with abundant phenocrysts (plagioclase + amphibole + pyroxene + biotite). The Neo-Sabalan rocks are slightly more evolved and include dacites (plagioclase + amphibole ± alkali-feldspar ± quartz). All Sabalan rock types share a common accessory assemblage (oxides + apatite + zircon). Previous K/Ar dating suggested protracted eruptive activity between 5.6 and 1.4 Ma, and a two stage evolution. The ubiquitous presence of zircon crystals has allowed us to utilize high spatial resolution and sensitivity U-Pb geochronology using SIMS. Initial results confirm that the edifice was built in two main stages with maximum ages of 4.5-1.3 Ma and 0.481-0.188 Ma. These stages are separated by a major caldera collapse event at 0.545-0.301 Ma deduced from U-Pb zircon ages of valley-filling ignimbrites. Because pre-eruptive residence of zircon is common, these ages are considered maximum ages for the eruption. These results indicate that eruptions occurred more recently than previously indicated by K/Ar dating. (U-Th)/He zircon dating is underway to quantify zircon residence and eruptive recurrence.