ERUPTIVE HISTORY OF THE SUNSET CRATER VOLCANIC CHAIN, NORTHERN ARIZONA

The San Francisco Volcanic Field, host to over 600 volcanic vents, is one of several late Cenozoic basaltic volcanic fields that lie along the southern margin of the Colorado Plateau. Volcanism began ~6 Ma in the western part of the field near Williams, AZ and has since migrated eastward to just east of Flagstaff, AZ. Most recently (~900 a), eruptive activity along a 10 km SE trending fissure produced a series vents, collectively referred to as the Sunset Crater Volcanic Chain (SCVC). The early stages of the eruption formed Rows of Cones, Gyp Crater, and Vent 512 with one lava flow. Shortly thereafter the eruption became focused on the northern end of the fissure building Sunset Crater Volcano, a 300 m tall cinder cone, and effusing the Bonito and Kana’a lava flows.

The eruptive products are all alkali olivine basalt with Mg numbers ranging from 54 – 59, elevated HFSE, and lower LILE consistent with a moderately evolved mantle derived OIB-like source. The earlier group has slightly higher Ti and Zr suggesting they formed from a smaller amount of partial melt than later erupted material. The amount of crustal contamination in the later melts may be greater as indicated by slightly higher La/Ta ratios. Fractional crystallization of olivine ± clinopyroxene further modified these melt compositions. The absence of a Eu anomaly suggests plagioclase was not involved in fractional crystallization, thus it must have occurred near the base of the crust at pressures greater than ~8 kb.

With the exception of clinopyroxene, mineral compositions are identical across the SCVC. Clinopyroxene, generally diopside, in earlier formed vents is enriched in Na and Al consistent with crystallization from a melt derived from a smaller percentage of partial melting. High Fo contents (87-71) in olivine represent values that are in equilibrium with mantle peridotite suggesting the melt rapidly ascended to the surface without much evolution.

Thus, the SCVC is a complex monogenetic system that resulted from partial melting of an OIB-like mantle source. As the eruption continued, further melting, greater crustal contamination, and fractional crystallization modified the melt composition to produce the later, chemically distinct Sunset Crater. These melts ascended quickly to produce a short-lived eruption that lasted for months to perhaps a few years.