GEOCHEMISTRY AND AGE DETERMINATIONS OF LAVA FLOWS IN THE NORTHEASTERN SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

The San Francisco Volcanic Field, located in northern Arizona, is one of several late Cenozoic volcanic fields located along the southern margin of the Colorado Plateau. Intermittent volcanic activity produced over 600 vents beginning in the western portion of the field approximately 6 Ma. Through time, this activity progressed eastward, culminating with the eruption of Sunset Crater (~900 a). In the northeastern portion of the field at Wupatki NM, sporadic volcanic activity over the last million years produced ten distinct lava flows including the Black Point, Citadel, Arrowhead Sink, Red House Basin, and Gem City, Woodhouse Mesa, Wukoki, and Grand Falls flows as well as the Doney Crater cinder cones and flows. Geochemical analyses suggest that these flows were derived from partial melting of an OIB-like parental material. Differences in trace element signatures show that there were at least seven individual volcanic events, each attributed to variable amounts of partial melting and subsequent modification by AFC processes. These melts ascended quickly producing short-lived eruptions.

The ages of five flows were reevaluated using ⁴⁰Ar/³⁹Ar age dating techniques and yielded ages that were significantly younger than older K-Ar ages. This likely due to the presence of excess Ar inherited from incompletely degassed magma from the mantle source region and/or the result of contamination contained in xenocrysts. In western Wupatki, the Black Point flow yielded an age of 0.873 ± 0.008 Ma, two samples from the Citadel Flow yielded ages of 0.85 ± 0.02 Ma and 0.87 ± 0.04 Ma, and the Red House Basin flow yielded an age of 0.89 ± 0.17 Ma. The similarities in geochemisty and age suggest that these flows may represent a single eruptive event. The Arrowhead Sink flow, although in close proximity to the above flows, is much younger (0.61 ± 0.03 Ma). In eastern Wupatki, the Wukoki flow yielded disparate ages, likely the result of significant excess Ar, a problem exacerbated by the younger age of this flow. Thus, the isochron age (0.15 ± 0.07 Ma) is interpreted to represent a maximum age. The Woodhouse Mesa flow yielded a significantly older age, 0.96 ± 0.03 Ma. These age dates, coupled with the field relationships for the younger and more weathered flows, are consistent with seven episodes of volcanic activity at Wupatki.