Abstract

Thumb Butte is an isolated outcrop of volcanic rock that forms a prominent landmark on the southwestern edge of Prescott, AZ. Underlain by Proterozoic granodiorite and surrounded by seemingly unrelated basaltic flows of low viscosity and great areal extent, Thumb Butte’s geological and petrogenetic affinities have been uncertain. A goal of this study was to determine these relationships, through field study, petrology/geochemistry and determination of its age. A groundmass concentrate yielded an ⁴⁰Ar/³⁹Ar age of 14.79 ± 0.05 Ma. Geochemical analysis shows Thumb Butte to be a latite, with exceptionally high levels of Sr and Ba. Its alkaline chemistry and distinctive trace-element signature, as well as its mid-Miocene age, seem to place Thumb Butte securely within the Hickey Formation, a suite of largely basaltic volcanic rocks erupted in Arizona’s Transition Zone between roughly 15 and 10 Ma. Hickey Formation volcanic rocks typically exhibit compositional trends that start alkaline and intermediate in composition and become tholeiitic and basic through time. Further studies are needed to confirm the hypothesis that Thumb Butte is most likely an early phase in an eruptive cycle that became more typically basaltic through time in this area. Thumb Butte’s age and location fit the temporal trend of northeastward volcanic migration across the Transition Zone onto the Colorado Plateau.