Abstract

The Lone Mountain laccolith in the Madison Range of southwestern Montana consists of andesitic adakite that intruded at 49.0±0.2 Ma. Mineral thermobarometry indicates that the magma began to crystallize at pressure-temperature (P-T) conditions of ∼8 kbar and ∼950°C. Hornblendite xenoliths show lower P-T of equilibration (<6 kbar, <910°C). Sr-Nd-Pb isotopic data are consistent with a dominantly Archean source for both the host rock and the xenoliths. Rocks with similar adakitic compositions occur within the dominantly Eocene Absaroka Volcanic Field (AVF). These samples highlight the role of crustal melting in a plagioclase-poor, amphibole ± garnet source in generating some AVF magmas. We conclude that (1) the Lone Mountain adakite is of Eocene age and is part of the AVF, (2) the AVF extends as far west as the Gravelly Range near Virginia City, and (3) although previous studies suggest that movement along the Spanish Peak fault system ended before the intrusion of Lone Mountain adakite in the Late Cretaceous, our revised date shows that the movement may have continued until the Eocene.

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