Three porphyritic dacite plugs from the western Absaroka volcanic belt in northwestern Wyoming have been studied, and their petrography, chemistry, and ages are treated in terms of the regional igneous geology of the Absaroka volcanic field.
The dacite from the northernmost plug studied, Bunsen Peak, is 47.6 ± 1.9 m.y. old, as dated by the fission-track method on apatite; the Birch Hills dacite is 40.5 ± 2.6 m.y. old, as dated by the fission-track method on apatite; and the dacite from the southernmost plug, Washakie Needles, is 38.8 ± 1.6 m.y. old, as dated by the fission-track method on sphene. It appears from the dates available that the oldest igneous activity in the Absaroka volcanic field occurred at the northwestern end about 53.5 m.y. ago. The activity migrated to the southeast, ending about 38.8 m.y. ago at the Washakie Needles.
The Absaroka volcanic field has been subdivided into two belts. The western belt is composed of normal calc-alkalic igneous rocks, and the eastern belt is composed of potassium-rich rocks. When the available analyses of the province are treated in terms of the system quartz-plagioclase-orthoclase, it becomes apparent that the rocks of the two belts lie on two distinct differentiation trends. The trend for rocks of the western belt is best explained by fractional crystallization of plagioclase from an intermediate magma. The trend for rocks of the eastern belt is best explained by crystallization of both plagioclase and potassium feldspars. The mafic members of the eastern belt rocks are similar to shoshonitic rocks.