Earth and Planetary Sciences ETDs

Publication Date




TI1e Washakie Needles, Bunsen Peak, and Birch Hills dacites are three of many plugs which intrude the Eocene flows and volcaniclastic rocks of the Absaroka-Gallatin volcanic province. The dacites contain phenocrysts of plagioclase (oligoclase to andesine), biotite, and resorbed quartz. The dacites of the Washakie Needles and Bunsen Peak also contain phenocrysts of hornblende. The rocks of Bunsen Peak and the Birch Hills have two feldspar phases (oligoclase to andesine and sanidine) and quartz in the groundmass. The Washakie Needles dacite groundmass is composed of one feldspar (andesine to anorthoclase) and quartz. The fission track ages of the dacites are: Bunsen Peak, 47.6 (+-) 1.9 m.y. (apatite); Birch Hills, 40.5 (+-) 2.6 m.y. (apatite); and the Washakie Needles, 38.8 (+-) 1.6 m.y. (sphene).

A review of the literature concerning the origin of dacites in­dicates that they are part of the calc-alkalic suite and the melts which produce them are probably generated in the lower crust or upper mantle. Resorbed quartz phenocrysts are probably crystallized from a melt under high load pressure. The amount of water-bearing phases, biotite and hornblende, indicates that the melt crystallized under equilibrium water-vapor pressure.

The Absaroka-Gallatin volcanic province is primarily composed of andesite and other members of the calc-alkalic suite. Minor amounts of potash-rich absarokites, shoshonites, and banakites are present. Radio­metric ages of extrusive and intrusive rocks of the province show that the site of igneous activity moved southeastward through Eocene time. Structural, stratigraphic, -and chemical data indicate the province can be divided into at least four subprovinces. The temporal and spatial configurations of the subprovinces suggest that this calc­alkalic suite is not entirely analogous to island arc calc-alkalic suites.

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Albert Masakiyo Kudo

Second Committee Member

Martin Prinz

Third Committee Member

J. Paul Fitzsimmons

Fourth Committee Member


Document Type


Included in

Geology Commons