sup 40^Ar/^sup 39^Ar Geochronology and Geochemical Reconnaissance of the Eocene Lowland Creek Volcanic Field, West-Central Montana

We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in west-central Montana. ...Ar/...Ar age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff-forming eruptions at 52.9 ± 0.14 and 51.8 ± 0.14 Ma. These dates span the age range of vigorous Eocene igneous activity in the Kamloops-Absaroka-Challis belt. The LCVF evolved upward from basal rhyolites (SiO... > 71 wt % ) to dacites and andesites (SiO... < 62 wt %). Compositional change parallels a transition from early explosive volcanism to late effusive activity. Four geochemical components can be detected in the rocks. A component with ...Pb/...Pb < 16.5 and ... near -15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with ... below -10 are dominated by a second component; hydrous rocks with ...Pb/...Pb > 18.3 and ... > -9 contain a third component; and an andesite with low Nd content and ... near -9 probably contains a fourth component. The first three components probably derive from the lower and middle crust, whereas the fourth is probably from the lithospheric mantle. (ProQuest: ... denotes formulae/symbols omitted.)