V.O. Ispolatov S. S. Harlan L.W. Snee F.O. Dudas 2010 We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in westcentral Montana. ⁴⁰Ar/ ³⁹Ar 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 ₂>71 wt%) to dacites and andesites (SiO ₂ > 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 ²⁰⁶Pb/²⁰⁴Pb < 16.5 and _epsilon;Nd near-15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with _epsilon;Nd below-10 are dominated by a second component; hydrous rocks with ²⁰⁶Pb/ ²⁰⁴Pb > 18.3 and _epsilon;Nd>-9 contain a third component; and an andesite with low Nd content and _epsilon;Nd 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. ?? 2010 by The University of Chicago. application/pdf 10.1086/651523 en ⁴⁰Ar/³⁹Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana article