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

摘要

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 52.90.14 and 51.80.14 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 1 71 wt%) to dacites and 2 andesites (SiO ! 62 wt%). Compositional change parallels a transition from early explosive volcanism to late effusive 2 activity. Four geochemical components can be detected in the rocks. A component with and Nd 206Pb/204Pb ! 16.5 near 15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with Nd below 10 are dominated by a second component; hydrous rocks with 206Pb/204Pb 1 18.3 and 1 9 contain a third Nd component; and an andesite with low Nd content and 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.