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40 Ar/ 39 Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana

F.O. Dudas·V.O. Ispolatov·S. S. Harlan·L.W. Snee
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Abstract

We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in westcentral Montana. 40 Ar/ 39 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 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 206 Pb/ 204 Pb < 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 206 Pb/ 204 Pb > 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.

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F.O. Dudas, V.O. Ispolatov, S. S. Harlan, L.W. Snee. 2010. 40 Ar/ 39 Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana. https://doi.org/10.1086/651523

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