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

作者：Dudas Francis Oe^*; Ispolatov Vladimir O; Harlan Stephen S; Snee Lawrence W

来源：Journal of Geology, 2010, 118(3): 295-304.

DOI：10.1086/651523

We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in west-central Montana. Ar-40/Ar-39 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 (SiO2 > 71 wt%) to dacites and andesites (SiO2 < 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-206/Pb-204 < 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 Pb-206/Pb-204 > 18.3 and epsilon(Nd) > -9 contain a third component; and an andesite with low epsilon(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.

• 出版日期2010-5