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New ⁴⁰Ar/³⁹Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.

¹ U.S. Geological Survey, Mail Stop 980, Box 25048, DFC, Lakewood, CO 80225, U.S.A.
² Department of Environmental Science and Policy, George Mason University, MS 5F2, 4400 University Drive, Fairfax, VA 22030-4444, U.S.A.

^* Correspondence should be addressed to: kkellogg{at}usgs.gov

Detailed ⁴⁰Ar/³⁹Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by 69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at 69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at 79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. ⁴⁰Ar/³⁹Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including Eocene–Oligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 ± 0.18 Ma and 49.42 ± 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a 30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 ± 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.

Key Words: ⁴⁰Ar/³⁹Ar dating • Laramide • Madison Range • Madison Valley • Montana • Norris • paleomagnetism • Red Mountain • Virginia City volcanic field

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