⁴⁰Ar/³⁹Ar ages of metamorphic rocks from the Tobacco Root Mountains region, Montana

Abstract

Measurements of 60 single-grain, UV laser microprobe 40 Ar/ 39 Ar total gas ages for hornblende from metamorphic rocks of the Tobacco Root Mountains in southwest Montana yield a mean age of 1.71 ± 0.02 Ga. Measurements of 40 Ar/ 39 Ar step-heating plateau ages of three bulk hornblende samples from the Tobacco Root Mountains metamorphic rocks average 1.70 ± 0.02 Ga. We believe that these and the K/Ar or 40 Ar/ 39 Ar ages reported by previous workers are cooling ages from a 1.78 to 1.72 Ga, upper-amphibolite to granulite facies, regional metamorphism (Big Sky orogeny) that affected the northwestern portion of the Wyoming province, including the Tobacco Root Mountains and adjacent ranges. Based on the 40 Ar/ 39 Ar data, this 1.78-1.72 Ga metamorphism must have achieved temperatures greater than ~500 °C to reset the hornblende 40 Ar/ 39 Ar ages of samples from the Indian Creek Metamorphic Suite, which was previously metamorphosed at 2.45 Ga, and of the crosscutting metamorphosed mafi c dikes and sills (MMDS), which were intruded at 2.06 Ga. Biotite and hornblende from the Tobacco Root Mountains appear to give the same 40 Ar/ 39 Ar or K/Ar age (within uncertainty), indicating that the rocks cooled rapidly through the interval from 500 to 300 °C. This is consistent with a model of the Big Sky orogeny that includes late-stage tectonic denudation that leads to decompression and rapid cooling. A similar cooling history is suggested by our data for the Ruby Range. Three biotite samples from the Ruby Range yield 40 Ar/ 39 Ar step-heating plateau ages with a mean of 1.73 ± 0.02 Ga, identical to the best-estimate (near-plateau) age for a hornblende from the same rocks. Two samples of the orthoamphibole, gedrite, from the Tobacco Root Mountains were studied, but did not have enough K to yield a reliable 40 Ar/ 39 Ar age. Several biotite and three hornblende samples from the region yield 40 Ar/ 39 Ar dates signifi cantly younger than 1.7 Ga. We believe these samples were partially reset during contact metamorphism by Cretaceous (75 Ma) intrusive rocks. Hydrothermal alteration associated with ca. 1.4 Ga rifting led to growth of muscovite with that age in the Ruby Range, but this alteration was apparently not hot enough to reset biotite and hornblende ages there.

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

“…No extensional structures have been identifi ed within the Tobacco Root Mountains (Harms et al, 2004, this volume, Chapter 10). This is not unexpected given the uniformity of metamorphic grade observed throughout the Spuhler Peak, Indian Creek, and Pony-Middle Mountain Metamorphic Suites and MMDS (Cheney et al, 2004a, this volume, Chapter 6) and the consistent time of cooling demonstrated across the Tobacco Root Mountains and adjacent ranges (Brady et al, 2004b, , this volume, Chapter 7, and references therein). The Tobacco Root Mountains must have been positioned in the footwall plate of any detachment systems active during decompression and rapid cooling.…”

“…4). 40 Ar/ 39 Ar ages of 1.71 Ga from hornblende in the Spuhler Peak and Indian Creek Metamorphic Suites (Brady et al, 2004b, this volume, Chapter 7) confi rm cooling through the hornblende closure temperature (~500 °C) immediately after fi nal growth of monazite and upper amphibolite-facies M2 metamorphism. This is a record of rapid quenching following M2 isothermal decompression, which is consistent with tectonic unroofi ng of the deeper parts of the Big Sky orogen, including the Tobacco Root Mountains.…”

“…Cooling through the hornblende and biotite blocking temperatures occurred between 1.74 and 1.71 Ga ( 40 Ar/ 39 Ar dating) (Roberts et al, 2002;Brady et al, 2004b, this volume, Chapter 7) and provides a minimum metamorphic age in the Highland and Ruby Ranges.…”

“…4) (Cheney et al, 2004a, this volume, Chapter 6, 2004bBrady et al, 2004b, this volume, Chapter 7). Metamorphism started with a higher-pressure, higher-temperature (>1.0 GPa and >700 °C) phase (M1).…”

Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

“…Note that thin‐skinned Cordilleran thrust faults impinge on Laramide basement structures in the Highland and Tobacco Root Mountains. Box in Tobacco Root Mountains (TRM) indicates area of the Brady et al () study referred to in text.…”