Absolute Age of Pre‐cambrian Rocks in Wyoming and Montana*

Giletti, Bruno J.; Gast, P. W.

doi:10.1111/j.1749-6632.1961.tb35502.x

Cited by 29 publications

(2 citation statements)

References 10 publications

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“…Early geochronological studies of the metamorphic rocks of Tobacco Root Mountains and adjacent ranges were based principally on K/Ar studies of micas or Rb/Sr studies of micas and whole rocks Wehrenberg, 1959, 1960;Giletti and Gast, 1961;Giletti, 1966Giletti, , 1968. These investigators consistently found ages of 1.6-1.7 Ga, which were interpreted by Giletti (1966) as evidence of a regional metamorphism of that age.…”

Section: Previous Geochronologymentioning

confidence: 99%

In situ ion microprobe 207Pb/206Pb dating of monazite from Precambrian metamorphic suites, Tobacco Root Mountains, Montana

Cheney¹,

Webb²,

Coath³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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The Precambrian rocks of the Tobacco Root Mountains have been separated into three suites: the Indian Creek Metamorphic Suite, the Pony-Middle Mountain Metamorphic Suite, and the Spuhler Peak Metamorphic Suite. The Indian Creek and Pony-Middle Mountain Metamorphic Suites are quartzofeldspathic gneiss suites that contain variable amounts of metasupracrustal rocks. The Spuhler Peak Metamorphic Suite contains primarily mafi c rocks and is possibly ocean crust. Metamorphosed mafi c dikes and sills that intruded the Indian Creek and Pony-Middle Mountain Meta morphic Suites, but not the Spuhler Peak Metamorphic Suite, indicate juxtaposition of the Spuhler Peak Metamorphic Suite with the other two suites after intrusion of the dikes at ca. 2060 Ma. All rocks have been deformed and metamorphosed together, initially at pressures greater than 1.0 GPa and temperatures greater than 750 °C, followed by differential reequilibration at ~0.6 GPa on a clockwise pressuretemperature path. Two-hundred-seventy-two 207 Pb/ 206 Pb spot ages of monazite grains from seventeen Spuhler Peak Metamorphic Suite, fi ve Pony-Middle Mountain Metamorphic Suite, and eight Indian Creek Metamorphic Suite rocks have been obtained from the University of California at Los Angeles ion microprobe. Based on the distribution of the ages, the samples can be divided into three groups. (1) All seventeen Spuhler Peak Metamorphic Suite, one Pony-Middle Mountain Metamorphic Suite, and two Indian Creek Metamorphic Suite samples have relatively homogeneous spot age populations that vary from ca. 1720 to ca. 1780 Ma. (2) A group of seven Indian Creek and PonyMiddle Mountain Metamorphic Suite samples has spot ages from monazite grains that form an array near 2450 Ma. (3) A group of four Indian Creek and Pony-MiddleMountain Metamorphic Suite samples are bimodal in that they contain spot ages from both the 1720-1780 Ma group and the 2450 Ma array. There are younger and a few older spot ages in these samples that likely represent mixed age domains, the former between the 1720-1780 Ma and the 2450 Ma age domains and the latter between older detrital grain cores and the 2450 Ma array.Monazite grains in the matrix have similar ages to those that occur as inclusions in garnet and kyanite. Thus, the monazite in these rocks, as well as the peak metamorphic minerals, either grew or re-equilibrated during the higher-pressure on July 24, 2015 specialpapers.gsapubs.org Downloaded from 152 J.T. Cheney et al.

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Section: Previous Geochronologymentioning

confidence: 99%

In situ ion microprobe 207Pb/206Pb dating of monazite from Precambrian metamorphic suites, Tobacco Root Mountains, Montana

Cheney¹,

Webb²,

Coath³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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“…Giletti and Gast (1961) using Rb/Sr data for micas and Giletti (1966Giletti ( , 1968 using K/Ar and Rb/Sr data for micas and whole rocks showed that the entire northwestern portion of the Wyoming province was heated at 1.6-1.7 Ga, including the Tobacco Root Mountains and adjacent Ruby Range and Highland Mountains. Giletti (1966) presented a map of the Wyoming province showing a line that divided the 1.6-1.7 Ga terrane from the rest of the Wyoming province.…”

Section: Introductionmentioning

confidence: 99%

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

Brady¹,

Kovaric²,

Cheney³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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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.

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Field Trip Guidebooks

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The DePass deposit occurs in a mineralized Proterozoic mafic dike of tholeiitic affinity. The dike at the shaft is about 15.6 m (50 ft) wide and mineralized across the entire width. The dike contains pyrite, chalcopyrite, milky quartz, and hematite. Late Archean pegmatites along the southern margin of the supracrustal belt are about 2.5 Ga (Giletti and Gast, 1961). A few of these pegmatites were mined as recently as the 1970's for feldspar. Some complex pegmatites contain muscovite, microcline, quartz, beryl (including rare aquamarine), columbite-tantalite, tourmaline, gamet, petalite, apatite, clevelandite, and lepidolite (McLaughlin, 1940). Copper Mountain area stop descriptions (Hausel, and Graff) Geology stop #1, Gold Nugget Camp. The Gold Nu~et Camp area includes rocks of metamorphic Umts II and III. In the Gold Nu~et Camp a shallow-dipping narrow quartz veIn is hosted by metapelite and extensively invaded by granite. Narrow banded iron-formation crops out a short distance south. Geology stop #2, DePass mine area. The DePass mine was developed by 2,440 m (11,000 ft) of workings in a Proterozoic basalt dike (fig. 32). The dike rock is stained by abundant secondary copper silicates, carbonates, and oxides, and is impregnated by disseminated sulfides. Reported copper values range from a trace to 3.5 percent Cu and include a trace to 0.14 oz/ton Au (4.8 ppm). REFERENCES CITED

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Absolute Age of Pre‐cambrian Rocks in Wyoming and Montana*

Cited by 29 publications

References 10 publications

In situ ion microprobe <sup>207</sup>Pb/<sup>206</sup>Pb dating of monazite from Precambrian metamorphic suites, Tobacco Root Mountains, Montana

In situ ion microprobe <sup>207</sup>Pb/<sup>206</sup>Pb dating of monazite from Precambrian metamorphic suites, Tobacco Root Mountains, Montana

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

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