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Isotopic ages of rocks and minerals from the Stillwater Complex, Montana

1969; American Geophysical Union; Volume: 74; Issue: 12 Linguagem: Inglês

10.1029/jb074i012p03226

2156-2202

Ronald W. Kistler, John D. Obradovich, Everett D. Jackson,

earthquake and tectonic studies

Journal of Geophysical Research (1896-1977)Volume 74, Issue 12 p. 3226-3237 Isotopic ages of rocks and minerals from the Stillwater Complex, Montana R. W. Kistler, R. W. KistlerSearch for more papers by this authorJ. D. Obradovich, J. D. ObradovichSearch for more papers by this authorE. D. Jackson, E. D. JacksonSearch for more papers by this author R. W. Kistler, R. W. KistlerSearch for more papers by this authorJ. D. Obradovich, J. D. ObradovichSearch for more papers by this authorE. D. Jackson, E. D. JacksonSearch for more papers by this author First published: 15 June 1969 https://doi.org/10.1029/JB074i012p03226Citations: 20AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Abstract The potassium-argon age of one whole-rock specimen from the chilled border phase of the Stillwater complex is 3.2±0.2 b.y., and that of another is 2.3±0.2 b.y. Rubidium-strontium and potassium-argon ages of micas and potassium-argon ages of plagioclase vary between 2.7 and 2.0 b.y. Four whole-rock samples analyzed for rubidium-strontium and Sr87/Sr86 do not define a unique isochron and indicate the rocks have been open to these elements sometime during the history of the complex. A first interpretation of these data is that the mica and plagioclase ages record the time of crystallization of the complex; a second is that these mineral ages record the time of the last thermal metamorphism of the complex. Ages obtained by routine potassium-argon dating of augite and bronzite from the complex are 7.1 and 8.4 b.y., respectively, and show that both pyroxenes contain excess radiogenic argon. Increment heating argon extractions on these pyroxenes demonstrate that argon is held in two positions in the minerals. Argon in the first position can be completely removed by heating the pyroxenes in a vacuum at 800°C, whereas argon in the second position is removed only on fusion of the minerals. From this we conclude that the major component, as well as possibly all the excess radiogenic argon, in the Stillwater pyroxenes is held at the interfaces between host crystals and exsolution lamellas. References Amirkhanov, Kh. I., S. B. Brandt, E. N. Bartnitskey, Radiogenic argon in minerals and its migration, Ann. N. Y. Acad Sci., 91, 235–275, 1961. Butler, J. R., Geologic evolution of the Beartooth Mountains, Montana and Wyoming, 6, Cathedral Peak area, Montana, Bull. Geol. Soc. Am., 77, 45–64, 1966. Catanzaro, E. J., J. L. Kulp, Discordant zircons from the Little Belt (Montana), Beartooth (Montana), and Santa Catalina (Arizona) Mountains, Geochim. Cosmochim. Acta, 28, 87–124, 1964. Fenton, M. D., G. Faure, Another age determination of the Stillwater complex, Montana, by the total-rock Rb-Sr method, Geol. Soc. Am. Ann. Meeting Program, North Central Sec., Iowa City, Iowa, 24–25, 1968. Gast, P. W., J. L. Kulp, L. E. Long, Absolute age of early Precambrian rocks in the Bighorn Basin of Wyoming and Montana, and south-eastern Manitoba, Trans. Am. Geophys. Union, 39, 322–334, 1958. Kirsten, Y., W. Gentner, K-Ar-Altersbestimmungen an Ultrabasiten des baltischen Schildes, Z. Naturforsch., 21a, 119–126, 1966. Kistler, R. W., Potassium-argon ages of volcanic rocks in Nye and Esmeralda counties, Nevada, Geol. Soc. Am. Mem., 110, 251–262, 1968. Kistler, R. W., F. C. W. Dodge, Potassium-argon ages of coexisting minerals from pyroxene-bearing granitic rocks in the Sierra Nevada, California, J. Geophys. Res., 71, 2157–2161, 1966. Kistler, R. W., J. D. Obradovich, E. D. Jackson, Isotopic ages of rocks and minerals from the Stillwater complex, Montana, Geol. Soc. Am., 80th Ann. Meeting Program, New Orleans, La, 120–121, 1967. Livingston, D. E., P. E. Damon, R. L. Mauger, R. Bennett, A. W. Laughlin, Argon 40 in cognetic feldspar-mica mineral assemblages, J. Geophys. Res., 72, 1361–1375, 1967. Page, N. J., E. D. Jackson, Preliminary report on sulfide and platinum-group minerals in the chromitites of the Stillwater complex, Montana, U. S. Geol. Surv. Profess. Paper, 575-D, D123–D126, 1967. Peterman, Z. E., B. R. Doe, A. Bartel, Data on the rock GSP-1 (granodiorite) and the isotope-dilution method of analysis for Rb and Sr, U. S. Geol. Surv. Profess. Paper, 575B, B181–B186, 1967. Schoen, Robert, D. E. Lee, Successful separation of slit-size minerals in heavy liquids, U. S. Geol. Surv. Profess. Paper, 501-B, B154–B157, 1964. Schwartzman, D. W., B. J. Giletti, ‘Excess’ argon in minerals from the Stillwater complex, Montana (abstract), Trans. Am. Geophys. Union, 49, 359, 1968. Stueber, A. M., V. R. Murthy, Strontium isotope and alkali element abundances in ultramafic rocks, Geochim. Cosmochim. Acta, 30, 1243–1259, 1966. Citing Literature Volume74, Issue1215 June 1969Pages 3226-3237 ReferencesRelatedInformation