The Age of the Igneous Rocks of the Stillwater Complex of Montana

M. D FENTON; G FAURE

doi:10.1130/0016-7606(1969)80[1599:TAOTIR]2.0.CO;2

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Research Article| August 01, 1969

The Age of the Igneous Rocks of the Stillwater Complex of Montana

M. D FENTON;

M. D FENTON

Department of Geology, The Ohio State University, Columbus, Ohio

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G FAURE

Department of Geology, The Ohio State University, Columbus, Ohio

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Author and Article Information

M. D FENTON

Department of Geology, The Ohio State University, Columbus, Ohio

G FAURE

Department of Geology, The Ohio State University, Columbus, Ohio

Publisher: Geological Society of America

Received: 09 Dec 1968

First Online: 02 Mar 2017

Online ISSN: 1943-2674

Print ISSN: 0016-7606

GSA Bulletin (1969) 80 (8): 1599–1604.

https://doi.org/10.1130/0016-7606(1969)80[1599:TAOTIR]2.0.CO;2

Abstract

Seven mafic and ultramafic rocks from the Stillwater Complex of Montana were analyzed for an Rb-Sr age determination. Five of these form a linear array on an isochron diagram. The slope of the isochron indicates an age of 2450 ± 210 m.y. for these samples, assuming Rb⁸⁷ = 1.39 × 10⁻¹¹ yr⁻¹. The initial Sr⁸⁷/Sr⁸⁶ ratio is 0.7029 ± .0006 and the Rb⁸⁷/Sr⁸⁶ ratios range from 0.048 to 0.243. The other two rocks, serpentinized harzburgites from the basal zone, do not fit the isochron. One possible explanation for this anomalous behavior is that these rocks became contaminated with radiogenic Sr⁸⁷ during serpentinization and had initial Sr⁸⁷/Sr⁸⁶ ratios of 0.7060 and 0.7094, respectively.