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The AAPG/Datapages Combined Publications Database
Earth Science Bulletin (WGA)
Abstract
Rb-Sr and K-Ar Age Determinations from the Precambrian Rocks of the Jardine - Crevice Mountain Area, Southwestern Montana
Abstract
Rb-Sr whole rock studies of the Precambrian rocks of the Jardine—Crevice Mountain area, Southwestern Montana show that the Crevice Mountain granite intruded older metasediments at about 2.66 + 0.08 b.y. (billion years) ago. The metasediments may be as old as 3.3 b.y.
The Crevice Mountain granite exhibits some foliation. K-Ar age studies on minerals from a whole rock sample indicate a strong thermal metamorphism at about 1.6 to 1.8 b.y. A Rb-Sr feldspar-whole rock isochron yields a date of 1.85 b.y., but muscovite from the same rock yields an older age. It is suggested that the 1.6–1.8 b.y. metamorphic event was of sufficient strength to drive off radiogenic AR40 accumulated until that time, but too weak to completely re-equilibrate the radiogenic Sr87 isotopes.
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