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The AAPG/Datapages Combined Publications Database

Earth Science Bulletin (WGA)

Abstract


Earth Science Bulletin
Vol. 1 (1968), No. 2. (June), Pages 5-9

Rb-Sr and K-Ar Age Determinations from the Precambrian Rocks of the Jardine - Crevice Mountain Area, Southwestern Montana

Douglas G. Brookins

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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