G.L. Davis H. L. James Lyman Thomas Aldrich 1965 <p><span>More than 100 independent isotopic&nbsp;</span>ages<span>&nbsp;have been determined for&nbsp;</span>minerals<span>&nbsp;from an area in northern&nbsp;</span>Michigan<span>&nbsp;about 35 miles square. Granites, pegmatites, and metamorphosed sedimentary and volcanic&nbsp;</span>rocks<span>&nbsp;have yielded Rb-Sr&nbsp;</span>ages<span>&nbsp;for feldspar, muscovite, and biotite, K-Ar&nbsp;</span>ages<span>&nbsp;for hornblende, muscovite, biotite, and feldspar, and U-Pb and Th-Pb&nbsp;</span>ages<span>&nbsp;for zircon. It was anticipated that we would learn from the measurements both the intrusive and&nbsp;</span>metamorphic<span>&nbsp;history of the area and would be able to place limits on the&nbsp;</span>age<span>&nbsp;of the Precambrian sediments in this area. The conclusions may be summarized as follows: (1) Granites and pegmatites with approximate&nbsp;</span>ages<span>&nbsp;of 2,700 and 1,900 m.y. have been found. (2) The major&nbsp;</span>mineral<span>-forming&nbsp;</span>metamorphic<span>&nbsp;event occurred between 1,800 and 2,000 m.y. ago. (3) The Precambrian sedimentary&nbsp;</span>rocks<span>&nbsp;called Animikie are older than 1, 900 m.y. and younger than 2,700 m.y. (4) Biotite Rb-Sr and K-Ar&nbsp;</span>ages<span>&nbsp;and muscovite K-Ar&nbsp;</span>ages<span>&nbsp;were strongly modified by the equivalent of a rise in temperature approximately 1,350 m.y. ago, although no&nbsp;</span>mineral<span>-forming event in this interval has been observed. (5) The K-Ar system was further affected by a thermal rise at about 1,100 m.y. This later event is probably recorded geologically by a few diabase dikes.The resistance to post-formation thermal events shown by the various&nbsp;</span>minerals<span>&nbsp;and decay systems tested may be classified as follows (lowest to highest): feldspar K-Ar, biotite K-Ar, muscovite K-Ar, biotite Rb-Sr, zircon U</span>²³⁸<span>-Pb</span>²⁰⁶<span>, zircon Th-Pb</span>²⁰⁸<span>, zircon U</span>²³⁵<span>-Pb</span>²⁰⁷<span>, muscovite Rb-Sr, feldspar Rb-Sr. The few hornblende K-Ar measurements indicate that the resistance of this system is about comparable to that of muscovite and feldspar Rb-Sr. The Pb</span>²⁰⁷<span>-Pb</span>²⁰⁶<span>&nbsp;</span>age<span>&nbsp;derived from the two U-Pb&nbsp;</span>ages<span>&nbsp;is somewhat more resistant to change than the feldspar Rb-Sr&nbsp;</span>ages<span>.</span></p> application/pdf 10.1093/petrology/6.3.445 en Oxford University Press Ages of minerals from metamorphic and igneous rocks near Iron Mountain, Michigan article