Richard F. Marvin Robert E. Zartman 1971 <div class="widget widget-SplitView widget-instance-SplitView_Article"><div class="article"><div class="widget widget-ArticleMainView widget-instance-ArticleMainView_Split"><div class="content-inner-wrap"><div class="article-body"><div id="ContentTab" class="content active"><div class="widget widget-ArticleFulltext widget-instance-ArticleFulltext_Split"><div class="module-widget"><div class="widget-items" data-widgetname="ArticleFulltext"><p id="15273334" class="abstract-title jumplink-heading">A geochronologic study of several intrusive bodies of alkalic granite from eastern Massachusetts yields the following radiometric ages (in millions of years).</p><table border="0" class="mce-item-table"><tbody><tr><td>Rock unit</td><td>Amphibole&nbsp; &nbsp; &nbsp;<br>K-Ar</td><td>Whole-rock <br>Rb-Sr isochron&nbsp; &nbsp; &nbsp;</td><td>Pb²⁰⁷/Pb²⁰⁶<br>zircon</td></tr><tr><td>Quincy Granite</td><td>430-458</td><td>313 ± 22</td><td>437 ± 32</td></tr><tr><td>Cape Ann Granite&nbsp; &nbsp; &nbsp;</td><td>374-397</td><td>435 ± 6</td><td>452 ± 10</td></tr><tr><td>Peabody Granite</td><td>350-403</td><td>367 ± 24</td><td>435 + 12<br>445 ± 22</td></tr></tbody></table><div><div class="article-section-wrapper js-article-section js-content-section "><p>The zircon ages show only mild internal discordancies and most diffusion models used to discuss U-Th-Pb isotopic systematics would give a true age about equal to that of the Pb²⁰⁷/Pb²⁰⁶<span>&nbsp;</span>value. When plotted on a concordia diagram, the zircon data indicate a time of emplacement of 450 ± 25 m.y. for all three granite bodies. This Late Ordovician age for post-tectonic granites suggests that revisions in the commonly accepted geologic interpretation of eastern Massachusetts may be required. Previously, because of their massive, undeformed nature, the alkalic rocks were generally regarded as being Mississippian or Devonian in age and younger than the Early to Middle Devonian Acadian orogeny.</p><p>The amphibole K-Ar and whole-rock Rb-Sr systems have responded in a complex way to postcrystallization disturbances. We interpret the pattern of ages for the Quincy Granite to reflect a late Paleozoic(?), low-temperature alteration that accompanied extensive faulting, and the patterns of ages for the Cape Ann and Peabody Granites to reflect a Devonian heating possibly related to contact metamorphism by a nearby mafic pluton.</p></div></div></div></div></div></div></div></div></div></div></div> application/pdf 10.1130/0016-7606(1971)82[937:RALOOT]2.0.CO;2 en Geological Society of America Radiometric age (Late Ordovician) of the Quincy, Cape Ann, and Peabody Granites from eastern Massachusetts article