Richard F. Marvin John W. McGonigle David H. McIntyre Cleaves Lincoln Rogers Dennis P. Cox 1977 <p>Potassium-argon ages of hornblende, biotite, and whole rocks in Puerto Rico include the following: 126 m.y. for amphibolites in the southwest part of the island; 109 m.y. for the oldest known quartz diorite plutons; 88 to 65 m.y. for emplacement of various quartz diorite and granodiorite batholiths and stocks; 38 to 46 m.y. for intrusions of quartz diorite porphyry stocks and their hydrothermal alteration and mineralization. These intrusions are the youngest known igneous rocks in Puerto Rico. </p><p>In addition, a 35-m.y. age was determined for hornblende and biotite from the Virgin Gorda batholith. </p><p>Although widespread hydrothermal alteration as old as 75 m.y. is known in Puerto Rico, all important copper deposits are related to the 38- to 46-m.y. old intrusions.</p> application/pdf en U.S. Geological Survey Potassium-argon geochronology of some metamorphic, igneous, and hydrothermal events in Puerto Rico and the Virgin Islands article