Potassium-argon geochronology of some metamorphic, igneous, and hydrothermal events in Puerto Rico and the Virgin Islands

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.

In addition, a 35-m.y. age was determined for hornblende and biotite from the Virgin Gorda batholith.

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.