Potassium-argon age dating of plutonic rocks has confirmed the existence of two periods of intrusion, uppermost Cretaceous (Campanian)-Paleocene (75-58 m.y. ago) and Eocene-earliest Oligocene (46-35 m.y. ago), in west central Puerto Rico. All of the older ages are confined to the mesozonal Utuado batholith, composed principally of tonalite, granodiorite, and granite, which was emplaced in Cretaceous volcanogenic rocks of the central igneous subprovince. Eocene emplacement ages were obtained for small quartz diorite and tonalite stocks, many of which are porphyritic, intruded into or immediately adjacent to a trough of lower Tertiary volcanogenic rocks that is subparallel to the southwestern margin of the Utuado batholith. Where the Eocene plutons intruded the southwestern margin of the batholith, partial or complete resetting of the ages of the batholithic rocks has occurred. Porphyry copper mineralization is associated with a number of Eocene tonalite and quartz diorite porphyries in two areas along the southwestern margin of the Utuado batholith. Apparent ages for a wide variety of altered rocks (44-39 m.y. ago) in which potassium-bearing and ore minerals are cogenetic, from the Piedra Hueca, Cala Abajo, and Sapo Alegre deposits in the Rio Vivi area and from the Tanama and Helecho porphyries, 13 km to the northwest, are essentially indistinguishable from each other and from the minimum ages of unaltered Eocene plutonic rocks nearby. The apparent ages of samples with postmineralization leonhardite veins (mean 34.9 m.y.) may be due to reheating during the submergence of Puerto Rico which began in the middle Oligocene. A hiatus of plutonic activity, lasting at least 12 m.y. (58-46 m.y. ago), corresponds to a shift from calc-alkaline magmas, related to subduction, to low potassium varieties localized along preexisting structures. The Eocene magmas are believed to have been generated at shallow depth by partial melting of oceanic crust trapped between rigid blocks of Cretaceous volcanogenic material as they approached each other in response to a change of convergence directions between the Atlantic and Caribbean plates.