Petrographic, chemical, and isotopic study of intrusive rocks related to porphyry copper mineralization in the northern Caribbean area and comparison of these data with similar information on other porphyry copper-related intrusions in island arc and craton regions yield support for the generally accepted, though inadequately tested, generalization that porphyry copper-related intrusions in island arcs are poorer in K-feldspar than are their craton counterparts. Although most of the mineralized island arc intrusions are quartz diorites, there are more potassic intrusions including quartz monzonites and syenites. All of these intrusions are lower in potassium than their craton counterparts and even the quartz monzonites lack K-feldspar phenocrysts. Island arc intrusions also appear to be depleted in lead and rubidium and to exhibit initial 87 Sr/ 86 Sr ratios of .705 or less, which are lower than those measured for intrusions in cratonic settings. Copper and possibly zinc are enriched in the island arc intrusions. Chlorine contents are similar to accepted averages for craton intrusions, and fluorine is depleted in the quartz diorites and enriched in granodioritic island arc intrusions. These data indicate that island arc porphyry copper-related intrusions are compositionally more primitive than their craton counterparts and greatly restrict the involvement of epicrustal material in the development of these magmas.