The Arden pluton, consisting of anorthosite, charnockite, and related rocks, is located within the granulite-facies Wilmington Complex of northern Delaware and southeastern Pennsylvania. Samples from the Arden body have a Rb-Sr whole-rock age of 502 ± 20 m.y. and an initial 87Sr/86Sr ratio of 0.7065. The data indicate primary crystallization about 500 m.y. ago and metamorphism roughly 440 m.y. ago, in early Paleozoic time. Thus, the formation of massif-type anorthosites and related rocks is apparently not restricted to a limited part of Proterozoic time and, therefore, does not require conditions unique to Precambrian time.