The Wilmington complex occupies about 100 square miles in Delaware and adjacent Pennsylvania and Maryland and is bordered by the Wissahickon formation (lower Paleozoic?) and the Port Deposit granodiorite. Its southern edge is covered by Coastal Plain sediments. It consists of a banded gneiss comprising mafic bands of calcic plagioclase, hypersthene, augite, and commonly hornblende and thick felsic bands of quartz and andesine with minor pyroxene. In the central portion the gneiss has been invaded by a granite, probably metasomatic, consisting of orthoclase, oligoclase-andesine, biotite, and pyroxene. To the southwest the banded gneiss grades into amphibolites. Amphibolites near the gneiss contain calcic plagioclase, hornblende, and traces of hypersthene. Those farther away are hypersthene-free, and the plagioclase is replaced in part by epidote. Small bodies of amphibolite within the adjacent schist show similar characteristics.
Veins of granite, apparently related to the Port Deposit granodiorite, intrude the southwestern part of the complex. A coarse-grained undeformed gabbro stock has intruded the gneiss north of Wilmington; a similar body of gabbro occurs as a group of low hills surrounded by Coastal Plain sediments.
The relationship between the chemical composition of the rocks and the mineral phases present indicates that the central granite body and the banded gneiss crystallized or re-crystallized under granulite-facies conditions. The amphibolite near the gneiss represents upper almandine amphibolite facies (probably almandine-sillimanite subfacies), and the more distant amphibolites, having undergone greater stress, probably represent the almandine-diopside-hornblende subfacies. The complex is probably along the axis of most intense metamorphism of the Appalachian(?) Revolution.
Structurally, the complex is characterized by the banding (S1) and foliation (S2) which are commonly, but not always, parallel. Structures in the complex generally parallel structures in the Wissahickon formation, indicating simultaneous deformation.
Recrystallization of the mafic portion of the banded gneiss was accompanied by the introduction, probably metasomatic, of the felsic material. Mafic bands may be transformed into felsic bands by the introduction of Si and Na and the removal of Ca, Fe, Mg, and Al representing a transfer of approximately 7 per cent material by volume. This “potassium-free granitization” may have a definite spatial relationship to the true granitization that produced the central granite. The origin of the mafic materials which comprise the gneiss and the amphibolites may have been (1) sedimentary (impure limestone); (2) volcanic (basaltic tuffs or flows); (3) hypabyssal (a lopolith); or (4) plutonic (a gabbro). Volcanic origin is tentatively suggested. If this is the case the rocks may be correlative with metabasalts of nearby Cecil County, Maryland, and with the Catoctin-South Mountain metabasalts in western Maryland, both of which underlie the Glenarm series.