The Halifax County complex: Oceanic lithosphere in the eastern North Carolina Piedmont

The Halifax County complex, in the Eastern Slate Belt of North Carolina, consists of the low-grade metamorphic equivalents of the following lithologic groups: (1) an ultramafic group composed of peridotite, pyroxenite, and dunite, locally preserving cumulate textures; (2) gabbroids consisting of leucogabbro, anorthosite, and gabbro; (3) local quartz diorite and plagiogranite; and (4) a relatively large volume of porphyritic and massive basaltic rocks. Within the complex, the lithologies conform approximately, from west to east, to the sequence: peridotite and dunite-pyroxenite-anorthosite and leucogabbro-gabbro–diorite-basaltic rocks. On its west side, the complex is separated from metavolcanic rocks and metagraywackes of the Eastern Slate Belt by the late Paleozoic Hollister mylonite zone; to the east, the complex disappears beneath sediments of the Atlantic Coastal Plain. Relict primary silicate and oxide minerals include olivine of Fo₇₉, clinopyroxene with compositional range of Wo_44–49 En_44–49 Fs_5–9 in the ultramafic rocks and Wo_34–49 En_42–48 Fs_8–21 in the gabbroids, plagioclase of as much as An₉₃ in the leucogabbros, Cr-rich magnetite, and ilmenite. Major- and trace-element analyses of samples from each of the rock groups show an Fe-enrichment trend extending through the mafic sequence of gabbroids and basaltic rocks. The chemical characteristics of the basaltic rocks compare favorably with those of tholeiitic ocean-floor basalts and still more favorably with island-arc tholeiites. Field relationships, plus lithologic, geochemical, and mineralogical evidence, suggest that the complex originated as (early Paleozoic?) oceanic material. Either it represents the lower portions of a lithospheric section produced at a mid-ocean ridge or in a marginal basin, or, perhaps more likely, it was formed in the early stages of the development of an intraoceanic volcanic arc. We believe the Halifax complex is representative of the underpinnings of the Eastern Slate Belt volcanic arc.