Edward F. Stoddard

The Carolina Zone: overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the eastern Flank of the southern Appalachians

Abstract The Carolina Zone is an amalgamation of mainly Neoproterozoic–Early Paleozoic metaigneous-dominated terranes that are clustered along the eastern flank of the southern Appalachians. These terranes are distinguished from other divisions of the orogen by a commonality in gross geologic content and by their close spatial association. They are considered exotic relative to Laurentia on the basis of stratigraphic and tectonic evolution, paleontology, and position in the orogen analogous to that of exotic terranes in the northern Appalachians. They are probably peri-Gondwanan in origin. Within this first-order identity, the terranes exhibit remarkable heterogeneity, with respect to deposition, magmatism, and tectonothermal overprint. The depositional–magmatic history of the zone is viewed in three broad stages, including: (I) pre-600 Ma, (II) ca. 590–560 Ma, and (III) younger than ca. 550 Ma. Although each stage records significant felsic volcanism, there are few compelling stratigraphic linkages between terranes. Stage III plutonism may form a link between the two largest terranes in the zone. The isotopic evolution of the zone reflects the stratigraphic heterogeneity; isotopically juvenile magmatism in some terranes is coeval with more crustally evolved magmatism in others. The tectonothermal history of the zone is heterogeneous, producing a patchwork of suprastructural and infrastructural elements of different ages. Major tectonothermal events responsible for this pattern span the Neoproterozoic–earliest Cambrian, the Late Ordovician–Silurian, and the late Paleozoic. Evidence for regionally extensive events in the zone is sparse and such a fundamental concept as its time of accretion to Laurentia is speculative. The central Piedmont shear zone, a late Paleozoic ductile thrust that defines the western limit of the Carolina Zone, marks the final emplacement of the zone against Laurentian elements.

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 Fo 79 , 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 An 93 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.

Metamorphosed melange terrane in the eastern Piedmont of North Carolina

The Falls Lake melange crops out in the eastern Piedmont of North Carolina between the Carolina slate belt and the Raleigh belt. The melange is composed of mafic and ultramafic blocks and pods of diverse shapes and sizes, dispersed without apparent stratigraphic continuity, in a matrix of pelitic schist and biotite-muscovite-plagioclase-quartz gneiss. Textures and structural relationships suggest formation by a combination of sedimentary and tectonic processes, perhaps in the accretionary wedge of a convergent plate margin. The Falls Lake melange and the overlying late Proterozoic to Early Cambrian volcanic-arc terrane of the Carolina slate belt were thrust upon a probable continental terrane of the Raleigh belt before overprinting by late Paleozoic folding and metamorphism.