Volcanic Evolution of the Southern Slate Belt of Georgia and South Carolina: A Primitive Oceanic Island Arc

Petrological and geochemical data have been obtained from silicic metavolcanic rocks, associated metamorphosed basic dikes, and overlying pyroclastic and sedimentary rocks in the Lincolnton, Georgia-McCormick, South Carolina area. The study area is within the Little River series, a western continuation of the Carolina Slate belt. The lower stratigraphic sequence is composed of massive, coarse to medium-grained units locally termed "metadacite" but which chemically might be termed quartz keratophyres. Part of these units represent massive flows or massive tuffs, but some coarsely crystalline portions may be intrusive domes formed in the core of a volcanic center. These are overlain by a sequence of pyroclastic tuffs, crystal tuffs, and lapilli tuffs which grade vertically into argillites. Prominant unconformities are not recognized in these sequences. Chemical analyses and petrographic relationships of dikes and massive volcanics which probably represent magmatic associations demonstrate that these lithologies are similar to those of the primitive island arc tholeiite association. All units are more sodic than normally reported from continental calc-alkaline volcanic belts, or andesitic island arcs. Alteration of the pyroclastic units has caused an increase in K₂O, MgO and total Fe₂O₃, and a depletion in Na₂O and CaO. These chemical changes may be related to devitrification and alteration in a marine environment. The petrogenetic suite is similar to that found in primitive island arcs throughout the world where oceanic crust is being subducted under oceanic crust without the presence of a thick continental basement. Modern day analogs suggest a model for the Slate belt during this volcanic phase in which the primitive island arc formed separated from a continent by a marginal basin. The position and direction of the related subduction zone is problematic.