Numerous northwest-trending (average, N40°W) diabase dikes of Mesozoic age intrude the Piedmont of northwestern South Carolina. Samples of the diabase were analyzed for major- and trace-element abundances and mineral chemistry. All samples are olivine tholeiite, containing 4 to 25% modal olivine. Plagioclase comprises 39 to 64% and pyroxene, dominantly augite, 13 to 45% of the diabases. Two contrasting pyroxene trends are represented: (1) augite → subcalcic augite (± low-Ca pyroxene), and (2) augite → ferroaugite (no low-Ca pyroxene). These are attributed to modest (about 2 wt%) differences in SiO₂ and iron oxide in the diabase magmas. The diabases also contain 1 to 2% titanomagnetite and up to 0.6% chromite. Textural relations suggest that chromite and olivine were first to crystallize, followed by plagioclase and then pyroxene and titanomagnetite. Major element variation trends cannot be satisfactorily accounted for by fractionation involving observed microphenocryst compositions. Wide ranges in REE patterns and in Th/Hf, Ti/K, and other incompatible element ratios further preclude derivation of the entire suite from a single parent magma by near surface fractional crystallization. As a group, the dikes span the gamut from light rare earth element (LREE) depleted (chondrite-normalized La/Sm <1), with chondrite-normalized Th/Hf <0.5 and very low K₂O (<0.1 wt%), to LREE enriched (La/Sm >2) with Th/Hf >2 and K₂O >0.35 wt%. The observed spread in geochemical values suggests significant heterogeneity in the mantle source.