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...