A narrow linear pattern of magnetic anomalies, trending in a northeasterly direction across the southeastern part of the Avalon Peninsula of Newfoundland, was interpreted earlier as a trace of a major diabase dike of possibly Triassic age, part of the early Mesozoic dike system of the Appalachians. Several isolated outcrops of diabase found recently by J. Hodych along the magnetic anomaly line over a distance of 70 km yield K–Ar isochron ages of 201 and 191 Ma, thus confirming the earlier interpretation.The Avalon diabase is a quartz-normative tholeiite, chemically similar to the 'high-Ti' Appalachian diabase; however, its MgO content is higher than normal for such rocks, approaching that of a rift-related ocean-floor basalt. The Avalon rocks consist of 45-50% plagioclase (An60–70), 40–45% magnesian augite (Ca38Mg52Fe10), 2–3% olivine (Fo78–82), and 3–4% Fe-Ti oxides. Alteration is of deuteric origin; there is no trace of albitization or low-grade metamorphism. The original titanomagnetite has been oxidized to very fine lamellar intergrowth of magnetite and ilmenite; mole fractions of Usp and Hm calculated from bulk analyses of the oxide minerals indicate an equilibration temperature of 970 ± 60 °C and oxygen fugacity (log10f(O2)) of −12.3 ± 1.The Avalon diabase comprises probably two or more dikes intruded into a fissure over 110 km long. These dikes extend the known system of early Mesozoic diabase dikes of the Appalachians by 800 km to the north-east, probably to the edge of the continental shelf.