The Picton and Varty Lake ultramafic dikes that intrude Ordovician limestone near Belleville, Ontario, consist of olivine or serpentinized pseudomorphs and Ti-phlogopite phenocrysts set in a groundmass of spinel, perovskite, serpentine, calcite, apatite, and chlorite. Phlogopite exhibits a complex chemical variation and late-stage Ba enrichment (up to 6.8 wt.% BaO). Spinels are chemically zoned and commonly exhibit an atoll texture in which a magnesian–aluminous chromite core is separated by a silicate zone from late-stage magnesian–aluminous titanomagnetite chemically similar to kimberlitic spinel. Atoll spinels from the Picton dike also have a zone of titanian pleonaste surrounding a central grain of magnesian–aluminous chromite at the centre of the atoll structures. Both dikes are extremely silica undersaturated, with high K/Na and Mg/(Mg + Fe2+) ratios. Petrographic and chemical features of the Picton and Varty Lake dikes preclude simple classification, as both dikes have aspects indicative of kimberlitic and lamprophyric affinities. K–Ar age determinations, indicating a Jurassic age of emplacement for both dikes, are supported by paleomagnetic data from the Picton dike. The Varty Lake dike yielded no stable paleomagnetic remanence directions; limestone adjacent to the dike retains a Paleozoic remanence possibly indicating a very low emplacement temperature. The emplacement of both dikes may be related to Grenville-age basement faults reactivated in Jurassic time.