Grenvillian gneiss and marble of the Buckingham area, Quebec were intruded by a syenite–monzonite–diorite suite, overlain by trachyandesite, and, finally, intruded by diabase. The three igneous events are tentatively dated as late Proterozoic, Cambro-Proterozoic, and Cambro-Ordovician.The trachyandesites were extruded subaerially from en echelon vents or fissures. These rocks are highly oxidized and locally enriched in potassium, thought to be the result of late volcanic, gaseous, or hydrothermal transfer. They are preserved in a small, northeast-trending graben, part of the St. Lawrence Valley Rift System. This occurrence of Cambro-Proterozoic volcanic rocks is the first reported for the Canadian Shield of central Canada.