Abstract
Three distinct groups of alkaline rocks can be recognized, on the basis of silica content, in the Monteregian Hills, Quebec: strongly undersaturated, slightly undersaturated to critically saturated, and oversaturated. The first two groups were derived from basanite and alkali picrite magmas, respectively, and yield distinct ages of intrusion clustered about 118 Ma and 136 Ma. The age data are not readily explained by the translation of the North American plate across a fixed mantle hotspot, but because the alkali picrite magmas were derived from greater mantle depths than the basanite magmas, the data do suggest an upward migration of the zone of melting. This change in the depth of melting may be a response to the upward transport of a metasomatic fluid, perhaps with a coupled rise in mantle isotherms.
