The petrology of gabbroic sills in the volcanic series of Roy and McKenzie Townships, Chibougamau Region, Quebec.

Metadiabase sills constitute up to 27 percent of a steeply-dipping assemblage of ancient volcanic rocks in Roy and McKenzie townships, Chibougamau region, northwest Quebec. Three rock types are represented. They are, from lower to upper parts of the sills, metapyroxenite, metadiabase, and diabase-pegmatite. Comparison is made with “metadiorite” and “metagabbro” sills in an adjoining area. Chemical and planimetric analyses show that layering of the sills is the result of magmatic differentiation which is characterized by iron-enrichment. The metadiabase sills are similar to the intrusive phase of plateau-type basaltic magma in chemical composition and in trend of differentiation