Electron microprobe and textural data are used to deduce a crystallization history for plagioclase in the Grand Manan Island (Canada) diabase sheet. The plagioclase crystals consist of a prominent core (An_79-67) surrounded by a distinctly more calcic zone (An_82-76), which grades into a normally zoned mantle (An_73-40). This zoning pattern is interpreted as reflecting the elevation of a dry diabase magma from at least 10 to 20 km to near surface conditions.