The upper part of the Sudbury Nickel Irruptive, referred to locally as the micropegmatite, is recognized as consisting of two distinctive rock types. One is granophyre-rich (the granophyric micropegmatite) and the other is rich in plagioclase (referred to as the plagioclase-rich rock). Field observations suggest that the granophyric micropegmatite intrudes and engulfs the plagioclase-rich rock. Modally the latter rock is distinguished from the former by its higher plagioclase and ferromagnesian mineral content; it resembles the upper part of the oxide-rich gabbro, a unit that immediately overlies the norite of the Irruptive.Augite of the plagioclase-rich rock is MgO-rich in comparison with that of the granophyric micropegmatite and has FeO:(FeO + MgO) atomic ratios similar to those in the oxide-rich gabbros and upper norite. Its CaO content, however, is higher than that in the norite and closer to that in the oxide-rich gabbro. Whole rock chemical data indicate that the plagioclase-rich rock is similar to the upper part of the oxide-rich gabbro in the normative anorthite content of its plagioclase and in total Na 2 O, K 2 O, and CaO.It is concluded that the plagioclase-rich rock is a phase of the upper part of the oxide-rich gabbro which was split off from the remainder of this unit by a later injection of granophyric micropegmatite. This micropegmatite is thought to be an extreme differentiate of the Irruptive that collected at its top, toward the center of the structure.