Petrology of the upper irruptive rocks, Sudbury, Ontario

Abstract

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.