Significance of pyroxene megacrysts for massif anorthosite petrogenesis: constraints from the Labrieville, Quebec, pluton

Newly discovered orthopyroxene and clinopyroxene megacrysts (OPMs and CPMs) in the Labrieville massif provide additional constraints on megacryst origins and their significance in massif anorthosite petrogenesis. Field characteristics of megacrysts indicate that they formed contemporaneously with the other minerals in the same outcrops. Furthermore, bulk OPMs have X_Mg similar to matrix pyroxene. In comparison, OPMs from the Saint Urbain massif are more magnesian, whereas lamellae and matrix plagioclase are more anorthitic. These systematic differences between the two massifs suggest that the En content of OPMs and the An content of plagioclase are strongly controlled by local magma composition. Collectively, these results can be explained best by the in-situ growth of megacrysts, together with or after adjacent plagioclase. -from Authors