Petrogenesis of the Labrieville alkalic anorthosite massif, Grenville Province, Quebec

The Labrieville massif (~ 1010 Ma) is an eroded dome consisting of three zones: (1) a foliated inner core of pink anorthosite (~ An₃₀Or₁₃, ~ 2100 ppm Sr, ∼ 1300 ppm Ba) with minor leuconorite; (2) a foliated outer core of pink anorthosite (~ An₃₅Or₁₀, ~ 1900 ppm Sr, ~ 850 ppm Ba) with more abundant leuconorite; (3) a border of foliated to massive green leucogabbro. Antiperthitic plagioclase, orthopyroxene, hemoilmenite, and small amounts of biotite are present in all rocks. The border zone contains additional clinopyroxene, magnetite, and apatite. Labrieville is more alkalic and Sr- and Ba-rich than almost all other massif anorthosites. The widespread presence of biotite and the high hematite content of hemoilmenite suggest crystallization from a relatively oxidizing and water-bearing magma. Collectively, these features imply that Labrieville originated from a magma that differed considerably from typical tholeiitic liquids. Major- and trace-element variations suggest that all rock types represent mixtures of cumulus minerals, with negligible trapped melt. The compositionally distinct zones imply differentiation during crystallization. Although plagioclase compositions become more evolved inward, other indicators (e.g. X_Mg in pyroxene) suggest differentiation outward from core to border. We favor the latter interpretation, and suggest that the increase in plagioclase An content reflects a decline in pressure during crystallization.