Anorthosites of diverse magma types in the Puttuaaluk Lake area, Nain complex, Labrador

• Published in: Canadian Journal of Earth Sciences
• Main Author: Ranson, W. A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1981
• Subjects: General Earth and Planetary Sciences; Nain
• Online Access: http://dx.doi.org/10.1139/e81-003; http://www.nrcresearchpress.com/doi/pdf/10.1139/e81-003

Helikian anorthositic rocks in the Puttuaaluk Lake area, Labrador are in contact with granitic rocks of similar age and basement granulites of Archean (?) age. Internal variation within the anorthositic rocks has led to their subdivision into anorthosite, leuconorite, and oxide-rich anorthosite, which suggests three separate intrusive events and at least three magma types. The major differences among the anorthositic rocks are that anorthosite typically carries augite, leuconorite carries orthopyroxene ± augite, and oxide-rich anorthosite carries ilmenite ± magnetite with minor orthopyroxene. Plagioclase compositions plotted in the system Ab–An–Or lie within 3 mol% Or of the Ab–An join, and regression analyses for anorthosite and leuconorite show a slight decrease of K with decreasing An. Hence a distribution coefficient D K plag–liq ≥ 1.0 is implied for the anorthositic parent magmas.The average modal composition of the leuconorite, assuming 1% ilmenite, is 23% orthopyroxene and 76% plagioclase (volume percent). This approaches the cotectic composition (25% Opx, 75% Plag) for leuconorite estimated for the system OI–Plag–SiO 2 after correction for the effects of Ab, Fe 2+ , and pressure. It is concluded that the leuconorite unit represents on the average a cotectic composition or a composition slightly more plagioclase-rich than the cotectic. It is likely that the leuconorite magma composition lay in the plagioclase field near the Plag–Opx join and the average rock composition. The low color index of the anorthosite unit, with an average modal composition, assuming 1% ilmenite, of 6% pyroxene and 93% plagioclase, suggests that crystallization from a magma equivalent to the rock composition is highly unlikely. Flotation of plagioclase from a leucogabbroic magma could account for the extreme plagioclase richness of the anorthosite unit. The oxide-rich anorthosite with its calcic plagioclase and abundant ilmenite cannot be the logical daughter product of either the anorthosite or the leuconorite and is more likely ...