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DOUGLAS SMITH, Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador, Journal of Petrology, Volume 15, Issue 1, 1974, Pages 58–78, https://doi.org/10.1093/petrology/15.1.58
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Abstract
Unusually iron-rich pyroxene and olivine occur in rocks associated with the Nain anorthosite massif, Labrador. Adamellite and granodiorite contain orthopyroxene (inverted from pigeonite) as iron-rich as Ca6Fe82Mg12; comparison with experimental data suggests a minimum pressure of crystallization of 5 kb. Some of these iron-rich pyroxene crystals have broken down, apparently upon decreasing pressure, to yield intergrowths of less iron-rich orthopyroxene (near Ca7Fe72Mg21), ferroaugite, fayalite (near Fo9), and quartz. Other rocks, monzonites, contain pyroxenes with calcium-poor cores and ferroaugite rims, as well as crystals composed of broad lamellae of ferroaugite and orthopyroxene in sub-equal proportions. Analysis of one such crystal with unusually thin and closely spaced lamellae yielded a bulk composition of Ca24Fe58Mg18. Such pyroxenes probably crystallized near or above the crest of the augite-pigeonite two-phase region, probably above 925 °C. This high temperature suggests that the monzonites crystallized from relatively dry magmas. If they represent a residual fraction derived from the same magma as the anorthosite, then that magma must have been nearly anhydrous.
Pigeonite rather than orthopyroxene was the primary magmatic Ca-poor pyroxene in most of the Nain rocks studied here. Nucleation rates apparently were low under subsolidus conditions, and low-Ca pigeonite (Ca2Fe78Mg20) is present in grains where orthopyroxene did not nucleate as pigeonite cooled and exsolved ferroaugite. Iron-rich orthopyroxene (Ca2Fe79M19) crystallized instead of pigeonite in a Greenland quartz syenite that contains more abundant hydrous phases.
