Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador
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 Ca 6 Fe 82 Mg 12 comparison with experimental data suggests a minimum pressure of crystallization...
| Published in: | Journal of Petrology |
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| Main Author: | |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1974
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| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/15/1/58 https://doi.org/10.1093/petrology/15.1.58 |
| Summary: | 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 Ca 6 Fe 82 Mg 12 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 Ca 7 Fe 72 Mg 21 ), ferroaugite, fayalite (near Fo 9 ), 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 Ca 24 Fe 58 Mg 18 . 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 (Ca 2 Fe 78 Mg 20 ) is present in grains where orthopyroxene did not nucleate as pigeonite cooled and exsolved ferroaugite. Iron-rich orthopyroxene (Ca 2 Fe 79 M 19 ) crystallized instead of pigeonite in a Greenland quartz syenite that contains more abundant hydrous phases. |
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