Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador |
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| Authors: | SMITH DOUGLAS |
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| Institution: | Department of Geological Sciences, The University of Texas at Austin Austin, Texas 78712 |
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| Abstract: | Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca6Fe82Mg12; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca7Fe72Mg21), ferroaugite, fayalite (near Fo9), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca24Fe58Mg18.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca2Fe78Mg20) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca2Fe79M19) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases. |
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