A comparison of amphiboles from medium- and low-pressure metabasites

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Authors:Hynes, A.
Author Affiliations:Primary:
McGill Univ., Dep. Geol. Sci., Montreal H3A 2A7, Canada
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 81(2), p.119-125. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1982
Note:In English. 30 refs.; illus. incl. 2 tables
Summary:A comparison of published metabasite amphibole analyses from medium- and low-P metamorphic terrains shows that there is no systematic variation in NaM4, Al or Alvi as a function of P. This may be due to blurring of the differences by variation in oxidation state, or by analytical differences between laboratories; it is not due to variable Mg/Fe in whole rocks. Differences that can be recognized are generally higher Ti/Al ratios in the low-P amphiboles, and a very poorly developed compositional gap between actinolite and hornblende, compared with a well-developed gap at medium P. All three features can be explained by the configuration of cation-exchange equilibria at the greenschist/amphibolite facies boundary. Enrichment in Ti at low P is due to the positive slope of reactions partitioning Ti into the amphibole. The composition gap in amphiboles at medium P is due to overstepping of the tschermakite-enriching equilibrium. At low P this overstepping still occurs, but the equilibrium tschermakite-content is much lower for a given amount of overstepping. The low-grade appearance of oligoclase at low P is due to convergence of the tschermakite- and anorthite-enriching equilibria with decreasing P. [P.Br.]
Subjects:Actinolite; Alkali metals; Aluminum; Amphibole group; Amphibolite facies; Chain silicates; Clinoamphibole; Facies; Feldspar group; Framework silicates; Geochemistry; Greenschist facies; Hornblende; Ion exchange; Low pressure; Metabasite; Metaigneous rocks; Metals; Metamorphic rocks; Metamorphism; Mineral composition; Miscibility gap; Petrology; Phase equilibria; Plagioclase; Pressure; Regional metamorphism; Silicates; Sodium; Titanium; Tschermakite; Middle pressure
Abstract Numbers:83M/3588
Record ID:1985080249
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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