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An enigmatic report of unmixing of calcic plagioclase to oligoclase and anorthite finally resolved

Patrick J. O'Brien and J. Desmond C. McConnell
An enigmatic report of unmixing of calcic plagioclase to oligoclase and anorthite finally resolved
European Journal of Mineralogy (December 1999) 11 (6): 915-918

Abstract

A new explanation is presented for a reported replacement, in a metagabbro, of calcic plagioclase (An (sub 50-55) ) by discrete more calcic (An (sub 65-93) ) and more sodic (An (sub 18-33) ) plagioclase phases, by a process of exsolution. In contrast to a possible exsolution origin from unstable incommensurate plagioclase structures during long, slow cooling, it is proposed here that a two-stage metamorphic process is the most likely cause for the presence of the two different feldspars. An initial high-pressure breakdown of magmatic Ca-plagioclase produced branching bundles of lath-shaped zoisite crystals set within a residual more sodic plagioclase. Later breakdown of the zoisite, at lower pressure, gave rise to the present-day occurrence of discrete anorthite (or Ca-rich plagioclase) laths in a more sodic feldspar matrix.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 11
Serial Issue: 6
Title: An enigmatic report of unmixing of calcic plagioclase to oligoclase and anorthite finally resolved
Affiliation: Universitaet Bayreuth, Bayerisches Geoinstitute, Bayreuth, Federal Republic of Germany
Pages: 915-918
Published: 199912
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 13
Accession Number: 2000-026825
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200009
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