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Crystallization sequences of coexisting andalusite, kyanite, and sillimanite, and a report on a new locality; Lesjaverk, Norway

Donna L. Whitney and Willa J. Samuelson
Crystallization sequences of coexisting andalusite, kyanite, and sillimanite, and a report on a new locality; Lesjaverk, Norway
European Journal of Mineralogy (July 2019) 31 (4): 731-737

Abstract

Rocks containing three Al (sub 2) SiO (sub 5) polymorphs - andalusite (And), kyanite (Ky), and sillimanite (Sil) - represent disequilibrium resulting from polymorph instability driven by changing pressure-temperature (P-T) conditions during related or unrelated tectonometamorphic events. There are six possible crystallization sequences. The two most common sequences of seventeen localities surveyed are: (1) Ky --> Sil --> And (six localities), and (2) And --> Ky --> Sil (six localities), including one locality for which two different sequences have been proposed. The two most common crystallization sequences have very distinct P-T conditions and paths and differ in whether low-P/moderate- to high-T (And zone) metamorphism preceded or postdated Ky-Sil zone conditions. Ky --> Sil --> And sequences may form by a continuum of tectonometamorphic events that drive isothermal decompression from Ky zone conditions through the Sil zone to shallow (And zone) crustal levels, such as during extension magmatism that ultimately results in thin, hot crust. In sequences with early andalusite (And --> Ky --> Sil), formation of andalusite is typically unrelated to the later Barrovian metamorphism that formed Ky-Sil at moderate P-T conditions. Two other sequences have been reported but are much rarer: (3) Ky --> And --> Sil (three localities), and (4) And --> Sil --> Ky (two, possibly three localities). In these, Ky-zone metamorphism occurred as a distinct tectonometamorphic episode before or after the one that formed And-Sil. In cases in which Al (sub 2) SiO (sub 5) polymorphs occur in veins or shear zones, the effects of deformation and fluids, including the possibility of non-lithostatic pressure effects, must be considered. Three-Al (sub 2) SiO (sub 5) polymorph-bearing rocks from Lesjaverk, Norway, have not previously been described in the literature, and are interpreted here as forming by the sequence And --> Ky --> Sil. Metamorphic conditions determined using Zr-in-rutile thermometry indicate peak T = 600-640 degrees C at moderate P near the Ky-Sil boundary, similar to the peak conditions reported for other localities with the same crystallization sequence. These results are consistent with the finding that And --> Ky --> Sil rocks record lower P-T conditions than the other reported sequences.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 31
Serial Issue: 4
Title: Crystallization sequences of coexisting andalusite, kyanite, and sillimanite, and a report on a new locality; Lesjaverk, Norway
Affiliation: University of Minnesota-Minneapolis, Department of Earth Sciences, Minneapolis, MN, United States
Pages: 731-737
Published: 20190705
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 26
Accession Number: 2019-063930
Categories: Igneous and metamorphic petrologyMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, geol. sketch map
N62°12'27" - N62°12'27", E08°31'43" - E08°31'43"
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201933
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