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Contrasting Fe-Ca distributions and related microtextures in syenite alkali feldspar from the Patagonian Andes, Chile

Published online by Cambridge University Press:  05 July 2018

Satoshi Nakano ,
J. Akai  and
N. Shimobayashi
Satoshi Nakano*
Affiliation:
Department of Natural Science, Faculty of Education, Shiga University, Hiratsu 2-5-1, Otsu, Shiga 520-0862, Japan
J. Akai
Affiliation:
Department of Geological Sciences, Faculty of Science, Niigata University, Ikarashi-2, Niigata 950-2181, Japan
N. Shimobayashi
Affiliation:
Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
*
*E-mail: nakano@sue.shiga-u.ac.jp
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Abstract

Contrasting distribution patterns of Fe and Ca have been found by electron microprobe analysis (EMPA) mapping of alkali feldspar in a quartz syenite from the Patagonian Andes, Chile. They comprise mainly mantle zoning (Fe-rich, Ca-poor rims and Fe-poor, Ca-rich interiors) and corresponding patchy zoning in grain interiors. The rims are dominantly of turbid, patch microperthites associated with abundant micropores, but there remain clear, optically featureless regions almost free of micropores. The interiors are intricate mixtures of optically clear, featureless regions, and turbid, patch microperthite regions. The clear, featureless regions (Or31 –47) are of remaining exsolution lamellar cryptoperthites. The zoning patterns of Fe and Ca formed by large-scale transport over the feldspar grain during the high-temperature fluid stage. They have been modified by successive transport of Fe and Ca during the later hydrothermal development of patch microperthites and finally by K-feldspathization and albitization. Cathodoluminescence images correspond to the spatial distribution patterns of Fe overprinted by these multi-stage reactions. The original composition of the alkali feldspar before the subsolidus reactions is estimated to have been ~Or34Ab65An1, and the present bulk composition after the reactions is Or40Ab59An0.5.

Keywords

Chilesyenitealkali feldsparFe-Ca distributionszoningperthite coarseningfluidcathodoluminescenceTEM
Type
Research Article
Information
Mineralogical Magazine , Volume 69 , Issue 4 , August 2005 , pp. 521 - 535
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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