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Chemistry and microstructure of orthoamphiboles from cordierite-amphibole rocks at Outokumpu, North Karelia, Finland

Published online by Cambridge University Press:  05 July 2018

Peter J. Treloar  and
Andrew Putnis
Peter J. Treloar
Affiliation:
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ
Andrew Putnis
Affiliation:
Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ
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Abstract

Orthoamphiboles from the cordierite-amphibole-bearing rocks at Outokumpu span a large part of the gedrite-anthophyllite solid-solution series. The amphiboles grew at temperatures near to or just above that of the solvus crest with post-metamorphic cooling resulting in sub-solvus exsolution, the distribution of which is compatible with an asymmetric solvus in which the anthophyllite limb is the steeper. Metamorphic conditions in the area imply that the crest of the solvus is unlikely to be at a temperature in excess of about 600 °C Electron microprobe traverses and Al-Kα scans show that the amphibole compositions vary rapidly both along and across grains. These variations can be explained by varying availability of Na controlling the extent of the edenitic substitution in the solid solution series. In this case this variability in Na availability may be explained by the external control of μNa through the fluid phase.

Type
Research Article
Information
Mineralogical Magazine , Volume 45 , Issue 337 , 1982 , pp. 55 - 62
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1982

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