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Exsolution and alteration microtextures in alkali feldspar phenocrysts from the Shap granite

Published online by Cambridge University Press:  05 July 2018

Martin R. Lee ,
Kim A. Waldron  and
Ian Parsons
Martin R. Lee
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
Kim A. Waldron
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
Ian Parsons
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland
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Abstract

Alkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to > 1 µm length. Cryptoperthites developed at < 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular microcline.

Keywords

alkali feldsparargon systematicsdeuteric alterationdislocationselectron microscopyetchingexsolutiongraniteorthoclasemicroclineperthite
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 394 , March 1995 , pp. 63 - 78
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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Footnotes

*

Present address: Department of Geology, Colgate University, Hamilton, NY 13346, USA

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