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Petrogenesis of migmatites in the Huntly-Portsoy area, north-east Scotland

Published online by Cambridge University Press:  05 July 2018

J. R. Ashworth
J. R. Ashworth*
Affiliation:
Department of Physics, University of Essex, Wivenhoe Park, Colchester
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Summary

Migmatites are described from the Sillimanite-potash-feldspar Zone of the aureole around the Newer Basic suite of synorogenic intrusions. The lowest-grade migmatites are trondhjemitoid (characterized by the assemblage quartz-plagioclase-biotite) or muscovite-granitoid (quartz-plagioclase potash-feldspar-muscovite-sillimanite-biotite). With increasing grade, a transition occurs to cordierite-granitoid assemblages (quartz-plagioclase-potash-feldspar-cordierite-garnet-sillimanite-biotite), which persist to the highest grades observed, where there are also noritoid migmatites (quartz-plagioclase-orthopyroxene-cordierite-biotite). The trondhjemitoids are texturally simple because the minerals did not undergo dehydration reactions. Textural immaturity and consistently cotectic modal compositions indicate that their leucosomes originated as melts. Scatter of plagioclase compositions suggests that the partial melting occurred in small closed systems. The other migmatites have more fusible compositions, so it is deduced that they also underwent partial melting. Retrograde reaction textures are used to infer the sequence of reactions, involving muscovite and biotite, by which melting proceeded during prograde evolution. Whereas the fugacity of water probably varied among spatially associated trondhjemitoid leucosomes, in the muscovite-granitoids it was constrained to an approximately constant value, at given pressure and temperature, by the buffering effect of the mineral assemblage.

Type
Research Article
Information
Mineralogical Magazine , Volume 40 , Issue 315 , September 1976 , pp. 661 - 682
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1976

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Footnotes

1

Present address: Dept. of Geological Sciences, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET.

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