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Metasomatism in the North Qôroq centre, South Greenland: cathodoluminescence and mineral chemistry of alkali feldspars

Published online by Cambridge University Press:  03 November 2011

D. A. Rae  and
A. D. Chambers
D. A. Rae
Affiliation:
Department of Geological Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, U.K.
A. D. Chambers
Affiliation:
Department of Geological Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, U.K.
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Abstract

The North Qôroq centre consists of a number of units of syenite, all undersaturated, with later units in sharp intrusive contact with earlier ones. Chemical and mineralogical characteristics have been determined by primary crystal-silicate liquid equilibria and also later, extensive, metasomatic alteration probably involving an alkali-rich aqueous phase. Cathodoluminescence studies on feldspars, the dominant phase present, reveal the development of red-luminescing albite in all the rocks affected by a late-stage fluid phase in contrast with the blue luminescence shown by unaltered perthites. Spectral and chemical investigation of the feldspars confirms earlier views that red luminescence is caused by Fe3+, an impurity in the feldspar structure, but also indicates that, for red luminescence to be seen, the blue luminescent peak must be greatly reduced or absent. The cause of the blue luminescence is less obvious but may well be related to an oxygen defect centre. Such a centre is apparently absent in red-luminescing albites and hence not formed during growth involving late-stage fluids. The fluids evolved from highly peralkaline, undersaturated magmas and metasomatised less evolved syenites forming feldspathoids, particularly sodalite. Once equilibrium had been attained the fluid behaved principally as a passive phase on its passage through the rocks, aiding the unmixing of the alkali feldspars.

Keywords

albitefeldspathoidfluid-phaseGardarperthite
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 79 , Issue 1 , 1988 , pp. 1 - 12
Copyright
Copyright © Royal Society of Edinburgh 1988

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