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The metasomatic development of zoned ultramafic balls from Fiskenaesset, West Greenland

Published online by Cambridge University Press:  05 July 2018

M. B. Fowler ,
H. R. Williams  and
B. F. Windley
M. B. Fowler
Affiliation:
Dept. of Geology, Imperial College, London SW7 2BP
H. R. Williams
Affiliation:
Dept. of Geology, The University, Leicester, LE1 7RH
B. F. Windley
Affiliation:
Dept. of Geology, The University, Leicester, LE1 7RH
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Abstract

This paper presents new data on the metasomatic development of zoned ultramafic balls from Fiskenaesset, West Greenland. Field and petrographic evidence indicate retrogression of original ultrabasic inclusions in acid country rock gneisses to serpentinemagnesite assemblages, followed by regional metamorphism and consequent development of the zonal structure upon metasomatic re-equilibration via a supercritical aqueous fluid phase confined to grain boundaries. The balls show varying degrees of deviation from an ideal sequence (antigorite-talc-tremolite-hornblende-chlorite-country rock) intimated by non-equilibrium thermo-dynamics, the currently accepted conceptual framework for the discussion of diffusion metasomatism.

Major and trace-element variations behave in a similar manner to those reported from other zoned ultramafic balls but fail to define the original country rock ultrabasic discontinuity unambiguously. It is tentatively suggested that application of the apparently systematic deviations from ideality may provide additional evidence.

The presence of a hornblende zone and absence of a continuous biotite zone are two significant differences from other zoned ultramafic bodies. The former may suggest increased mobility of the aluminium species at Fiskenaesset, the latter a lower temperature of formation or smaller K2O content of the gneissic hosts.

Type
Research Article
Information
Mineralogical Magazine , Volume 44 , Issue 334 , June 1981 , pp. 171 - 177
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1981

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Footnotes

*

Present address: Dept. of Geological Sciences, Brock University, St. Catherines, Ontario, Canada LZS 3A1.

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