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The conditions of metamorphism of a grossular—wollastonite vesuvianite skarn from the Omey Granite, Connemara, western Ireland, with special reference to the chemistry of vesuvianite

Published online by Cambridge University Press:  05 July 2018

Y. Ahmed-Said  and
Bernard E. Leake
Y. Ahmed-Said
Affiliation:
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK
Bernard E. Leake
Affiliation:
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK
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Abstract

The Fountain Hill skarn, which was produced by thermal metamorphism of impure limestone in the Omey Granite aureole, consists of wollastonite, calcite, grossular-andradite garnet, diopside, B-free vesuvianite and small amounts of albite, K-feldspar and quartz. Zoning in vesuvianite is, overall, independent of birefringence but oscillatory concentric zoning in the mineral is controlled mainly by Ti. In addition to extensive within-site substitutions (e.g. F for OH in the OH-sites, Fe2+ for Mg, Fe3+ for Al, Ti for Al etc. in the Y-sites), there are significant cross-site combined substitutions involving Y-X and Y-Z but not X-Z sites so that ideal solution models for this mineral are not applicable. The thermodynamic mole fraction of Hoisch (1985) is modified to account for the excess of the ΣY cations in the Y sites and can be applied to both B-bearing and B-free vesuvianites. Using the thermodynamic dataset of Holland and Powell (1990) and taking into account the existence of andalusite, sillimanite and corundum in associated pelites, leads to the conclusion that the metamorphic conditions were about 640±20°C and 3.3±0.3 kb at 0.15±0.05 XCO2.

Keywords

activitygrossularhornfelsthermodynamic mole fractionvesuvianitewollastonite
Type
Petrology
Information
Mineralogical Magazine , Volume 60 , Issue 401 , August 1996 , pp. 541 - 550
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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Footnotes

*

Previous address: Oum-Toub (Skikda), P.O. Box 56, 21450, Algeria

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