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

Y. Ahmed-Said* and Bernard E. Leake
Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK
*Previous address: Oum-Toub (Skikda), P.O. Box 56, 21450, Algeria

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: activity • grossular • hornfels • thermodynamic mole fraction • vesuvianite • wollastonite

Mineralogical Magazine; August 1996 v. 60; no. 401; p. 541-550; DOI: 10.1180/minmag.1996.060.401.01
© 1996, The Mineralogical Society
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