Muscovite breakdown and corundum growth at anomalously low fH2o: a study of contact metamorphism and convective fluid movement around the Omey granite, Connemara, Ireland
| Autor: | Colin C. Ferguson, Sanaa I. Al-Ameen |
|---|---|
| Rok vydání: | 1985 |
| Předmět: |
Calcite
010504 meteorology & atmospheric sciences Muscovite Metamorphic rock Geochemistry Metamorphism Skarn Corundum engineering.material Anorthite 01 natural sciences Andalusite chemistry.chemical_compound chemistry Geochemistry and Petrology engineering Geology 0105 earth and related environmental sciences |
| Zdroj: | Mineralogical Magazine. 49:505-514 |
| ISSN: | 1471-8022 0026-461X |
| DOI: | 10.1180/minmag.1985.049.353.03 |
| Popis: | In the aureole of the late Caledonian Omey granite corundum develops in the Dalradian country rocks in a zone up to 250 m from the granite contact. The distribution of andalusite and K-feldspar in pelites and calcite+wollastonite+grossularite in marbles is consistent with inner-aureole metamorphic conditions of 615±25°C at 2.5±0.25 kbar, and ⋍ 0.85. Corundum develops from the reaction muscovite → corundum + K-feldspar+H2O and first appears over 100 m further from the granite than the assemblage wollastonite + grossularite + anorthite. Experimentally determined equilibria can be satisfied only if for the corundum-producing reaction was less than 0.6 and perhaps as low as 0.4. Corundum always grows within large muscovite crystals;fH2O within the crystal lattice is unrelated to that in the grain-boundary fluid of the surrounding rock.Although whole-rock oxidation ratios are irregularly distributed within the aureole they are uniformly low in corundum-bearing rocks. Reducing conditions probably resulted from localized flow of H2O-rich fluid away from the granite in a diffuse channelway that contains most of the corundum localities and also a distinctive skarn. Although corundum growth within muscovite is sealed off from the external water vapour conditions, it is suggested that movement of H2O down a thermal gradient (and hence down an fH2O gradient at constant pressure) promotes the escape of (OH)− from the muscovite lattice and so allows the corundum reaction to proceed. |
| Databáze: | OpenAIRE |
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