The Coire Dhuinnid fault zone contains emerald green chromian illite-ankerite-quartz rocks that are similar in appearance to the fuchsite(or mariposite)-carbonate-quartz parageneses that are commonplace in Archaean greenstone belts but which are rather rare in Phanerozoic rocks. The chromian illite contains 2.3 wt.% Cr 2 O 3 , low K 2 O (7.1-7.6 wt.%) and high H 2 O+ (5.7 wt.%), and it is a 1M polytype with < or =10% of an illite/smectite interstratification indicative of a formation temperature of c. 175-200 degrees C. The host rocks contain high concentrations of Ni and Cr, and show low concentrations of Ti, Nb, Y and Zr, suggesting a former primitive mafic protolith (boninitic magma?); they are considered to be retrograde remnants of Lewisian rocks. The latter, and the associated rocks of the Moine Series, have been affected by CO 2 metasomatism that was accompanied by the addition of Ca(+Sr), Fe and Mg, and by the removal of Na from, and the addition of H 2 O to the Moine metasediments. Radiogenic isotope studies of mineral separates and whole rock from sample no. 43 yielded ages of 483+ or -2 Ma (Ar-Ar dating on Cr illite), 413+ or -12 Ma (K-Ar dating on Cr illite), and 322+ or -9 Ma (Rb-Sr dating on minerals and whole rock); the significance of this discrepant pattern is discussed.

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