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Geological Society, London, Special Publications; 1979; v. 8; p. 375-384;
DOI: 10.1144/GSL.SP.1979.008.01.43
© 1979 Geological Society of London

4. Metamorphism

Oxide-sulphide-silicate mineral phase relations in some thermally metamorphosed graphitic pelites from Co. Donegal, Eire

B. P. Atkin

Department of Geology, University of Nottingham, University Park, Nottingham

A chemical control over the oxide-sulphide-silicate phase relations of the graphitic pelites from the aureoles of Donegal granites is demonstrated. Kyanite-bearing rocks (M/FM ratio >0.54) have the ore assemblage of rutile + pyrite or pyrrhotite and kyanite-free rocks have the assemblage ilmenite + pyrrhotite. The stabilisation, during prograde metamorphism of pyrrhotite over pyrite and ilmenite over rutile accompanied by an increase in the Mg content of ferromagnesium phases is also demonstrated. Phase relations between oxide and silicate phases are illustrated by AFMT(i); tetrahedral diagrams and sulphide-silicate phases by AFMS tetrahedral diagrams. Retrograde reactions especially the replacement of ilmenite by rutile and pyrrhotite by marcasite are also described.