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Journal of the Geological Society; 1986; v. 143; issue.2; p. 237-252;
DOI: 10.1144/gsjgs.143.2.0237
© 1986 Geological Society of London

Article

Geochemistry of Dalradian pelites from Connemara, Ireland: new constraints on kyanite genesis and conditions of metamorphism

C. C. FERGUSON1 & S. I. AL-AMEEN2

1 Department of Geology, Birkbeck College (University of London), London W1P 1P A, England
2 Department of Geology, University of Nottingham, Nottingham, NG7 2RD, England

Whole-rock and mineral analyses of Dalradian pelites from W Connemara show that kyanite pelites are not unusually Mg-rich as previously supposed. Their chemistry is transitional, especially in Ca content, between overlying Fe-rich staurolite pelites and underlying Ca-rich rocks. The flysch basin in which they were deposited was established relatively early in the west, its sediment fill including carbonate-rich influxes derived from an adjacent swell to the east which later subsided to form an enlarged basin. Kyanite has a very restricted spatial distribution because its development was controlled by Fe deficiency associated with the carbonate-rich influxes, which in turn led to relatively high XMg during metamorphism. P–T estimates for the metamorphic peak are 650° ± 25 °C and ~8kbar, about 50 °C and 3kbar greater than for similar rocks in the east. The difference can be related to structural level with respect to regional-scale folds, which may have developed as a huge backfolded complex during the emplacement of the Connemara allochthon.





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