Abstract

Dr B. W. D. Yardley writes: the paper by Ferguson & Al-Ameen (1986) is welcome for the detailed information it provides for the first time on the regional metamorphism in western Connemara. There are, however, a number of problems with the temperature and pressure determination made in the paper and these have important repercussions for both the determination of the thermal history of western Connemara and the comparisons made with the region to the east described by Yardley et al. 1980.

Geothermometry. The authors rely primarily on the garnet–biotite cation exchange geothermometer, as did Yardley et al. 1980. Unfortunately, however, they have used a different calibration of the thermometer (due to Hodges & Spear 1982) from that on which the earlier study was based. Yardley et al. (1980) relied primarily on Ferry & Spear (1978). The effect of this is that for the rocks in question the Hodges & Spear (1982) calibration yields temperatures that are consistently higher (by at least 30°) than if the Ferry & Spear (or most other) calibrations had been used. This effect accounts entirely for the temperature differences claimed by Ferguson & Al-Ameen between rocks of similar grade in east and west Connemara.

It is hard to decide which of the two calibrations is actually correct, and indeed their discrepancy might best be seen as an indication of the uncertainty that actually attends estimates of metamorphic temperature. Nevertheless, there are several independent lines of evidence which suggest that the temperature estimates of Ferguson &

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