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U–Pb and Rb–Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland
J. A. Evans & N. J. Soper write: We comment on two papers on Caledonian geology recently published in the Journal; Dempster et al. 1995 and Cliff et al. 1996. These papers investigate two inter-linked themes, the timing of the Grampian/Caledonian orogeny in the Dalradian and the role of the Newer Gabbro suite in the metamorphism. The former paper is concerned with NE Scotland, the latter with western Ireland; they arrive at startlingly different conclusions.
Dempster et al. 1995 present mica-whole-rock Rb-Sr ages from metamorphic rocks along the Buchan coast which they interpret as a record of cooling after the main Buchan tectonometamorphism. They infer a minimum age of 505 Ma for the D1–D2 orogenic peak, the oldest apparent age recorded, and favour a pre-590 Ma age for D1, the age of Ben Vurich intrusion (Rogers et al. 1989). On this basis they conclude that emplacement of the gabbros was not responsible for the high heat flow Buchan metamorphism (the traditional interpretation) but, somewhat counter-intuitively, for development of the higher pressure Barrovian assemblages by a process of ‘magmatic loading’.
In contrast, the conclusion reached by Cliff et al. 1996 from a combined U–Pb and Rb–Sr study in Connemara is that the low pressure metamorphism in this part of the Dalradian tract is linked with the syn-orogenic gabbros but that advective heat pulses were short-lived and thus show complex relationships to the deformation. All the new isotope data presented in the Irish paper are compatible with mid-Ordovician peak
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