The cooling history of Connemara, western Ireland, from K-Ar and Rb-Sr age studies

Abstract

K-Ar age determinations are reported on 26 hornblendes, 17 coexisting muscovites and biotites, nine muscovites and nine biotites from the Dalradian Connemara schists, metagabbros and gneisses. These are supplemented by 19 Rb-Sr whole-rock-mica ages and four 40 Ar/ 39 Ar incremental heating ages on micas. A long (at least 75 Ma) and complex cooling history without a simple pattern of ages has been obtained. The ages are not correlated with the grain-sizes or the chemical compositions of the analysed minerals, or the topographic elevation from which the samples came, but there is an irregular increase of hornblende K-Ar ages (to 480 ± 10 Ma) with increase of metamorphic grade southwards towards the metagabbros and gneiss. The preservation of the complex cooling pattern is ascribed to the action of syn- and post-metamorphic slides or faults which moved slices of the massif upwards by varying amounts. Closely spaced samples give results which rule out simple tilting of the massif. Rapid cooling ( c. 30° Ma −1 ) at 480 ± 10 Ma (Arenig-Llanvirn) during the F3 fold phase was followed by slower cooling until 455 ± 10 Ma (Caradoc), when renewed rapid uplift occurred, possibly correlated with the southward thrusting of Connemara and with synchronous renewed uplift in the Scottish Dalradian rocks. Widespread biotite K-Ar and Rb-Sr ages of 440 ± 5 Ma (end-Ordovician) throughout Connemara and the Dalradian rocks of Scotland are ascribed to the major Taconic tectonic event, which docked the Dalradian rocks against the flanking folded Ordovician successions of the Midland Valley and the South Mayo Trough. Biotites giving K-Ar ages of about 415 Ma may have been reset during the late Silurian folding of the cover rocks. The importance of very close sampling integrated with structural studies is emphasized.