The Connemara region is a world-class example of a regional-scale, high-temperature metamorphic terrain. Its rock record documents formation of a bi-vergent orogenic wedge and associated calkalkaline magmatism in a an arc–continent collisional setting (Grampian orogeny), for which a protracted evolution was inferred based on a >75 Ma spread in U–Pb, Rb–Sr, and K–Ar mineral ages. In contrast, geological field observations imply a simple relationship between syntectonic magmatism, bi-vergent deformation, and Barrovian-type metamorphism. We explore the significance of the spread in apparent cooling ages using 40Ar/39Ar mica thermochronometers of varying grain sizes and composition, collected across metamorphic grades ranging from staurolite to upper sillimanite. We integrated geological and previously published geochronological evidence to identify a 32 Ma range (ca. 475–443 Ma) of permissible cooling ages and distinguished them from those dates not related to cooling after high-temperature metamorphism. Variations in 40Ar/39Ar dates at a single locality are ≤10 Ma, implying rapid cooling (≥6–26 °C/Ma) following metamorphism and deformation. A distinct cooling age variation (≥15 Ma) occurs on the regional scale, consistent with spatial differences in the metamorphic, magmatic, and deformational evolution across Connemara. This cooling record relates to a lateral thermal gradient (30 °C/km) in an evolving arc–continent collision, rather than to differential unroofing of the orogen. Our results imply that the large (≥50 Ma) spread in thermochronometers commonly observed in orogens does not automatically translate into a protracted cooling history, but that only a small number of thermochronometers supply permissible cooling ages.
↵* Present address: Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, 80333 Munich, Germany.
↵† Present address: School of Earth and Space Exploration, Arizona State University, Phoenix, AZ, USA.
↵1 This paper is part of a special issue that honors the careers of Kevin C. Burke and John F. Dewey.
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- Received January 4, 2016.
- Accepted August 26, 2016.
- Published on the NRC Research Press Web site at http://cjes.nrc.ca on October 6, 2016.
- Published by NRC Research Press