Geological significance of40ar/39ar mica dates across a mid-crustal continental plate margin, Connemara (Grampian Orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

Anke M. Friedrich, Kip Hodges

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Abstract

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 using40Ar/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 in40Ar/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.

Original languageEnglish (US)
Pages (from-to)1258-1278
Number of pages21
JournalCanadian Journal of Earth Sciences
Volume53
Issue number11
DOIs
StatePublished - 2016

ASJC Scopus subject areas

  • Earth and Planetary Sciences(all)

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