TY - JOUR
T1 - 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
AU - Friedrich, Anke M.
AU - Hodges, Kip
N1 - Publisher Copyright:
© 2016, Canadian Science Publishing. All rights reserved.
PY - 2016
Y1 - 2016
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=84994807591&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84994807591&partnerID=8YFLogxK
U2 - 10.1139/cjes-2016-0001
DO - 10.1139/cjes-2016-0001
M3 - Article
AN - SCOPUS:84994807591
SN - 0008-4077
VL - 53
SP - 1258
EP - 1278
JO - Canadian Journal of Earth Sciences
JF - Canadian Journal of Earth Sciences
IS - 11
ER -