Geological significance of Ar-40/Ar-39 mica dates across a mid- crustal continental plate margin, Connemara (Grampian orogeny, Irish Caledonides), and implications for the evolution of lithospheric collisions

摘要

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 Ar-40/Ar-39 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 Ar-40/Ar-39 dates at a single locality are <= 10 Ma, implying rapid cooling (>= 6-26 degrees 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 degrees 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.