GEOLOGICAL SIGNIFICANCE OF 40AR/39AR MICA DATES ACROSS A MID-CRUSTAL CONTINENTAL ARC, CONNEMARA (IRISH CALEDONIDES) AND IMPLICATIONS FOR THE THERMAL EVOLUTION OF OROGENIC SYSTEMS

The Connemara region of the Irish Caledonides is a classical example of a regional-scale high-temperature metamorphic terrain. Its formation relates to continental arc magmatism in a convergent setting, for which a protracted evolution was inferred based on a > 75 Ma spread in U-Pb, Rb-Sr, and K-Ar mineral dates. Such a history is inconsistent with geological field observations, which imply a simple relationship between syntectonic magmatism and metamorphism. Here, we explore the significance of the large spread in apparent cooling ages using ⁴⁰Ar/³⁹Ar mica thermochronometers of varying grain sizes and composition, which we collected across all metamorphic grades. We integrated geological and previously published geochronological evidence to identify a 32 Ma range (c. 475 to 443 Ma) of permissible cooling dates and distinguished them from those ages not related to cooling after high-temperature metamorphism. Variations in ⁴⁰Ar/³⁹Ar cooling dates at a single locality are ≤ 10 Ma, implying rapid cooling (≥ 6 to 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 and magmatic evolution across the Connemara region. This cooling record relates to a lateral thermal-gradient in an evolving continental magmatic arc, 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. Our results build upon insights gleaned from high-precision U-Pb zircon-ages of the Connemara intrusives, which we had acquired previously with the help of Samuel Bowring.