⁴⁰Ar/³⁹Ar and U-Pb geochronological constraints on the thermal and tectonic evolution of the Connemara Caledonides, Western Ireland

Abstract

The Connemara region of the Irish Caledonides is a classical example of a regional-scale high-temperature metamorphic terrain. Its formation was related to intrusion of a compressional continental magmatic arc, for which a protracted thermal 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 field observations which suggest a simple relationship between metamorphism and syntectonic magmatism. This study was designed to explore the significance of the large spread in apparent ages using higher resolution U-Pb and 40Ar/ 39Ar geochronometers. The results indicate that arc magmatism, sillimanite-grade metamorphism, anatexis, and late fluid infiltration spanned only about 12 million years. Cooling following the metamorphic peak was actually relatively rapid at 35*C/Ma until about 460 Ma, then 214*C/Ma until 450 Ma. Regional differences in 40Ar/ 39Ar cooling ages of >15 Ma are related to spatial and temporal variations in magmatism, metamorphism, and deformation, rather than differential unroofing of the orogen. 40Ar/ 39Ar dates older than the onset of magmatism or younger than a regional Silurian unconformity represent the combined effects of excess 40Ar contamination, metasomatism, thermal resetting or alteration related to post-orogenic pluton emplacement. This study shows that geochronologic data must be evaluated in the context of careful field mapping, structural and petrologic analysis. Geochronological data from Connemara suggest that arc magmatism related to the Grampian orogeny in this region spanned a brief interval between 475 and 462 Ma and was followed by rapid cooling. The oldest recognized Grampian processes included high P/T metamorphism, followed by intrusion of the Connemara Gabbros into Dalradian metasedimentary rocks, regional-scale ductile deformation, and sillimanite-grade metamorphism between 474.5 and 470.1 Ma. Voluminous -467 Ma quartz diorites only intruded in southern Connemara associated with more localized deformation, anatexis and metasomatism between 468 and 462 Ma. Intrusion of the 462.5 Ma Oughterard Granite marks the end of arc magmatism and contractional deformation at Connemara. The compressional continental magmatic arc at Connemara (the Grampian orogeny) was coeval with continental arc magmatism in Scotland and Newfoundland, and postdates ophiolite formation and obduction along strike in the Appalachian- Caledonian orogen.