Geochronological constraints on the magmatic, metamorphic and thermal evolution of the Connemara Caledonides, western Ireland

Abstract

The Connemara region of the western Irish Caledonides records the evolution of a short-lived mid-Ordovician (c. 475–463 Ma) continental magmatic arc that intruded into Dalradian metasedimentary rocks during regional ductile deformation. New U-Pb geochronological data for intermediate intrusive phases and anatexis in southern Connemara are consistent with the relative age succession suggested by field relationships. Calc-alkaline plutons (quartz diorites) were emplaced into the Dalradian sequence and into the Connemara Gabbros at about 467 ± 2 Ma, coeval with anatexis, which was manifested by a 467 ± 2 Ma granitic pegmatite and 468 ± 2 Ma leucosome formation in a metapelite. Metasomatic diopside rocks from west-central Connemara and a calcsilicate rock from southern Connemara yield U-Pb titanite dates of c. 462 ± 2 Ma and 463 ± 2 Ma, respectively. These dates postdate the metamorphic peak by at least five million years and indicate titanite crystallization at a late stage during the metamorphic history. On the basis of U-Pb and 40 Ar/ 39 Ar geochronological data we conclude that the cooling history of southern Connemara following anatexis was simple and brief, at a minimum rate of 35°C Ma" 1 between 468 and 460 Ma, and 14°C Ma -1 between 460 and 450 Ma.