Short-lived continental magmatic arc at Connemara, western Irish Caledonides: Implications for the age of the Grampian orogeny

U-Pb geochronological data from the Connemara region of Ireland indicate th at continental are magmatism along the southern margin of Laurentia was sho rt-lived, lasting only from ca, 475 to 463 Ma. Previous work has demonstrat ed that intrusive activity in Connemara was broadly synchronous with Grampi an mid-crustal deformation and upper amphibolite facies metamorphism of Neo proterozoic-lower Paleozoic Dalradian Supergroup rocks. The two oldest intr usions, the Currywongaun and Cashel-Lough Wheelaun gabbros, have U-Pb zirco n ages of 474.5 +/- 1.0 and 470.1 +/- 1.4 Ma, respectively, whereas the U-P b xenotime age of the postdeformational Oughterard granite is 462.5 +/- 1.2 Ma. Thus, the implied age of the Grampian orogeny in Connemara is substant ially younger than generally acknowledged, but consistent with other age co nstraints from Ireland and Scotland. A revised age for the Grampian orogeny helps to solve a long-standing controversy about the age of contractional deformation and are magmatism in Dalradian Supergroup rocks of the British Isles. This event now demonstrably postdates Neoproterozoico-lower Paleozoi c rift deposits and Laurentian passive margin sediments, a finding consiste nt with deposition of Dalradian Supergroup rocks in an extensional environm ent at or near the Laurentian margin. The Grampian orogeny is also now demo nstrably synchronous,vith the Taconian orogeny of the northern Appalachians , In Middle Ordovician time, the Laurentia-Iapetus plate boundary was chara cterized by ophiolite obduction, are-continent collision, and associated sh ort-lived subduction beneath Laurentia from at least the New England Appala chians to the Irish Caledonides.