Rapid arc-ophiolite formation during the Grampian-Taconic event: evolution of the Tyrone Igneous Complex, Northern Ireland

The Grampian-Taconic event of the broader Caledonian-Appalachian orogenic cycle records the accretion of oceanic tracts, arc terranes and microcontinental blocks the Laurentian margin during the early Palaeozoic. We present new field constraints, whole rock geochemistry, Sm-Nd isotope data and U-Pb zircon geochronology from the Tyrone Igneous Complex. The ca. 484-480 Ma Tyrone Plutonic Group forms the structurally lowest levels of the complex and preserves a tectonically dissected suprasubduction affinity ophiolite, obducted onto an outboard fragment of Laurentian microcontinental crust (the Tyrone Central Inlier). A new U-Pb zircon age of S-type muscovite granite from Tremoge Glen (c. 477 Ma) suggests ophiolite obduction was likely coeval with the emplacement of the Ballantrae ophiolite of Scotland at c. 480-478 Ma. A 5 Myr magmatic gap in the Tyrone Igneous Complex (c. 477-473 Ma) likely relates to a subduction polarity reversal at this time or jump into the Irish equivalent of the Taconic seaway. The structurally overlying ca. 473-469 Ma Tyrone Volcanic Group formed upon the Tyrone Central Inlier and records episodes of intra-arc rifting and backarc volcanism. Continental arc magmatism continued until c. 464 Ma through the emplacement of I-type granite and sheets of quartz-feldspar porphyry. In the British and Irish Caledonides, the Tyrone Volcanic Group correlates with Middle Ordovician continental volcanic arc rocks of Charlestown and Connemara, and the late ‘within-plate’ volcanic sequences at Ballantrae.