The Tyrone Igneous Complex is one of the largest areas of ophiolitic and arc-related rocks exposed along the northern margin of Iapetus within the British and Irish Caledonides. New U–Pb zircon data and regional geochemistry suggest that the Tyrone Plutonic Group represents the uppermost portions of a c . 480 Ma suprasubduction-zone ophiolite accreted onto an outboard segment of Laurentia prior to 470.3 ± 1.9 Ma. The overlying Tyrone Volcanic Group formed as an island arc that collided with the Laurentian margin during the Grampian phase of the Caledonidan orogeny. Early magmatism is characterized by transitional to calc-alkaline, light REE (LREE)-enriched island-arc signatures, with an increasing component of continentally derived material up sequence. Tholeiitic rhyolites with flat to U-shaped REE profiles and LREE-depleted basalts, located stratigraphically below a c . 473 Ma rhyolite of the upper Tyrone Volcanic Group, suggest initiation of intra-arc rifting at c . 475 Ma. Metamorphic cooling ages from the Tyrone Central Inlier imply arc–continent collision before 468 ± 1.4 Ma, with the emplacement of the Tyrone Volcanic Group onto the margin. A suite of 470.3 ± 1.9 Ma to 464.3 ± 1.5 Ma calc-alkaline intrusions are associated with the continued closure of Iapetus. Supplementary material: Geochemical data and petrography are available at http://www.geolsoc.org.uk/SUP18467 .

In order to understand the progressive growth of continental margins and the evolution of continental crust, we must first understand the formation of allochthonous ophiolitic and island-arc terranes within ancient orogens and the nature of their accretion. During the early Paleozoic closure of the Iapetus Ocean, diverse sets of arc terranes, oceanic tracts, and ribbon-shaped microcontinental blocks were accreted to the passive continental margin of Laurentia during the Grampian-Taconic orogeny. In the northern Appalachians in central Newfoundland, Canada, three distinct phases of arc-ophiolite accretion have been recognized. New field mapping, high-resolution airborne geophysics, whole-rock and Nd-isotope geochemistry, and U-Pb zircon geochronology within the Tyrone Volcanic Group of Northern Ireland have allowed all three episodes to now be correlated into the British and Irish Caledonides. The Tyrone Volcanic Group (ca. 475–469 Ma) is characterized by mafic to intermediate lavas, tuffs, rhyolite, banded chert, ferruginous jasperoid, and argillaceous sedimentary rocks cut by numerous high-level intrusive rocks. Geochemical signatures are consistent with formation within an evolving peri-Laurentian island-arc/backarc, which underwent several episodes of intra-arc rifting prior to its accretion at ca. 470 Ma to an outboard peri-Laurentian microcontinental block. Outriding microcontinental blocks played a fundamental role within the orogen, explaining the range of ages for Iapetan ophiolites and the timing of their accretion, as well as discrepancies between the timing of ophiolite emplacement and the termination of the Laurentian Cambrian–Ordovician shelf sequences. Accretion of the Tyrone arc and its associated suprasubduction-zone ophiolite represents the third stage of arc-ophiolite emplacement to the Laurentian margin during the Grampian-Taconic orogeny in the British and Irish Caledonides.

Recently identified Early Jurassic, Early Cretaceous, and Late Cretaceous granites of the Tengchong terrane, SW China, help to refine our understanding of the Mesozoic tectonic-magmatic evolutionary history of the region. We present new zircon U–Pb geochronological, Lu–Hf isotopic and geochemical data on these rocks. The zircon LA-ICP-MS U–Pb ages of the Mangzhangxiang, Laochangpo, and Guyong granites, and Guyong granodioritic microgranular enclaves are 185.6, 120.7, 72.9, and 72.7 Ma, respectively. Geochemical and Hf isotopic characteristics suggest the Mangzhangxiang and Laochangpo S-type granites were derived from partial melting of felsic crust and that the Guyong I-type granite and associated MMEs were generated through magma mixing/mingling. Mesozoic magmatism in the Tengchong terrane can be divided into three episodes: (1) the Triassic syn- and post-collisional magmatic event was related to the closure of the Palaeo-Tethyan Ocean, as represented by the Changning-Menglian suture zone; (2) the Jurassic to Early Cretaceous magmatism was related to the subduction of the Meso-Tethyan oceanic crust, as represented by the Myitkyina ophiolite belt; and (3) the Late Cretaceous magmatism was related to the subduction of the Neo-Tethyan oceanic crust, as represented by the Kalaymyo ophiolite belt.