Abstract

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.

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