Episodic arc-ophiolite emplacement and the growth of continental margins: Late accretion in the Northern Irish sector of the Grampian-Taconic orogeny

Abstract

In order to understand the progressive growth of continental margins and the evolution of continental crust, we must fi rst 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 micro conti nental 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 fi eld mapping, high-resolution airborne geo physics, 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 mafi c 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.

A review of geological origins and relationships in the Ballantrae Complex, SW Scotland

“…The TVG lavas show evidence of contamination by continental material (Draut et al 2009;Cooper et al 2011) and are thought to have been erupted in a peri-Laurentian island-arc to back-arc environment, which underwent several episodes of intra-arc rifting (Hollis et al 2012). Accretion to the outboard peri-Laurentian microcontinental block now represented by the Tyrone Central Inlier followed soon after obduction of the TPG, and was also complete by about 470 Ma.…”

“…All are interbedded with calc-alkaline, arc-derived volcaniclastic rocks that display strongly negative ƐNdt values. Lavas of island arc affinity are strongly calc-alkaline in the south-east of the TIC, but are dominantly tholeiitic and primitive in the north-east, where they are related to back-arc volcanism (Hollis et al 2012). Lavas of within-plate affinity occur at several stratigraphical levels within the TVG and show Fe-Ti-P enrichment.…”

“…Ma (Cooper et al 2011;Hollis et al 2012). The TVG lavas show evidence of contamination by continental material (Draut et al 2009;Cooper et al 2011) and are thought to have been erupted in a peri-Laurentian island-arc to back-arc environment, which underwent several episodes of intra-arc rifting (Hollis et al 2012).…”

“…The TVG is a variable, island arc to back-arc volcanic succession of lavas and other, mostly volcanogenic rocks, with compositions ranging from basaltic to rhyolitic (Hollis et al 2012). In the lower part of the succession, basalts of arc, back-arc and within-plate affinity are dominant.…”

“…U-Pb isotopic dates from the gabbros range approximately from 484 Ma (Hollis et al in press a) to 480 Ma (Cooper et al 2011), with obduction onto the Tyrone Central Inlier basement occurring prior to about 470 Ma. The latter date is constrained by the ages of postobduction intrusions that range between about 470 Ma and 464 Ma, which contain xenocrystic Proterozoic zircons, and have strongly negative ƐNdt values (Draut et al 2009;Cooper et al 2011;Hollis et al 2012). …”