Abstract:

The Ordovician strata of the South Mayo Trough in western Ireland were deposited as a conformable sequence, first in the forearc of a north-facing oceanic arc, then in a synorogenic basin above the arc–continent collisional Grampian orogen, then in an extensional hanging-wall basin above the exhuming Dalradian metamorphic complex and, finally, in an Andean-type margin successor basin. This depositional history, and its relationship to the Grampian orogeny, to the pre- and post-collision arcs, and to sources in the Laurentian foreland, is recorded in detrital heavy mineral suites, Ar/Ar ages of detrital white micas, and U–Pb ages of detrital zircons. Heavy minerals record the progressive extensional unroofing of an obducted ophiolite complex, then the rapid extensional unroofing of the Grampian Dalradian metamorphic complex. White mica ages give Neoproterozoic, Mesoproterozoic, and Neoarchaean ages in the lower part of the sequence then, at about 466 Ma, synchronously with the appearance of amphibolite-facies metamorphic detritus, a tightly unimodal Grampian metamorphic age. Zircons yield mainly Mesoproterozoic and Neoarchaean ages, indicating a Laurentian source, and a Grampian age of 460–468 Ma. Paralleling the appearance of Grampian-age micas, the colour of zircon changes from dominantly purple to dominantly colourless, reflecting the thermal annealing effect of metamorphism. However, the temperature of Grampian metamorphism was not high enough to reset the U–Pb isotopic systematics of detrital zircons.

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