SINGLE-CRYSTAL 40AR/39AR GEOCHRONOLOGY OF DETRITAL MUSCOVITE FROM THE SOUTH MAYO TROUGH, WESTERN IRELAND

Ordovician strata of the South Mayo Trough (SMT) of western Ireland record the deformation, metamorphism, and unroofing of the Grampian orogeny, a short-lived collision between the Early Paleozoic Laurentian margin and an outboard ophiolite/arc complex. We have determined 350 new single-crystal ⁴⁰Ar/³⁹Ar ages for detrital muscovites from the SMT. These ages, together with the results of a recent high-resolution heavy mineral study (Dewey and Mange, 1999, Geol. Soc. London Spec. Pub. 164), characterize the provenance of the SMT strata and the timing and magnitude of exhumation associated with the Grampian event in western Ireland.

Reinterpretation by Dewey and Mange (1999) of the hitherto enigmatic Killadangan “Formation” as a pre-collisional accretionary complex is strongly supported by the new ages. The Killadangan yielded a broad range of Proterozoic detrital muscovite ages, with peaks at 750-1000, 1400-1800, and 2100-2500 Ma. The complete absence of Paleozoic ages is consistent with derivation of the Killadangan sediments from the Laurentian craton, and assembly of the Killadangan complex prior to significant unroofing associated with the Grampian collision. Similarly, the lowermost (Arenigian) units of the SMT section (Letterbrock and Sheeffry Fms.) are also characterized by broad distributions of Proterozoic ages, with peaks at 520-600, 750-1000, 1300-1800, and 2200-2400 Ma. A minor component (<10%) of Early Ordovician muscovite ages in these units may herald the earliest pulse of collisional exhumation. The overlying Llanvirnian Derrylea Fm. is characterized by a rapid upward transition from complex, predominantly Proterozoic muscovite age distributions to nearly unimodal distributions defined by a narrow peak in the Ordovician. This transition is also marked by a sudden influx of key metamorphic minerals reflecting the exhumation of the adjacent Grampian metamorphic complex. Lag times of <10 m.y. in the upper Derrylea and correlative Rosroe and Maumtrasna Fms. imply unroofing of mid-crustal level source rocks at rates on the order of mm/yr. Unimodal Ordovician age distributions are also evident in the overlying Mweelrea and Derryveeny Fms. Lag times of <10 m.y. persist into the Derryveeny Fm., suggesting that exhumation rates remained high for at least 10 m.y. following the onset of collision.