Petrography of Ordovician and Silurian sediments in the western Irish Caledonides: tracers of a short-lived Ordovician continent-arc collision orogeny and the evolution of the Laurentian Appalachian-Caledonian margin

Abstract

Ordovician orogeny affected the Laurentian margin of the Appalachian-Caledonian Belt from the Southern Appalachians to the British Isles and is dated, stratigraphically, as post-uppermost Lower Cambrian and pre-upper Llandovery. Geochronological data favour a short Grampian orogeny from c . 470 to 460 Ma during the late Arenig-mid-Llanvirn, which is supported by the late Arenig-earliest Llanvirn termination of the Laurentian rifted margin carbonate shelf. The likeliest plate tectonic model for the Grampian Orogeny is of a continent-facing oceanic (Laurentia) arc, created as an infant mafic arc during the Middle Cambrian, evolving into an Early Ordovician intermediate-silicic arc with suprasubduction-zone ophiolites. This arc collided with the Laurentian margin and forearc ophiolites were obducted across the margin creating an orogen and accreting the arc to the margin. Continued plate convergence was accommodated by a flip in subduction polarity that terminated late orogenic retrocharriage, and led to the rapid final stages of uplift and unroofing of the orogen and to subduction accretion in the new mid-Ordovician trench. Presented in this paper are definitive new detrital heavy mineral evidence that supports and enhances this model of a short-lived orogenic event involving ophiolite obduction, and the rapid development and unroofing of a Grampian Barrovian metamorphic complex over c . 10 Ma. High-resolution heavy mineral analysis (HRHMA) has been undertaken on most of the Ordovician and Silurian stratigraphical units in the South Mayo Trough in western Ireland. It is shown that ophiolite unroofing began during the Arenig and that, by the early Llanvirn, a Barrovian complex was being eroded. The Killadangan ‘Formation’, of the Clew Bay Complex is confirmed as part of a pre-Grampian early Ordovician accretionary prism by both HRHMA and by quartz-felspar-lithic (QFL) analysis, which show it to have been derived from Precambrian rocks of the Laurentian margin. QFL analysis also shows that the Ordovician provenance of the South Mayo Trough sequence evolves from an undissected arc through a dissected arc to a recycled orogenic detrital pattern, except for the earliest sediments (Letterbrock), which were derived from the transitional continental sediments of the Killadangan accretionary prism, which, in turn, were derived from Laurentia. The problem of timing the Grampian deformation and metamorphism of the Dalradian is now regarded as being solved. It was an Arenig-Llanvirn event lasting c . 10 Ma, which occurred during and is recorded, faithfully, by the detrital heavy mineral assemblage in the conformable Ordovician sequence of the South Mayo Trough. The Miocene evolution of New Guinea is strikingly similar to the Arenig-Llanvirn evolution of the Laurentian margin and was, analogously, the short-lived result of the collision of an arc with the north Australian margin followed by subduction polarity flip.