PROVENANCE OF SEDIMENT IN SILURIAN BASINS OF WESTERN IRELAND BASED ON NEW U-PB DATING

Elements of the Grampian (Taconic) accretionary history of the Caledonian – Appalachian orogeny in western Ireland include Laurentian Dalradian strata, the Ordovician Clew Bay (CB) ophiolite – accretionary complex, and the Ordovician Nafooey arc/South Mayo Trough (SMT) forearc basin. Three Silurian basin successions, the Llandovery-Wenlock Croagh Patrick (CPs) and Killary-Joyces Country (KJCs) and the non-fossiliferous Louisburgh-Clare Island (LCIs), overstep the contacts between these various elements and record sedimentation during Grampian orogenic collapse. New U-Pb detrital zircon data from Silurian rocks address sediment provenance as a guide to basin evolution.

The LCIs comprises terrestrial to shallow-marine red beds. It unconformably overlies the CB complex on Clare Island and is faulted against the CPs on the mainland. The Strake Banded Fm yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician – Silurian ages . An interbedded, ca. 424 Ma tuff indicates the LCIs is younger than the CPs and KJCs.

The shallow-marine CPs unconformably overlies the CB complex and the northern part of the SMT. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, and no Ordovician grains. Up to 13% of the grains form a cluster at ca. 950 – 800 Ma, younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting.

The KJCs overlies the structural contact between the Nafooey arc and the Connemara Dalradian block and records a transgressive – regressive cycle. Three samples of the Lough Mask Fm show contrasting age spectra. Two samples, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source. One sample also includes 8% Silurian grains. The third sample overlies Dalradian basement and is dominated by Ordovician grains. Ca. 456 Ma ages are younger than any preserved Ordovician rocks in the region. Cambrian to Late Neoproterozoic grains represent Iapetan rifting or derivation from a peri-Gondwanan source in the late stages of Iapetus closure.

The broadly coeval CPs and KJCs have provenance independent of structural boundaries between basement Grampian elements yet have distinct Neoproterozoic sources.