DETRITAL ZIRCON PROVENANCE AND GRAMPIAN TERRANE AMALGAMATION IN WESTERN IRELAND

The traditional view of the Ordovician, Laurentian-margin geology of western Ireland holds that the Connemara Dalradian block was intruded by plutons of the Notre Dame arc and then emplaced to the south of the filled South Mayo Trough marginal basin by strike-slip faulting at the end of the Taconic Orogeny (i.e., ~460 Ma). Our detailed study of an interbedded ignimbrite-sandstone sequence in the upper part of the South Mayo Trough basin fill (Mweelrea Formation) includes detrital zircon analysis with a goal of understanding a possibly different emplacement history for the Connemara block.

U-Pb analysis of zircons in ignimbrite at the base of the Mweelrea Fm gives an age of 465 Ma. Within errors, this age is matched by granite clasts in an overlying, southerly derived conglomerate that are petrographically similar to intrusions of the same age in the Notre Dame arc of Connemara. Sandstones interbedded with this ignimbrite, in the southern Mayo Trough, yield detrital-zircon relative probability curves with peaks at ~475 and ~465. These ages reflect erosion of both ignimbrite in the succesion and older igneous rocks from the Notre Dame arc. Other age peaks are at ~1080, ~1850, and ~2700. Proterozoic ages are similar to inherited zircons in Notre Dame plutons and Neoproterozoic metasedimentary rocks in Connemara.

A northerly derived sandstone from the northern part of the basin has detrital zircons of similar Proterozoic and Neoproterozoic age, but differs markedly from sandstones in the southern basic by the presence of a strong peak at 490 – 515 Ma.

These results suggest that the Connemara terrane already lay to the south of the South Mayo Trough during Mweelrea deposition and Notre Dame arc magmatism. The concentration of ages between 490 – 515 Ma extends to older ages than are seen in the Notre Dame arc and leads us to speculate on the presence of early peri-Laurentian arc rocks such as are seen in the Baie Verte oceanic tract of Newfoundland.