USE OF DETRITAL ZIRCONS IN RECONSTRUCTING SOURCE AREAS IN CONNEMARA FOR ORDOVICIAN SEDIMENTS IN THE SOUTH MAYO TROUGH, 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 and sandstone sequence in the upper part of the South Mayo Trough basin fill (the 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 overlying conglomerate that are petrographically similar to intrusions of the same age in the Notre Dame arc of Connemara.

A relative probability curve of detrital zircons in Mweelrea sandstone yields spikes in age concentration at ~ 490 Ma, ~1100-1600 Ma, ~ 1850 Ma, ~2400 - 2600 Ma, and ~ 2700 Ma. The source of Archaean grains is unknown. Proterozoic ages match ages of inherited zircons in Notre Dame plutons and Neoproterozoic metasedimentary rocks in Connemara. Together with the data from the granite clasts, the presence of zircons with these Proterozoic ages suggest that the Connemara terrane already lay to the south of the South Mayo Trough during Mweelrea deposition and Notre Dame arc magmatism. A strong concentration of ages between 480 - 505 Ma extends to older ages than are seen in the Notre Dame arc. A potential source is early peri-Laurentian arc rocks similar to, but including material older than the Early Ordovician Lough Nafooey arc of South Mayo.