BASEMENT GEOLOGY AND TECTONIC EVOLUTION OF IRELAND AS DEDUCED FROM PB ISOTOPES

In Ireland, lead isotope data collected on Carboniferous-hosted Zn-Pb deposits can and has been interpreted to indicate that lead and other metals were derived from the underlying basement rocks. In order to test this hypothesis, whole rock lead isotope measurements were made on greywackes from the Longford-Down Inlier, which represent the along strike continuation of the northern part of the Caledonian basement underlying the base metal deposits. The lead isotope data define a linear array on a 207Pb/204Pb versus 206Pb/204Pb plot, with end points at (207Pb/204Pb) = 15.542, (206Pb/204Pbp) = 18.456 and (207Pb/204Pb)= 15.461, (206Pb/204Pb)= 17.973. No evidence of lead isotope overprinting by the mineralizing event could be found in the samples studied. When the lead isotope data are corrected for addition of radiogenic lead since the time of base metal mineralization (c. 350 Ma ago), there is a 1: 1 correlation between the lead isotopic composition of galenas in the base metal deposits and the Longford-Down grewyackes. These results support the hypothesis that lead and other metals were derived from the greywackes underlying the base metal mineralization. On a 208Pb/204Pb versus 206Pb/204Pb plot, neither whole rock greywacke nor galena lead isotope data define a linear array. The 208Pb/204Pb-206Pb/204Pb trend is best explained by tectonic or sedimentological mixing of three isotopically distinct lead isotope end members. These end members are: (1) old unradiogenic basement in NW Ireland (Lewisian complex precursor?), (2) Ordovician arc material from NW Ireland and (3) Ordovician arc material form SE Ireland. Isotopic modelling of these end members shows that the present-day mixing line seen in the Ordovician and Silurian sediments can be calculated back to values inferred for the Ordovician arc end members using the same values of μ as were used to back calculate the lead isotopic composition of the greywackes to the time of mineralization. This information is further confirmation of the sediment-mixing model presented. A tectonic model developed from these and established geological data suggest a late Ordovician closure of the Iapetus Ocean, and that the Silurian greywackes are a by-product of eroded Ordovician arc material from NW and SE of the Iapetus suture.