Isotope systematics of Precambrian marbles from the Lewisian complex of northwest Scotland: implications for PbPb dating of metamorphosed carbonates

We present Pb-isotopic data from the late-Archaean Lewisian marbles of Tiree, Iona, and Glenelg, each locality representing a different tectonic history since the mid-Proterozoic. Data from Tiree (unaffected Precambrian foreland to the Caledonian) yield an age of 1656 ± 23 Ma which is within error of broadly defined Laxfordian reworking events, both locally and throughout the Lewisian. Nd-isotopic data confirm a late-Archaean protolith, although low model μ1 values require considerably lower pre-Laxfordian UPb ratios. An age pf 1418 ± 56 Ma is obtained from the Iona marble (? foreland, buried beneath Caledonian nappes), again with a low μ1 and negative ϵNd(t). This age is too young for the Laxfordian and might indicate some disturbance of UPb systematics during Caledonian events. No meaningful age data are obtained from the Glenelg marbles (Lewisian inlier in Caledonian thrust sheet) where Grenvillian (∼ 1.1 Ga) eclogite metamorphism preceded Caledonian events and Pb-isotopic compositions appear not to have been homogenised. Leaching experiments for the Tiree and Iona marbles reveal that radiogenic Pb is hosted in silicate phases, with unradiogenic Pb in the carbonates, a reversal of the situation commonly observed in sedimentary carbonates. If a sedimentary precursor is assumed, redistribution of U and Pb probably occurred during high-grade metamorphism recorded by the age regression, accompanied by significant UPb fractionation to yield higher post-metamorphic UPb ratios. The significance of non-zero ThU ratios (between ∼ 1 and ∼ 3, post-Laxfordian) remains unclear, but may not have any bearing upon the age or nature of the protolith. Our study provides further evidence that Pb-isotopic studies of marbles can yield reliable geochronological information on metamorphic events in the absence of later tectono-metamorphic overprinting, and is therefore a potentially useful technique for reconnaissance geochronology in metasupracrustal belts. However, caution should be exercised in applying this technique to terrains with complex histories.