ANCIENT LAURENTIAN DETRITAL ZIRCON IN THE SOUTHERN UPLANDS TERRANE, BRITISH CALEDONIDES

The Southern Uplands terrane is a belt dominated by Ordovician-Silurian metasedimentary rocks in the British Caledonides. Lying between Laurentian rocks of the Scottish Highlands, and Avalonian rocks of England and Wales, the terrane has long been interpreted as representing deposition in the closing Iapetus Ocean. Suggested depositional settings include an accretionary fore-arc, and a back-arc environment attached to Laurentia. Deformation in the terane records convergence, with significant sinistral transpression, and is likely to have been diachronous; deformation in the north was initiated while deposition was still taking place in the south. The belt is divided into a series of NE-SW-striking fault-bounded tracts, distinguished by different sedimentary successions. In most, the stratigraphically lowest sediments are black, graptolitic shales, which pass northward and stratigraphically upward into lithic wackes; the time of transition is successively younger to the SE. In some tracts, different compositions can be distinguished; quartzose, continentally-derived wackes interdigitate with units derived from andesitic volcanic sources.

Zircons were extracted from quartzose wackes from several tracts in the northern part of the terrane, deposited in late Ordovician. U-Pb analyses were carried out on 60-80 grains by multicollector ICPMS using laser ablation with a beam diameter of 40-60 μ.

Zircons from a sample deposited early in the depositional history (Kirkcolm Fm.) display a range of U-Pb ages from Paleoarchean to late Ordovician, including the oldest such grain yet recorded from the British Isles. Neoarchean (2.8 - 2.5 Ga), Paleoproterozoic (2.0 - 1.7 Ga), and Mesoproterozoic (1.5 - 1.0 Ga) age populations suggest sources in Laurentia, such as the Grenville and Trans-Hudson orogens. The overall age distribution is comparable to metasedimentary rocks of the Laurentian margin of Iapetus in the Taconian orogen of W. Newfoundland. Samples with younger depositional ages show increasing amounts of Grenville-age zircon (ca. 1.0 Ga) and a general trend of reduced Archean input. Several analyses from the Glenlee Fm. plot on a discordia line suggesting overprinting of Archean zircon in the Early Paleozoic, consistent with tectonothermal reworking of Laurentian detritus in the Grampian orogen.