Detrital Avalonian zircons in the Laurentian Southern Uplands terrane, Scotland

• Published in: Geology (Boulder) 2003-07, Vol.31 (7), p.625-628
• Main Authors: Phillips, E. R, Evans, J. A, Stone, P, Horstwood, M. S. A, Floyd, J. D, Smith, R. A, Akhurst, M. C, Barron, H. F
• Format: Article
• Language: English
• Subjects: absolute age; assimilation; Avalonia; basins; Caledonian Orogeny; Caledonides; Caradocian; cathodoluminescence; clastic rocks; continental drift; continental margin; dates; detritus; Europe; experimental studies; Geochronology; Geology; Great Britain; Iapetus; laser ablation; laser methods; Laurentia; magmas; mass spectra; Neoproterozoic; nesosilicates; Ordovician; orthosilicates; paleogeography; Paleozoic; Plate tectonics; Precambrian; Proterozoic; provenance; sandstone; Scotland; sedimentary basins; sedimentary rocks; silicates; Southern Uplands; spectra; Stratigraphy; subduction; terranes; thermal ionization mass spectra; U/Pb; United Kingdom; Upper Ordovician; upper Precambrian; volcaniclastics; volcanism; Western Europe; zircon; zircon group
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/0091-7613(2003)031<0625:DAZITL>2.0.CO;2

The Silurian-Ordovician Southern Uplands terrane occupies a key position in the Caledonian orogen, yet its genesis is controversial. Marginal-basin, backarc, and forearc tectonic regimes have all been invoked as operative at the Laurentian margin of the Iapetus Ocean. Fresh andesitic detritus within turbidite sandstones has, until now, been assumed to provide evidence for an Ordovician suprasubduction volcanic arc, a central feature of most models. However, high-precision thermal-ionization mass spectrometer U-Pb and laser-ablation data for detrital zircons from the sandstone prove Neoproterozoic volcanism at 557±6 Ma (2σ) and probably also at 613±12 Ma (2σ). The complex crystallization history recorded by the zircons shows assimilation of 1043±7 Ma (2σ) Grenvillian basement into the andesite magma. The fact that no zircons have been found having ages that overlap the Caradocian depositional age of the host sedimentary rocks undermines all extant terrane models. The age profile of the detrital zircons is typical of Gondwana and Avalonia. This finding has important implications for the paleogeography of the Iapetus Ocean during the Ordovician, because the zircon data require the introduction of Avalonian detritus into a sedimentary basin marginal to Laurentia.