Evidence for a Late Cambrian juvenile arc and a buried suture within the Laurentian Caledonides of Scotland; comparisons with hyperextended Iapetan margins in the Appalachian Mountains (North America) and Norway

• Published in: Geology (Boulder) 2019-08, Vol.47 (8), p.734-738
• Main Authors: Dunk, M, Strachan, R. A, Cutts, K. A, Lasalle, S, Storey, C. D, Burns, I. M, Whitehouse, M. J, Fowler, M, Moreira, H, Dunlop, J, Pereira, I
• Format: Article
• Language: English
• Subjects: absolute age; Allochthonous deposits; Amphibolites; Appalachians; Caledonides; Cambrian; Dating; Den föränderliga jorden; Europe; Geochemistry; Geochronology; Geochronometry; Geological time; Geology; Geophysics; Gneiss; gneisses; Grampian Orogeny; Great Britain; hafnium; Heredity; Hf-177/Hf-176; Iapetus; igneous and metamorphic rocks; isotope ratios; isotopes; Laurentia; Lead; Low pressure; Magma; metals; metamorphic rocks; metamorphism; Mineral assemblages; Moine Supergroup; Mountains; Nd-144/Nd-143; neodymium; Neoproterozoic; nesosilicates; North America; northern Scotland; Ordovician; Orogeny; orthosilicates; P-T conditions; paleogeography; Paleozoic; Passive margins; Petrology; plate tectonics; Precambrian; Pressure; Proterozoic; Radiometric dating; rare earths; Rifting; Scotland; silicates; solid Earth (tectonophysics); stable isotopes; Strathy Complex; Subduction; Subduction (geology); suture zones; Tectonics; The changing Earth; U/Pb; United Kingdom; Upper Cambrian; upper Precambrian; Uranium; Western Europe; Zircon; zircon group
• ISSN: 0091-7613; 1943-2682; 1943-2682
• DOI: 10.1130/G46180.1

Uranium-lead (U-Pb) zircon dating establishes a late Cambrian (Drumian) protolith age of 503 ± 2 Ma for a trondhjemitic gneiss of the calc-alkaline Strathy Complex, northern Scottish Caledonides. Positive εHf and εNd values from trondhjemitic gneisses and co-magmatic amphibolites, respectively, and an absence of any inheritance in zircon populations support published geochemistry that indicates a juvenile origin distal from Laurentia. In order to account for its present location within a stack of Laurentia-derived thrust sheets, we interpret the complex as allochthonous and located along a buried suture. We propose that a microcontinental ribbon was detached from Laurentia during late Neoproterozoic to Cambrian rifting; the intervening oceanic tract closed by subduction during the late Cambrian and formed a juvenile arc, the protolith of the Strathy Complex. The microcontinental ribbon was reattached to Laurentia during the Grampian orogeny, which transported the Strathy Complex as a tectonic slice within a nappe stack. Peak metamorphic conditions for the Strathy Complex arc (650-700 °C, 0.6-0.75 GPa) are intermediate in pressure between those published previously for Grampian mineral assemblages in structurally overlying low-pressure migmatites (670-750 °C,