U-Pb geochronology of regional migmatites in East Sutherland, Scotland; evidence for crustal melting during the Caledonian Orogeny

• Published in: Journal of the Geological Society Vol. 156; no. 6; pp. 1143 - 1152
• Main Authors: Kinny, P. D, Friend, C. R. L, Strachan, R. A, Watt, G. R, Burns, I. M
• Format: Journal Article Conference Proceeding
• Language: English
• Published: London Geological Society of London 01-11-1999; The Geological Society of London; Geological Society; Geological Society Publishing House
• Subjects: absolute age; Caledonian Orogeny; crust; Crystalline rocks; dates; Earth sciences; Earth, ocean, space; Europe; Exact sciences and technology; Geochronology; Geological time; Geology; Great Britain; Highland region Scotland; igneous and metamorphic rocks; Igneous and metamorphic rocks petrology, volcanic processes, magmas; ion probe data; Isotope geochemistry. Geochronology; lithofacies; mass spectra; melting; metamorphic rocks; metamorphism; migmatites; migmatization; Moine Supergroup; monazite; nesosilicates; Ordovician; orthosilicates; overprinting; Paleozoic; petrography; Petrology; phosphates; Plate tectonics; Precambrian; Proterozoic; Scotland; SHRIMP data; silicates; spectra; Sutherland Scotland; U/Pb; United Kingdom; upper Precambrian; Western Europe; zircon; zircon group; Scotland; United Kingdom; Great Britain; Sutherland Scotland; Highland region Scotland; Western Europe; Scotland; Moine Supergroup; Caledonian orogeny; monazite; zircons; metapelite; Caledonian Orogeny; Caledonides; zircon; thrust fault; Taconic Orogeny; silicates; dating; migmatization; migmatites; cathodoluminescence; scanning electron microscopy; nesosilicates; nappes; geochemical inheritance; Europe; protoliths; Middle Ordovician; Ordovician; anatexis; U/Pb; metamorphic rocks; partial melting; Paleozoic; syntectonic processes; closure temperature; phosphates; cooling; metasedimentary rocks; concordia diagram
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/gsjgs.156.6.1143

Moine semi-pelitic metasediments near the base of the Kirtomy and Naver nappes, northern Scotland, were partially melted syn-tectonically under low T, high aH2O conditions. Migmatites from the Kirtomy and Naver nappes have been dated using SHRIMP U-Pb geochronology. Zircon rims which grew during migmatization have mean 206Pb/238U ages of 461±13 Ma (2σ) (Kirtomy) and 467±10 Ma (2σ) (Naver). These ages demonstrate the reality of a Middle Ordovician (Taconic) tectonothermal event in the Caledonides of northern Scotland. Monazites from the Kirtomy migmatite have a younger mean 206Pb/238U age of 431±10 Ma (2σ). This may date either a late, low amphibolite facies overprint associated with steepening of part of the nappe pile, cooling through the closure temperature of monazite for radiogenic Pb, or cessation of fluid flow along the Kirtomy Thrust. Original detrital zircons from the sedimentary precursors of both migmatites fall mostly into the age range: 1850-1000 Ma, demonstrating that deposition of the Moine sediments in Sutherland occurred after c. 1000 Ma, following the culmination of Grenvillian orogenic activity in the North Atlantic region. Only two Archaean grains were found, demonstrating that Lewisian gneisses of the foreland to the Caledonian were not the source.