U–Pb geochronology of regional migmatites in East Sutherland, Scotland: evidence for crustal melting during the Caledonian orogeny

Abstract

Moine semi-pelitic metasediments near the base of the Kirtomy and Naver nappes, northern Scotland, were partially melted syn-tectonically under low T, high a H 2 O conditions. Migmatites from the Kirtomy and Naver nappes have been dated using SHRIMP U-Pb geochronology. Zircon rims which grew during migmatization have mean 206 Pb/ 238 U 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 206 Pb/ 238 U 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.