No 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.

The East Greenland Caledonides occupy a crucial position in plate-tectonic reconstructions of the Late Mesoproterozoic to Early Neoproterozoic Grenville–Sveconorwegian belt. We present new field and isotopic data from the northern Stauning Alper which indicate that the 1050–930 Ma history of the area was characterized by deposition of extensive clastic sequences. Sources of detritus were dominated by rocks of Mesoproterozoic age, with only limited contributions from Archaean sources, suggesting deposition at a distance from the present Caledonian foreland. A Neoproterozoic granite (938±13 Ma) provides evidence for thermal perturbation at a time of extensional collapse and uplift recorded in NW Scotland, the Grenville Belt of Canada, Labrador and the Sveconorwegian of SW Sweden and southern Norway. Widespread anatexis in the northern Stauning Alper at c.  430–425 Ma resulted from both collisional melting and decompression melting on uplift contemporaneous with the early part of orogenic collapse of the Caledonian Fold Belt. Caledonian deformation was focused at the zone of most extensive granite emplacement. Isotopic evidence suggests that Caledonian granites, previously thought to be entirely post-kinematic, actually predate late Caledonian extension.