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[sup.40]Ar/[sup.39]Ar and Rb--Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb--Sr muscovite ages of c. 428, c. 421 and c. 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. [sup.40]Ar/[sup.39]Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb--Sr analyses of muscovites from the same samples: it is likely that this discrepancy results from a component of extraneous or `excess' argon. [sup.40]Ar/[sup.39]Ar hornblende ages and Rb--Sr and [sup.40]Ar/[sup.39]Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c. 440 Ma and c. 410 Ma. These ages are interpreted to date cooling during and following `[D.sub.2]' regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. [D.sub.2] structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c. 470-460 Ma. Syn-[D.sub.2] temperatures were generally [is greater than] 600 [degrees] C and sufficient to achieve more or less complete thermal rejuvenation of Rb--Sr and [sup.40]Ar/[sup.39]Ar isotopic systems in muscovite and hornblende, even in areas of low [D.sub.2] strain. Keywords: Caledonides, Sutherland, deformation, cooling, absolute age.

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 [+ or -] 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. Keywords: SHRIMP, Grenvillian Orogeny, Caledonian Orogeny, anatexis, extension.

Water-undersaturated partial melting at granulite facies conditions, followed by accumulation and upward migration of the resulting granitic melt, is one of the principle causes of crustal differentiation. Microfracturing caused by small, positive volume changes associated with waterundersaturated melting reactions may facilitate rapid extraction of melts from melting sites. We have successfully imaged annealed microfractures in residual quartz from three granulite facies migmatites. Annealed microfractures have been discovered in (1) quartz inclusions in peritectic garnet; (2) quartz inclusions in K-feldspar; and (3) quartz occurring as entrained grains. In the latter, oscillatory zoned quartz has overgrown fractures in the residual core, 'mdicating that fracturing occurred prior to melt crystallization and that postanatectic volume changes are not responsible for the observed fracturing. Fractures are generally [is less than] 5 gan wide, are parallel sided, and have low sinuosity. Crack densities are two to three times higher than experimental studies involving muscovite melting and approach theoretical interconnectivity thresholds for percolation, implying that reaction-induced microfracturing is a viable mechanism contributing to melt interconnectivity during anatexis under granulite facies conditions. Keywords: quartz, microcracking, migmatites, melt extraction.

Moine semi-pelitic metasediments near the base of the Kirtomy and Naver nappes, northern Scotland, were partially melted syn-tectonically under low T, high [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions. Migmatites from the Kirtomy and Naver nappes have been dated using SHRIMP U-Pb geochronology. Zircon rims which grew during migmatization have mean [sup.206]Pb/[sup.238]U ages of 461 [+ or -] 13 Ma (2[Sigma]) (Kirtomy) and 467 [+ or -] 10 Ma (2[Sigma]) (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 [sup.206]Pb/[sup.238]U age of 431 [+ or -] 10 Ma (2[Sigma]). 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. Keywords: Scotland, Moine Supergroup, Caledonian orogeny, zircons, monazite.