U-Pn geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D2 have been dated using SHRIMP UâPb geochronology. Zircon ages of 424 8 Ma (Vagastie Bridge granite), 420 6 Ma (Klibreck granite) and 429 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D2 deformation, during mid- to late Silurian time. Titanite ages of 413 3 Ma (Vagastie Bridge granite) and 416 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550â 500 8C. A mid- to late Silurian age for D2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of forelandpropagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arcâcontinent orogeny. The mid-to late Silurian D2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia

U-Pb geochronology of the Fort Augustus granite gneiss: constraints on the timing of Neoproterozoic and Palaeozoic tectonothermal events in the NW Highlands of Scotland

The West Highland granite gneiss suite in Inverness-shire, Scotland, represents a series of S-type, anatectic granites formed by partial melting of host Neoproterozoic metasediments of the Moine Supergroup. U-Pb (SHRIMP) dating of zircons from a member of the suite, the Fort Augustus granite gneiss, indicates that the granitic protolith to the gneiss was intruded at 870 +/- 30 Ma. This is indistinguishable from the published age determined by the same method for the Ardgour granite gneiss at Glenfinnan, thus supporting the assumption that the various members of the West Highland granite gneiss are part of a single intrusive suite. The spread of ages from the zircon cores (1626-947 Ma) is interpreted to indicate a Proterozoic source terrain for the Moine sediments that were later melted to form the granitic protolith. A U-Pb age of 470+/-2 Ma obtained for titanite in the Fort Augustus granite gneiss is interpreted to date amphibolite-facies metamorphism during the early to mid-Ordovician Grampian Orogeny, The emerging similarity in the timing of this event either side of the Great Glen Fault implies that this structure does not juxtapose crustal blocks with significantly different histories with respect to the Grampian Orogeny

Field relationships and petrology of leucocratic schists from the Ravns Storø group near Fiskenæsset

During field work in 1971-72 whilst mapping the Ravns Storø amphibolite belt (Andersen &amp; Friend, 1973) a series of leucocratic schists were found. These occur along the southceast side of the belt on the islands Íkátup nunâ, Sarqarigsup nunâ, Qagssikasaup nunâ and lkerasatsiaup nunâ. These rocks are initially striking because of their colour, paIe cream or yellowish, with the frequent development of spectacular garbenschiefer textures (Spry, 1969) in paIe brown, honey or colourless amphiboles (fig. 41).</jats:p

A metamorphosed basic dyke swarm in the vicinity of Sarqarigsup nunâ (Ravns Storø area), Fiskenæsset, southern West Greenland

Ancient dyke swarms have been used successfully as time markers throughout the Archaean of West Greenland, particularly in subdividing the early Archaean in the Godthåb region (McGregor, 1973). This paper describes a swarm of metamorphosed basic dykes which cut across the lithologicallayering and an early tectonic fabric in the Ravns Storø group of metavo1canic and plutonic rocks. Previously only sporadic occurrences of metamorphosed basic dykes have been noted in the Fiskenæsset region. The Sarqarigsup nuna swarm may thus have a significant part to play in the division of the events which have taken place in the area from the Frederikshåbs Isblink to Bjørnesund and perhaps even further north.</jats:p

Field relations of some Precambrian basic dykes in the vicinity of Sukkertoppen, central West Greenland

Various field relations of the basic dyke swarms near Sukkertoppen have been described (Ramberg, 1948; Berthelsen &amp; Bridgwater, 1960; Windley, 1970) and a preliminary chronology suggested (Bridgwater et al., 1976). The Kangâmiut dykes are noteable in that the central portion of some of the dykes has been converted into amphibolite and garnet amphibolite (Ramberg, 1948; Windley, 1970). Otherwise they appear as normal dolerites.</jats:p