U–Pb 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

Abstract

Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D 2 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 D 2 have been dated using SHRIMP U-Pb geochronology. Zircon ages of 424 AE 8 Ma (Vagastie Bridge granite), 420 AE 6 Ma (Klibreck granite) and 429 AE 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D 2 deformation, during mid-to late Silurian time. Titanite ages of 413 AE 3 Ma (Vagastie Bridge granite) and 416 AE 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c. 550-500 8C. A mid-to late Silurian age for D 2 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 arc-continent orogeny. The mid-to late Silurian D 2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…Within various sectors of the North Atlantic Caledonides it has therefore been possible to differentiate deformation fabrics and mineral assemblages formed during early phases of Ordovician arc-continent collision from those formed during Silurian continental collision (e.g. Kinny et al 1999Kinny et al , 2003Roberts 2003;Kocks et al 2006;Bird et al 2013).…”

The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs–Beinn Dearg area, NW Highlands

“…In north Sutherland, Holdsworth (1989) and Holdsworth et al (2001) SSE to south plunging lineations (suggesting NNW to northerly directed transport) for high-level thrusts, such as the Sgurr Beag and Naver thrusts, as well as in nearby syn-tectonic intrusions, are widely reported along the length of the Northern Highlands (Rathbone & Harris 1979;Kelley & Powell 1985;Grant & Harris 2000;Kinny et al 2003a). This would suggest that the early ductile Sgurr Beag-type thrusts did have a systematically different transport direction to the lower, Moine Thrust.…”

“…The main phase of ductile thrusting and associated folding and fabric development affecting the Morar Group probably occurred slightly earlier, in the interval 435 -430 Ma (Kinny et al 2003a;Goodenough et al in review). This would imply that all deformation episodes from D2 Ac onwards (Fig.…”

“…Syn-tectonic granites with SSE plunging lineations either side of the Naver Thrust have yielded Silurian (Scandian) U-Pb zircon crystallization ages of c. 430 -425 Ma (Kinny et al 2003a). This would suggest that Scandian thrusting did involve an early phase of NNWdirected transport, followed by a change to WNW-directed transport.…”

The Dalradian rocks of the northern Grampian Highlands of Scotland

“…Recent radiometric studies have confirmed the existence of Neoproterozoic (800-750 Ma) tectonothermal events within both the Moine and the oldest rocks of the Northern Grampian Highlands (Noble et al, 1996;Highton et al, 1999), yet in the higher Grampian, Appin and Argyll group rocks of the Dalradian succession, only Palaeozoic (470-450 Ma) Caledonian events are known (e.g. Rogers andPankhurst, 1993, Smith et al, 1999). Comparable dating studies now recognize discrete tectonothermal events and it remains one of the key issues of Highland geology to define the limits of these events.…”