Pelites of the Glen Urquhart serpentinite–metamorphic complex, west of Loch Ness (Anomalous local limestone–pelite successions within the Moine outcrop: III)

The c. 6 km2 complex comprises graphitic/pyritic, mainly kyanite-rich pelites, interbanded with metalimestones and invaded by foliated serpentinites. It is enveloped by typically Moinian psammites, semipelites and pelites. Previously regarded as a Lewisian inlier, or as integral to the Moine succession, or as a transitional Moinian–Dalradian or Dalradian–Durness Limestone facies, its allochthonous status is in fact suggested by (1) its singular lithology; (2) its unique association with a large serpentinite; (3) associated sharp and local gravity and magnetic anomalies; (4) a blastomylonitic slide zone separating the complex from its Moine envelope; (5) local and regional petrological differences between pelites of the complex and Moine succession. A previous model, in which muscovite-rich Moine pelites developed by K-metasomatism (‘granitisation’) of kyanite-schists within the complex, via (a) shimmerisation (conversion of kyanite to shimmer aggregate); and (b) coarsening of shimmer aggregate to muscovite, is ruled out by new whole-rock and microprobe data. These suggest shimmerisation was a near-isochemical, small-scale redistribution of alkalis and water [approximately kyanite + biotite + quartz + water = muscovite (shimmer aggregate) + chlorite], and show large compositional differences between shimmer aggregates and coarse muscovites. Rb–Sr isotopic data imply Late Caledonian (c. 459 Ma) metamorphism in the complex, but also imply a significant earlier history for the kyanite-schists. Overall, data not merely invalidate a Glen Urquhart ‘Lewisian inlier’, but also as yet preclude correlation of the complex with the Moine or Dalradian. It is attributed in the interim to a separate lithotectonic unit, the ‘Albynian’, which may lie between (or be diachronous with) some part of the Moinian or Dalradian successions.