The problem of anomalous local limestone-pelite successions within the Moine outcrop; I: metamorphic limestones of the Great Glen area, from Ardgour to Nigg

Abstract

Synopsis Small outcrops of folded calcareous metasediments, surrounded by otherwise lime-poor Moine rocks, occur along both sides of the Great Glen: (from SW to NE) in Ardgour, beside Loch Lochy, in Gleann Liath and Glen Urquhart, and near Inverness, Rosemarkie and Nigg. Further examples lie in Glen Dessarry to the NW. Outcrops cover from a few m 2 to several km 2 , and vary from pure limestones, with much rarer dolostones, to Ca-Mg-Fe-silicate ‘skarns’. The few previously described examples have been contradictorily assigned to the Lewisian, Moine, Dalradian, and even Durness Limestone. The mineralogical range in each outcrop is generally similar, indicating amphibolite-facies metamorphism. Several limestones are intimately associated with graphitic, kyanite or sillimanite-rich pelites (also atypical of the local Moine), and with the margins of synformal (D 2 or D 3 ), foliated intrusions. Chemically, the calcareous rocks are strikingly homogeneous, as revealed by cluster analysis, and by very high correlation coefficients between ‘immobile’ trace elements (Ti, P, Y, Zr, Nb, Ce, Th), between Cr, Co and Ni, and between K and Rb, Ba or Pb. Mg also correlates inversely with Sr, as the dolomite/calcite ratio varies. This homogeneity suggests that most of the calcareous rocks belong to a single, now dismembered lithostratigraphical formation. Over 100 analyses of Lewisian ‘limestones’, and over 400 covering most of the Dalradian limestone formations, show quite different and far less homogeneous geochemistries. In particular, the Great Glen limestones cannot be equated with the geographically nearest Dalradian limestones, in the Appin–Ballachulish–Lismore area, as revealed by discriminant and correlation analysis. Field and regional considerations argue that the limestones are unlikely to be either Cambro-Ordovician, or an integral part of the Moine succession. As a working hypothesis, they should be assigned to a separate, provisional lithostratigraphical assemblage, whose relationship to the Lewisian, Moinian and Dalradian remains to be established.