The comparative geochemistry of some Highland pelites (Anomalous local limestone–pelite successions within the Moine outcrop; II)

Synopsis Within the Moine outcrop, three types of ‘anomalous’ pelitic rocks form local, probably allochthonous successions (nearly all with limestones): (1) sillimanite-gneisses (± garnet, kyanite) in Ardgour, Glen Dessarry and Glen Loy, each intimately involved with major intrusions; (2) kyanite-schists (± garnet, staurolite) in Glen Lui, Glen Urquhart and Strathspey, each associated with major faults or slides; (3) fault-bounded biotite–muscovite pelitic gneisses in Gleann Liath. Different authors have assigned these successions variously to the Lewisian, Moinian, Dalradian, Cambro-Ordovician, or to an unknown lithotectonic unit (e.g. ‘Grantown series’). The sillimanite-gneisses are restites , carrying local cordierite, corundum, hypersthene, spinel etc. Compared with typical Highland pelites, they have more variable but higher contents of elements carried in refractory minerals (e.g. Al, Y, Zr, Nb, Ce, Th), and correspondingly lower contents of elements carried in fusible minerals (e.g. Si, Ca, Na, Rb). Their present chemistry cannot therefore reveal their stratigraphical affinities. The anomalous kyanite-schists and pelitic gneisses, however, appear to approximate sediment compositions, and are compared chemo-stratigraphically with mineralogically equivalent Lewisian, Moinian and Dalradian rocks, using discriminant analysis on 161 analyses from > 50 localities. Lewisian, Moinian and Dalradian kyanite-pelites are distinguishable not only from each other (e.g. Lewisian examples carry lowest Al, K, Ti, P, Rb, Y, Nb, Ce, Pb, and highest Fe, Mg, Ca and Cr), but also from the Glen Urquhart and Gleann Liath anomalous pelites. The latter are therefore tentatively assigned to a separate lithotectonic unit, the ‘Albynian’, so far only identified near the Great Glen fault. The Strathspey and Glen Lui kyanite-schists are chemically closest to Dalradian kyanite-schists, supporting the existing view that the ‘Grampian Moines’ may include Dalradian strata.