A metasedimentary outlier of the Laurentian continental margin preserved on the Tonian Loch Ness Supergroup, Scottish Caledonides

The identification of orogenic unconformities in the high-grade internal zones of orogens requires a multidisciplinary approach. In the northern Scottish Caledonides, isolated occurrences of marble and schist (some kyanite-bearing) do not appear to be integral to the Tonian Loch Ness Supergroup and their affinities are controversial. At Glen Urquhart, structural evidence rules out an allochthonous setting for a kyanite schist-marble succession. Petrological evidence indicates contrasting peak metamorphic conditions between kyanite schist (7-8 kbar and 650°C) and adjacent Tonian paragneiss (9 kbar and 700°C). The U-Pb ages obtained from the youngest detrital zircons within a kyanite schist yield a maximum depositional age of c. 728 Ma, overlapping the c. 725 Ma age of migmatisation of the paragneiss. The schist-paragneiss contact is thus interpreted as a tectonically modified unconformity. C-O isotope data from associated marbles and other marble occurrences across the Loch Ness Supergroup suggest correlation with marbles of the late-Cryogenian Easdale Subgroup (Dalradian Supergroup) east of the Great Glen Fault. Deposition of lower parts of the Dalradian Supergroup was likely restricted to east of the fault, but the basin widened so that younger strata progressively onlapped northwestward and were deposited unconformably on Tonian migmatitic basement.