Anomalies on the Cambro-Ordovician passive margin of Scotland

Abstract

Various hypotheses have tried to explain the relationships between the metamorphic blocks of Scotland and the craton edge of Laurentia. Whilst the recent dating of the basins, which preceded the metamorphism, is exciting, it overlooks the point that the blocks now sit where a passive margin to the Laurentian continent should be. Given that there should be c . 300 km or more of passive margin sediment and some distance of oceanic crust seaward of it implies that the blocks are allochthonous, having shifted in from some area along the Laurentian craton. The timing of the interaction between the blocks and the Laurentian craton is equivocal. The Moine, deposited between c . 900 Ma and 870 Ma may overlap with the deposition of the Torridonian, but, between 470 Ma and 420 Ma, underwent metamorphism and should have delivered sediment to the Ordovician ( c . 460 Ma) rocks on the craton. Moreover the relationship between the blocks is also questionable. The Dalradian sequence was deposited pre-720 Ma and terminated c . 780–730 Ma. So far, no Moine zircons have been recorded from the Dalradian Supergroup and this is explained by the Moine metamorphism being insufficiently high grade in order to produce zircon. It may also be that the Dalradian and Moine were not united at this time.