A Baltic heritage in Scotland: basement terrane transfer during the Grenvillian orogeny

Archean basement inliers within the Northern Highland Terrane (NHT), Scottish Caledonides, have been correlated with the Lewisian Gneiss Complex of the Laurentian foreland. New zircon U-Pb ages indicate that the NHT basement contains evidence for magmatism at 2823–2687 Ma, and 1772–1655 Ma. The first group compares with crystallization ages of the foreland Archean gneisses. However, the second group, and a supracrustal unit, formed ca. 100–250 myr after the youngest major phase of juvenile magmatism and sedimentation in the foreland. Also, there is no indication within the NHT basement of the Palaeoproterozoic mafic and felsic intrusions common within the foreland, leading us to conclude that there is no firm basis for correlation of the two crustal blocks. The Caledonian Moine Thrust, which separates the foreland and the NHT basement, is thought to have reworked a Grenvillian suture indicated by the presence of the ca. 1100–1000 Ma Eastern Glenelg eclogites. On the basis of the new isotopic data, we propose that the NHT basement was a fragment of Baltica that was emplaced onto Laurentia during the Grenvillian orogeny, representing a further example of basement terrane transfer in the circum-North Atlantic orogens.