In Shetland the main outcrops of Lewisian basement gneisses
of the Hebridean Terrane is on north-west Mainland and are cut in
places by foliated pegmatites. These gneisses are acid, banded, high-grade
orthogneisses with a minimum age of 2900 Ma and once were granites that had been
intruded by smaller bodies of gabbro. Subsequent burial to extreme depths by
mountain building events re-crystallised and deformed the granites and gabbros.
Since their uplift to higher crustal levels these rocks have been relatively
undisturbed and may represent the Caledonian foreland in Shetland.
Inliers of basement gneiss also
outcrop within the Caledonian cover rocks and are believed to represent the
ancient ‘Laurentian’ continental basement onto which the cover sediments were
deposited. Included in the Shetland basement are paragneisses that make up the
south coast of St Magnus Bay. These formed from the reworking of the continental
basement during the Grenville Orogeny (1100-1000 Ma) when continental
convergence formed the Rodinia supercontinent.