Synopsis

Small inliers of poorly known pre-Devonian metamorphic rocks are exposed in the west part of Mainland Orkney and on Graemsay. These rocks can be divided into three main groups. In order of decreasing age these are: (1) paragneisses (also including amphibolites); (2) foliated meta-granites, and (3) late, undeformed granitic and aplitic veins. There is evidence for three deformational episodes (D1–D3). D1 resulted in the gneissic foliation within the metasedimentary rocks and was associated with migmatization under mid- to upper amphibolite facies conditions. The meta-granites were probably intruded either prior to or during D2. This event resulted in the formation of a pervasive foliation within the meta-granites, as well as modification of SI fabrics within the host metasedimentary rocks. Metamorphic grade during D2 was probably within the amphibolite facies. D3 resulted in gentle, upright folding of earlier foliations. The paragneisses are correlated on lithological grounds with the high-grade Moine rocks of east Sutherland. Regional migmatization and D1 are assigned provisionally to the mid-Ordovician Grampian orogenic event recognized in east Sutherland. The meta-granites are assigned to the Silurian ‘Newer Granite’ suite and compared with the syn-tectonic plutons and sheets of central Sutherland. D2 and D3 are, therefore, tentatively assigned to the Scandian orogenic event.

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