Timing of strain partitioning and magmatism in the Scottish Scandian collision, evidence from the high Ba–Sr Orkney granite complex

Abstract

The Orkney granite complex dominates the outcropping basement on Orkney, Scotland. It comprises a grey and a pink variably foliated granite, and structurally younger pegmatites and aplites. Based on geochemical characteristics the granites are assigned to the Scottish high Ba–Sr granites. The granites are deformed by synmagmatic extensional east–west-trending mylonite zones. These are locally overprinted by similarly oriented extensional phyllonites and, in one case, by similarly oriented extensional faults. The grey and the pink granites are dated by zircon U–Pb CA-ID-TIMS to 431.93 ± 0.46 and 430.26 ± 0.92 Ma, respectively. An aplite cutting mylonitic granite and cut by phyllonite is dated to 428.50 ± 0.31 Ma. We interpret the shear zones to record north–south extension during emplacement and cooling of the granites, likely at a shallow crustal depth (4–12 km). The extension is best explained by a subsidiary pull-apart structure related to displacement on the Great Glen Fault. In this case, the Orkney granite complex dates transcurrent faulting to 432–429 Ma, coeval with the 431–429 Ma Moine Thrust. This indicates that strain partitioning and high Ba–Sr magmatism across the Scottish Highlands was an immediate response to attempted subduction of Avalonia beneath Laurentia during the Scandian collision.