Devonian subduction-related plutonism in the Shetland Islands, Scottish Caledonides: final closure and aftermath of the Iapetus Ocean

Revised U-Pb zircon geochronological data for ‘Newer Granite’ intrusions in the Shetland Islands, Scottish Caledonides, indicate that they are mainly Devonian in age. Calculation of emplacement ages is complex because many plutons contain significant antecryst populations. In the previous absence of geochemical and isotopic data, the Devonian plutons were attributed to pulses of localised lithospheric melting during strike-slip displacements along the Walls Boundary Fault, given the prevailing view that the Iapetus Ocean had closed significantly earlier. However, new geochemical evidence indicates that most of these plutons have a subduction-related or crustal melting signature, consistent with recent tectonic models that argue that Iapetus did not finally close until c. 400-390 Ma. We therefore attribute most Devonian plutonism in the Shetland Islands to subduction of the final vestiges of the Iapetus Ocean, contemporaneous with emplacement of the ‘Trans-Suture Suite’ in southern Scotland and northern England. Plutonism was terminated by the collision of combined Avalonia-Baltica with Laurentia at c. 400-390 Ma. The E-type MORB or OIB chemistry of the Sandsting Complex diorites is consistent with asthenospheric melting after slab foundering.