U–Pb zircon geochronology and geodynamic significance of ‘Newer Granite’ plutons in Shetland, northernmost Scottish Caledonides

Abstract

U–Pb zircon ages obtained from the late- to post-tectonic ‘Newer Granite’ suite in Shetland, northernmost Scottish Caledonides, indicate a significantly more protracted intrusion history than was inferred previously from K–Ar data. Emplacement of the Brae Complex ( c. 465 Ma), Graven Complex ( c. 440 Ma) and the Muckle Roe Granophyre ( c. 438 Ma) followed regional deformation and metamorphism of metasedimentary successions during the Grampian orogenic event, and is attributed to NW-directed subduction beneath Laurentia. The almost complete absence of plutons of this age along-strike in mainland Scotland suggests a change in subduction angle and/or the distance between the subduction zone and the Laurentian margin. Intrusion of the Ronas Hill Granite ( c. 427 Ma) was approximately coeval with displacement on the Moine Thrust in mainland Scotland, and therefore probably occurred during Baltica–Laurentia collision. A gap of c. 35 myr followed before emplacement of the Mangaster Voe Intrusion and Eastern Granophyre ( c. 390 Ma), and a further gap of c. 20 myr before emplacement of the Sandsting Complex ( c. 370 Ma). Both periods of magmatism are attributed to pulses of localized lithospheric melting in the vicinity of the Walls Boundary Fault during Devonian sinistral relative displacements between Laurentia and Baltica. Supplementary material: Petrological details, full analytical methods and data tables are available at https://doi.org/10.6084/m9.figshare.c.3655706