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 so was likely emplaced 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 localised lithospheric melting in the Article text vicinity of the Walls Boundary Fault during Devonian sinistral relative displacements between Laurentia and Baltica.

Evidence for an Early Silurian Synorogenic Basin Within the Metamorphic Hinterland of the North Atlantic Caledonides: Insights From the U‐Pb Zircon Geochronology of the Funzie Conglomerate, Shetland, Scotland

“…Second, three of the granitic clasts that yielded U‐Pb zircon ages of <465 Ma are characterized by gneissic fabrics consistent with high‐temperature recrystallization and deformation. However, none of the granitic intrusions on Shetland that were emplaced after 465 Ma contain gneissic fabrics (Lancaster et al, ), ruling out a local source. Third, the matrix of the Funzie Conglomerate shows no sign of derivation from a medium‐ to high‐grade Barrovian metamorphic terrane.…”

The mantle of Scotland viewed through the Glen Gollaidh aillikite

“…the very last intrusions attributed to Caledonian deformation occurred at the extension of the Great Glen Fault in West Shetland ca. 350-370 Ma (Lancaster et al 2017). At this point the Caledonian Orogeny had firmly entered an extensional phase.…”

“…However, the key lies with the almost coeval magmatism in Shetland along the extension of the Great Glen Fault. Late Caledonian magmatism in Shetland is attributed to localised melting of the lithospheric mantle as a consequence of sinistral displacements between Laurentia and Baltica (Lancaster et al 2017). However, the principle driver may be delamination of the subducted Avalonia slab (Lancaster et al 2017) and hence the same explanation could be attributed to the magmatism at Glen Gollaidh.…”

“…Late Caledonian magmatism in Shetland is attributed to localised melting of the lithospheric mantle as a consequence of sinistral displacements between Laurentia and Baltica (Lancaster et al 2017). However, the principle driver may be delamination of the subducted Avalonia slab (Lancaster et al 2017) and hence the same explanation could be attributed to the magmatism at Glen Gollaidh. Delamination as a driver to magmatism contrasts to aillikite magmatism in central West Greenland which is interpreted to have been triggered by rifting (Tappe et al 2009).…”

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…Significant magma pressures typically lead to space genera tion through inflation (or ballooning) of an in trusion, which may produce high strains within the outer part of the intrusion and/or the adjacent host rocks (Cloos, 1925;Johnson and Pollard, 1973;Hutton, 1988;Cruden, 1990;Molyneux and Hutton, 2000;Yan et al, 2011). Tectonic controls on emplacement are often associated with shear zones or faults and thus can be de rived through the study of regional structures (Hutton, 1982;Guineberteau et al, 1987;Brun et al, 1990;D'lemos et al, 1992;Petford et al, 2000;Rosenberg, 2004;Rabillard et al, 2014;McCarthy et al, 2015;Lancaster et al, 2017).…”