Caledonian and Pre-Caledonian orogenic events in Shetland, Scotland: evidence from garnet Lu–Hf and Sm–Nd geochronology

Abstract

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Tectonic significance of a supra-ophiolitic sedimentary cover succession, Unst, Shetland, Scottish Caledonides: insights from the U–Pb–Hf detrital zircon record

“…The transport direction is assumed to have been towards the west, parallel to L 1 (Cannat 1989;Flinn & Oglethorpe 2005;Flinn 2014). Lu-Hf, Sm-Nd, U-Pb and Rb-Sr mineral ages of c. 485-470 Ma date this event which correlates with the Grampian orogenic event recognised in mainland Scotland and Ireland (Cutts et al 2011;Walker et al 2020). Peak pressuretemperature (P-T) conditions obtained from metasedimentary rocks in the footwall of the ophiolite were c. 10 kbar and 775°C (Cutts et al 2011).…”

“…Geochemical data support its formation in a supra-subduction setting (Prichard & Lord 1988;O'Driscoll et al 2012O'Driscoll et al , 2018. Obduction is constrained by: a) a Lu-Hf garnet age of 491 ± 6 Ma (Walker et al 2020), b) a U-Pb zircon age of 484 ± 4 Ma (Crowley & Strachan 2015), and c) K-Ar hornblende ages of c. 480-465 Ma (Spray 1988), all obtained from the amphibolites that form the metamorphic sole.…”

“…Peak pressuretemperature (P-T) conditions obtained from metasedimentary rocks in the footwall of the ophiolite were c. 10 kbar and 775°C (Cutts et al 2011). Low-angle foliations were then reworked into a steep orientation across Yell and Mainland Shetland (Walker et al 2016(Walker et al , 2020.…”

“…'Grampian Lu-Hf, Sm-Nd and Rb-Sr mineral isochron ages of c. 450-445 Ma date 'Grampian II' metamorphism within the footwall of the ophiolite (Walker et al 2016(Walker et al , 2020. The tectonic driver of this event, which is also recorded in mainland northern Scotland, is uncertain but it could have resulted from either flat-slab subduction or terrane accretion (Bird et al 2013).…”

“…The tectonic driver of this event, which is also recorded in mainland northern Scotland, is uncertain but it could have resulted from either flat-slab subduction or terrane accretion (Bird et al 2013). Cutts et al (2011) calculated P-T conditions of c. 7.5 kbar and 550°C from garnets in NE Unst that have yielded a Grampian II age (Walker et al 2020).…”