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

Abstract

The spectacular kilometer‐thick Funzie Conglomerate is a deformed and metamorphosed high‐energy sedimentary deposit that occurs in Shetland (Scottish Caledonides). Its age and tectonic significance in relation to Early Ordovician (485–475 Ma) and Late Ordovician (450–445 Ma) accretionary orogenic events and the culminating Silurian (430–425 Ma) continental collision and closure of the Iapetus Ocean have hitherto been unknown. U‐Pb zircon dating indicates that the clast population of the conglomerate was derived from easterly sources comprising Neoproterozoic metasedimentary rocks and the northern extension of the Ordovician‐Silurian Midland Valley Arc that formed during Iapetus closure. A lower age limit of 440 Ma is indicated by the ages of the youngest matrix detrital zircon grains and the youngest granitic clast. Deformation and metamorphism of the conglomerate is attributed to sinistrally oblique extension at ca. 430 Ma that excised at least 10 km of crustal section. Sedimentation of the conglomerate is therefore constrained to a 10‐myr time span (440–430 Ma). The tectonic setting is interpreted as a synorogenic basin that received detritus from, and was buried by, advancing thrust nappes during the Early Silurian collision of Laurentia and Baltica. Tectonic burial was followed rapidly by ductile deformation during oblique extensional unroofing of the metamorphic pile and accounts for preservation of the Funzie Conglomerate as a rare example of a synorogenic basin within the metamorphic hinterland of the North Atlantic Caledonides.

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

“…On Fetlar, a late fault separates the Lower Nappe of the ophiolite and the Muness Phyllite from the Funzie Conglomerate (Fig. 1c) which contains abundant deformed clasts of quartzite as well as less common granitic types (Flinn 2014;Biejat et al 2018).…”

“…deformation fabric, the linear component of which plunges parallel to L 2 within the Muness Phyllite (Biejat et al 2018).…”

“…The Funzie Conglomerate is interpreted as a synorogenic deposit that was sourced from advancing Scandian thrust nappes (Biejat et al 2018). Sedimentation occurred after c. 440 Ma, the U-Pb zircon age of the youngest detrital components (Biejat et al 2018).…”

“…The Funzie Conglomerate is interpreted as a synorogenic deposit that was sourced from advancing Scandian thrust nappes (Biejat et al 2018). Sedimentation occurred after c. 440 Ma, the U-Pb zircon age of the youngest detrital components (Biejat et al 2018). Tectonic burial generated P-T conditions of c. 300-400°C and <4 kbar within the ophiolite, the Muness Phyllite and the Funzie Conglomerate (Cutts et al 2011) and was followed by sinistrally- (Biejat et al 2018).…”

“…Sedimentation occurred after c. 440 Ma, the U-Pb zircon age of the youngest detrital components (Biejat et al 2018). Tectonic burial generated P-T conditions of c. 300-400°C and <4 kbar within the ophiolite, the Muness Phyllite and the Funzie Conglomerate (Cutts et al 2011) and was followed by sinistrally- (Biejat et al 2018). The tectonic driver for the latter is unknown.…”