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.