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

The tectonic significance of the Muness Phyllite which overlies the Unst-Fetlar ophiolite in Shetland, Scottish Caledonides is poorly understood. U-Pb analyses of detrital zircons show that it was deposited after c. 469 Ma. Lower Palaeozoic grains have εHf values of -0.3 to +12.3 and were likely derived from the extension of the Midland Valley arc. Psammite clasts and the matrix of the Muness Phyllite contain Proterozoic and Archaean detrital zircons with age peaks of c. 1 Ga, 1.4 - 1.5 Ga, 1.6 - 1.7 Ga, 1.8 - 1.9 Ga and 2.7 Ga. These are consistent with ultimate derivation from NE Laurentia sources and were probably recycled from the Neoproterozoic East Mainland Succession that underlies the Mesozoic East Shetland Basin. The Muness Phyllite is interpreted to have been deposited soon after the Grampian I orogeny in a successor basin that overstepped, and received detritus from, the Midland Valley arc, the East Mainland Succession and the Unst-Fetlar ophiolite. It was then deformed and metamorphosed, probably at c. 450 Ma during the Grampian II orogenic event. The Muness Phyllite therefore provides a record of middle to late Ordovician tectonic events along the Scottish sector of the Laurentian margin following ophiolite obduction.