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The Unst Ophiolite is the best exposed of a chain of early Ordovician ophiolites in the Scottish Caledonides and is widely regarded as having formed in a supra-subduction zone setting within the Iapetus Ocean. Reinterpretation of sheeted dykes suggests that it formed as an oceanic core complex, presumably during subduction roll-back immediately prior to obduction onto the Laurentian margin. A new U-Pb zircon age of 484 ± 4 Ma for development of the metamorphic sole places a lower limit on the timing of obduction, which was subsequently followed by regional-scale crustal thickening and Barrovian metamorphism during the Grampian orogenic event. [ABSTRACT FROM AUTHOR]

Calc-alkaline magmatism in the Grampian Terrane started at c. 430 Ma, after subduction of the edge of continental Avalonia beneath Laurentia, and it then persisted for at least 22 Ma. Isotope dilution thermal ionization mass spectrometry U-Pb zircon dating yields 425.0 ± 0.7 Ma for the Lorn Lava Pile, 422.5 ± 0.5 Ma for Rannoch Moor Pluton, 419.6 ± 5.4 Ma for a fault-intrusion at Glencoe volcano, 417.9 ± 0.9 Ma for Clach Leathad Pluton in Glencoe, and, in the Etive Pluton, 414.9 ± 0.7 Ma for the Cruachan Intrusion and 408.0 ± 0.5 Ma for the Inner Starav Intrusion. The Etive Dyke Swarm was mostly emplaced during 418-414 Ma, forming part of the plumbing of a large volcano (≥2000 km³) that became intruded by the Etive Pluton and was subsequently removed by erosion. During the magmatism large volumes (thousands of km3) of high Ba-Sr andesite and dacite were erupted repeatedly, but were mostly removed by contemporaneous uplift and erosion. This volcanic counterpart to the 'Newer Granite' plutons has not previously been fully recognized. The intermediate magmas forming both plutons and volcanoes originated mainly by partial melting of heterogeneous mafic to intermediate lowermost crust that had high Ba-Sr derived from previous melting of large ion lithophile element (LILE)-enriched mantle, possibly at c. 1.8 Ga. This crustal recycling was induced by heat and volatiles from underplated small-degree melts of LILE- and light REE-enriched lithospheric mantle (appinite-lamprophyre magmas). The post-collision magmatism and uplift resulted from breakoff of subducted oceanic lithosphere and consequent rise of asthenosphere. [ABSTRACT FROM AUTHOR]