The U-Pb geochronology of Ordovician to Devonian plutonism in the Northern Highlands of Scotland

Ordovician-Devonian intrusions of the Scottish Northern Highlands Terrane have piecemeal geochronology and contested geodynamic significance. We review existing geochronological data and present new in-situ U-Pb zircon results from seven of these intrusions. Zircon growth predating emplacement is nearly ubiquitous, related to magma addition to the lower crust during Early Scandian compression. Emplacement ages, structures, and petrology vary in relation to major geodynamic processes on the Laurentian margin in the context of Baltica, Ganderia and Avalonia accretion: 1) ∼450-432 Ma Iapetus subduction and back-arc magmatism, 2) ∼432-423 Ma Ganderia accretion, lithospheric foundering, Scandian metamorphism and sinistral transpression, 3) ∼423-415 Ma Late Scandian exhumation and decompression and 4) ∼410-380 Ma Acadian subduction and collision processes. Only ∼423-415 Ma magmatism resulted in production of high radiogenic heat production intrusions of geothermal exploration interest. There is considerable potential in constraining the extent and timing of pre-emplacement zircon growth to relate crustal tectonics and magmatism during orogenesis.