Careen MacRae, Iain Neill , Edward D. Dempsey, Anna Bird, Mark Wildman, Liene Spruzeniece, Chloe Gemmell, Bailey Lathrop, Joshua Einsle

The Northern Highlands Terrane of the Scottish Caledonides contain a record of Palaeozoic collision magmatism which overlaps with Baltica – Laurentia convergence and the Scandian Orogeny. Here, there remain gaps in our understanding of the relationship between collision dynamics and the spatial-temporal distribution of Caledonian intrusions. In this study, the Strontian and Helmsdale plutons and the Abriachan stock are newly dated using laser ablation U-Pb zircon mass spectrometry. Strontian data (weighted mean 206Pb/238U results of 426.7 ± 1.8 Ma, 2; 425.6 ± 2.0 and 418.6 ± 6.3 Ma); corroborate previous findings that the complex was emplaced over the period ~427 - ~417 Ma. New emplacement dates are confirmed for Helmsdale (419.3 ± 3.3 Ma) and Abriachan (420.1 ± 3.4 Ma). Furthermore, 4 of 6 complexes so far analysed by in-situ geochronology methods preserve evidence for antecrystic zircon growth (Ross of Mull, Strontian, Cluanie, and Helmsdale). This finding substantiates the development of a lower crustal hot zone during Iapetus subduction beneath the Laurentian margin. Spatially limited amounts of mid-crustal emplacement occurred from ~448 - ~428 Ma, overlapping the onset of the Scandian Orogeny at ~437 Ma. Emplacement at this time was likely triggered by subduction, accelerated slab roll-back, sporadic hot zone remobilisation, and localised trans-tension. There followed two spatially greater phases of emplacement during the Scandian Orogeny. The most significant, around 425 Ma, is associated with sinistral faulting and may have been triggered by lithospheric delamination. A second phase, around 419 Ma, includes the Sanda facies of the Strontian pluton, previously associated with dextral Great Glen Fault motion. This phase may further correlate uplift at the end of the Scandian Orogeny, which only terminated by ~415 Ma, and the effect of peri-Gondwanan terrane accretion further to the south and west along the Laurentian margin.