Chloe Gemmell, Iain Neill , David Currie, Mark Wildman, Liene Spruzeniece, Careen MacRae, Joshua Einsle

The relationship between igneous activity and the geodynamics of subduction and collision is uncertain in northern Britain and Ireland, which lay on the Laurentian margin during accretion of Baltica and peri-Gondwanan terranes during the Palaeozoic Caledonian - Acadian orogenies. There is uncertainty over the relevance and timing of Iapetus slab breakoff beneath the Laurentian margin, and modest attention has been paid to typical characteristics of collision magmatism such as its trans-crustal nature and association with ore deposits. The Southern Uplands-Down-Longford terranes were an ocean-continent accretionary complex north of the Iapetus suture and present a natural laboratory to better constrain some of these issues. The terrane is cut by plutonic, hypabyssal, and volcanic complexes whose emplacement overlaps the docking of peri-Gondwanan terranes. Together with existing geochronological data, new laser ablation U-Pb data for the Carsphairn, Bengairn, and Cheviot plutons and the Black Stockarton Moor sub-volcanic complex in Scotland, allow us to identify populations of antecrystic and emplacement-related zircons. The complexes have records of antecrystic zircon growth up to ~30 Myr prior to emplacement, and some plutonic facies could be derived largely from remobilised crystal mushes formerly stored in the lower crust. Crystallisation of magmas began at ~424 Ma as the Iapetus slab began to roll back from the Laurentian margin, permitting melting of the mantle beneath the accretionary complex. The timing of breakoff is potentially constrained to ~420-418 Ma by the cessation of sedimentation in the accretionary prism and the onset of magmatism directly above the suture. Subsequent emplacement from 417 – 387 Ma may be driven by a combination of under-thrusting of Gondwanan lithosphere, sub-lithospheric convection, and a trans-tensional upper plate regime.