title: Timing and distribution of post-subduction magmatism in the Southern Uplands of Scotland: new insights from U-Pb zircon data of Caledonian igneous bodies in the Southern Uplands Terrane
Creators: name: Gemmell, Chloe Angel Rebekah
Publisher: name: University of Glasgow
Year: 2025

description: The Southern Uplands of Scotland and its equivalent Down-Longford Terrane in Northern Ireland were part of a Palaeozoic accretionary complex on the Laurentian margin that actively accreted during the Ordovician and Silurian (~455 – 420 Ma). The Terranes were widely cross-cut by Devonian-aged ‘Caledonian’ intrusions related to subduction of the Iapetus oceanic slab and associated collision of periGondwanan Terranes with Laurentia (~428 – 423 Ma). However, geochronological data is lacking for several magmatic bodies in the Southern Uplands Terrane. Consequently, the timing of specific geodynamic and tectonic events and the overall architecture of the resultant magmatic systems is uncertain. In this study, I conducted U-Pb zircon dating and partial zircon trace element analysis using laser ablation mass spectrometry for several under-studied intrusions, namely the Cairnsmore of Carsphairn pluton, the Black Stockarton Moor subvolcanic complex, the Bengairn and Cheviot plutons. I apply a cautious texturally and in part chemically constrained approach to determine an appropriate time of emplacement for these magmatic bodies and the total duration of magmatism, based on the occurrence of zircon cores interpreted as reflecting antecrystic zircon growth. Zircon is a robust mineral that is inherited from basement rock and/or newly crystallises from magmas at different depths throughout the crust (Roberts and Spencer, 2015), thus zircon records total durations of magmatism from deeper levels to emplacement. Zircon data can be integrated regionally to identify magmatic patterns (i.e., time of onset of magmatism vs emplacement) that in turn can be linked to geodynamic processes that are suspected within a specific region (Oliver et al. 2008; Yilmaz Şahin et al. 2014).

Results presented in this study support and advance previous work by confirming textural and geochronological evidence for long-lived pre-emplacement evolution of magmas, probably in a lower crustal hot zone setting. The earliest onset of zircon crystallisation at ~424 Ma reflects the start of Iapetus slab rollback following the onset of continental collision between Laurentia and peri-Gondwanan Terranes. A potential southwards younging in the onset of magmatism from northern bodies (e.g., Carsphairn) to southern bodies (e.g., Cheviot) is supportive of rollback. There is no obvious spatial pattern in emplacement, rather individual emplacement events appear to be randomly distributed, probably due to local tectonic controls and associated pathways for melt migration. Overall, little emplacement occurred prior to ~415 Ma, but an upsurge in emplacement from hereon is coeval with a proposed switch from transpression to transtension; such an event might be concurrent with slab breakoff. Findings of this study imply that the Midland Valley and Grampian Highlands Terranes would benefit from further zircon studies of ‘Caledonian’ intrusions to define total durations of zircon growth and aim for high precision emplacement ages. This in turn would enable comparison between the geodynamic reconstruction presented here and geodynamic models for the Grampian Highlands and Midland Valley, to better understand the geodynamics over a broader sector of the Caledonian Orogen.