Scottish Highlands Caledonian Granites: a fresh look at hot zone origins, emplacement and their relationship to Pb-Zn-carbonate mineralisation

Plutons formed during the latter stages of the Caledonian Orogeny are a prominent feature of the landscape of the Northern Highlands of Scotland. Despite their prominence, and in rare cases mineralisation (Strontian) or high heat producing properties (Helmsdale), various intrusions lack critical analysis of their timing, emplacement mechanisms and geodynamic significance. For example, published emplacement ages are typically from small air abrasion isotope dilution studies of the 1970’s-1990’s¹. These have recently been argued to risk bias towards high quality grains which potentially grew during lower crustal processing of parental magmas². Here, we are conducting U-Pb zircon re-dating of six intrusions associated with the Great Glen Fault system: Glen Loy, Linnhe, Abriachan, Cluanie, Strontian and Helmsdale. Through a combination of extensive zircon picking, cathodoluminescence imaging and laser ablation mass spectrometry on multiple points per zircon we aim to reduce this selection bias.  

Initial results, with titanite geochronology to follow, indicate that Glen Loy and Cluanie pre-date Iapetus slab breakoff and are therefore related to subduction beneath the Laurentian margin. All plutons studied so far demonstrate evidence of zircon growth which pre-dates final emplacement. We argue that, Iapetus subduction and Baltica-Laurentia collision were responsible for the generation of a lower crustal hot zone beneath the Northern Highlands. This hot zone lasted from ~450-430 Ma, prior to the upsurge in magmatism which followed slab breakoff. Re-dating of the ‘outer’ granodiorite facies of the Strontian pluton has produced a probable emplacement age at least 10 Myr younger than the previous accepted age of ~425 Ma. This finding raises questions about a) whether previous results reflected antecrystic zircon and titanite and b) the association of pluton emplacement with the timing of left-lateral motion on the Great Glen Fault system. 

In addition, few Northern Highlands plutons are significantly mineralised, except for the Pb-Zn-hosting carbonate veins at the Strontian pluton. However, we do not know the age of mineralisation or its metal distributions, particularly any metals which have been designated as critical to society since surveys in the 1980's. In this study, we have also developed a workflow in collaboration with the Critical Minerals Intelligence Centre of the British Geological Survey to date mineralisation using U-Pb methods on calcite, and to compare results with U-Pb apatite dating of a mafic sub-volcanic dyke at the Strontian pluton, suspected to be Permian-Carboniferous in age. We will further address the distribution of metals using a combination of optical petrology, electron microscopy, laser rastering and focused ion beam nano-tomography. This further addresses the above knowledge gaps with correlative cm- to nano-scale and three-dimensional insights into the mineralisation process, a strategy that can be replicated for other potential critical element bearing deposits.