Caledonian hot zone magmatism in the ‘Newer Granites’: insight from the Cluanie and Clunes plutons, Northern Scottish Highlands

Abstract

Scottish ‘Newer Granites’ record the evolution of the Caledonides resulting from Iapetus subduction and slab breakoff during the Silurian–Devonian Scandian Orogeny, but relationships between geodynamics, petrogenesis and emplacement are incomplete. Laser ablation U–Pb results from magmatic zircons at the Cluanie Pluton (Northern Highlands) identify clusters of concordant Silurian data points. A cluster with a weighted mean 206 Pb/ 238 U age of 431.6 ± 1.3 Ma (2 σ confidence interval, n  = 6) records emplacement whilst older points (clustered at 441.8 ± 2.3 Ma, n  = 9) record deep crustal hot zone magmatism prior to ascent. The Cluanie Pluton, and its neighbour the c. 428 Ma Clunes tonalite, have adakite-like high Na, Sr/Y, La/Yb and low Mg, Ni and Cr characteristics, and lack mafic facies common in other ‘Newer Granites’. These geochemical signatures indicate the tapping of batches of homogenized, evolved magma from the deeper crust. The emplacement age of the Cluanie Pluton confirms volumetrically modest subduction-related magmatism occurred beneath the Northern Highlands before slab breakoff, probably restricted by crustal thickening during the c. 450 Ma Grampian 2 event. Extensive new in situ geochemical–geochronological studies for this terrane may further substantiate the deep crustal hot zone model and the association between Caledonian magmatism and potentially metallogenesis. The term ‘Newer Granites’ is outdated as it ignores geochronology, petrography and tectonic setting. Hence, ‘Caledonian intrusions’ would be a more appropriate generic term to cover those bodies related to either Iapetus subduction or to slab breakoff. Supplementary material: Full geochronological, whole rock geochemical and geobarometric data are available at https://doi.org/10.6084/m9.figshare.c.6305927