Testing petrogenetic models for contemporaneous mafic and felsic to intermediate magmatism within the “Newer Granite” suite of the Scottish and Irish Caledonides

Abstract

Granitoid batholiths dominated by felsic to intermediate compositions are commonly associated with mafic plutons and enclaves; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is unclear. Here, we reviewed the age and lithogeochemical and Nd-Sr isotopic compositions of some classic plutonic rocks emplaced in the Northern Highlands, Grampian and Connemara terranes of the Caledonide orogen of Scotland and Ireland. The Northern Highlands terrane consists mostly of Neoproterozoic metasedimentary rocks of the Moine Supergroup and is located north of the Great Glen fault. The Grampian terrane also consists of Neoproterozoic metasedimentary rocks (Dalradian Supergroup) and is located south of the Great Glen fault in both Scotland and Ireland. Amphibolite-facies metasedimentary rocks in the Connemara terrane are correlated with the Dalradian Supergroup, and the terrane is bounded by splays of the Highland Boundary and Southern Uplands faults. These three terranes were intruded by Silurian–Devonian mafic and felsic to intermediate plutonic rocks that display field evidence for mingling and mixing and have a similar range (between ca. 437 and 370 Ma) in emplacement ages. This range implies they were intruded during and after the late Caledonian Scandian orogenic event that resulted from the mid- to late Silurian collision of amalgamated Avalonia and Baltica with Laurentia and the final closure of the Iapetus Ocean. Our review supports the contention that the Great Glen fault represents a major compositional boundary in the Silurian lithosphere. Felsic to intermediate plutons that occur north of the Great Glen fault are more enriched in light rare earth elements and Ba-Sr-K compared to those to the south. Isotopic compositions of these late Caledonian plutonic rocks on both sides of the Great Glen fault indicate that metasomatism and enrichment of the subcontinental lithospheric mantle beneath the Northern Highlands terrane occurred just prior to emplacement of late Caledonian plutons. Within the same terrane, mafic and felsic to intermediate rocks display similar trace-element and rare earth element concentrations compatible with models implying that fractionation of a mafic magma played an important role in generating the felsic to intermediate magmas. The onset of slab failure magmatism may have been diachronous along the length of the collision zone. If so, slab failure may have propagated laterally, possibly initiating where promontories collided.

Reply to Dewey and Ryan Comment on Searle, MP (2021) ‘Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation’

“…The Newer granites are also unlikely to represent products of slab-break-off (Atherton & Ghani, 2002), or of a discrete Andean-type plate margin, because they intruded across all terranes from the Northern Highlands south to the Southern Uplands and northern England, not along a plate boundary as would be expected in an Andean subduction zone setting. The 430-408 Ma high-Ba, high-Sr 'granites' have compositions ranging from ultramafic and mafic to granites and syenites, and are more similar to A-type alkaline complexes with associated appinites, and mantle-derived lamprophyres (Fowler et al 2008;Neilson et al 2009;Miles et al 2016;Archibald et al 2022).…”

“…Some of the Northern Highlands granites are more similar to alkaline complexes with associated appinites, and mantle-derived lamprophyres, than to subduction-related magmas (Fowler et al 2008;Neilson et al 2009;Goodenough et al 2011;Miles et al 2016;Archibald et al 2022). The highly undersaturated alkaline source for the syenites (e.g.…”

“…The original suggestion of Kennedy (1946) that the Strontian granite intruding the Moine rocks NW of the Great Glen fault, and the Foyers granite intruding the northern Dalradian rocks, SE of the Great Glen fault, were originally one and the same pluton, offset sinistrally by 105 km, remains a distinct possibility, and if correct is the only known offset geological marker along the Great Glen fault. Both granites show a tonalite-granodiorite-adamellite composition, both have similar Nd-Sr isotopic compositions, both are associated with hornblende-bearing appinites (Marston, 1971;Archibald et al 2022) and both have indistinguishable U-Pb zircon ages (425 ± 3 Ma for Strontian; 424 ± 4 Ma for Foyers granite; Rogers & Dunning, 1991;Oliver et al 2008). Figure 3 shows a reconstruction of Northern Scotland assuming a c. 100 km sinistral offset along the Great Glen fault.…”

Logan Medallist 7. Appinite Complexes, Granitoid Batholiths and Crustal Growth: A Conceptual Model

“…Geochemical data are consistent with a genetic connection between mafic and felsic magmas. For example, in the Scottish and Irish Caledonides, high-K shoshonitic appinite bodies are associated with Ba-Sr rich syenite and granite with adakitic affinities to the north of the Great Glen Fault, whereas appinite and coeval granite plutons have low-K calc-alkaline compositions to the south of the Great Glen Fault (Archibald et al 2022). Although they share similar depletions in HREE and HFS elements (e.g.…”

“…Despite their mineralogical simplicity, geochemical analyses indicate that mafic to intermediate rocks range from high-K shoshonitic to low-K calcalkaline compositions. Intriguingly, high-K shoshonitic rocks are associated with high-K, Ba-Sr rich granite with adakitic affinities whereas granite associated with low-K calc-alkaline mafic compositions are also low-K calc-alkaline, implying a genetic relationship between coeval mafic and felsic compositions (Murphy 2020;Archibald et al 2022).…”