The chemical character of the Late Caledonian Donegal Granites, Ireland, with comments on their genesis

Abstract

The Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470–460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407–402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all 'I' type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba–Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba–Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoffon Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite–lamprophyre suite.

A U–Pb age for the Late Caledonian Sperrin Mountains minor intrusions suite in the north of Ireland: timing of slab break-off in the Grampian terrane and the significance of deep-seated, crustal lineaments

“…They all belong to the same high-K, calc-alkaline series (Ghani & Atherton 2008). Low 87 Sr/ 86 Sr ratios (<0.707, typical of an enriched source), and the presence of amphibole and titanite, are consistent with their I-type, metaluminous nature.…”

“…Low 87 Sr/ 86 Sr ratios (<0.707, typical of an enriched source), and the presence of amphibole and titanite, are consistent with their I-type, metaluminous nature. No geochemical domains are apparent in the Grampian terrane of the Irish Caledonides, where high-Ba-Sr plutons can be found with their low Ba-Sr equivalents (Ghani & Atherton 2008). However, two distinct geochemical groupings have been identified (after Ghani & Atherton 2008), as follows.…”

“…In the Irish Caledonides, major and trace element geochemical characteristics of the Donegal granitoids have been discussed in detail by Azman & Atherton (2008) and Ghani & Atherton (2008) as summarized below. They all belong to the same high-K, calc-alkaline series (Ghani & Atherton 2008).…”

“…1b), Thorr and Fanad plutons. These contain very high Ba and Sr, high Na, P and Zr, and typically low Rb, Th, Nb, Rb/Zr and Rb/Sr (Tarney & Jones 1994;Ghani & Atherton 2008). Hornblende is common in these rocks, at up to 15%.…”

“…In the Grampian terrane of Scotland, between the Great Glen and Highland Boundary faults, large volumes of intermediate to acid magmatism were generated after cessation of subduction processes at c. 430 Ma (Soper et al 1992;Neilson et al 2009). Late Caledonian intrusions of northern Britain and Ireland record the arrival of Ganderia-Avalonia, its underthrusting and slab break-off (Ghani & Atherton 2008). Many of the larger Late Caledonian granitoids, such as the Donegal Batholith, have provided a natural laboratory for generations of geologists to test models of granite emplacement (see Chew & Stillman 2009), as well as the underling petrogenetic processes (i.e.…”