The Late Silurian to Upper Devonian Galway Granite Complex (GGC)

The Late Silurian to Middle Devonian calc-alkaline Galway Granite Complex (GGC) occupies a key location in the Irish and UK Caledonides. The GGC consists of a suite of Earlier Plutons (~424 Ma) i.e., the Roundstone, Inish, Omey and Letterfrack Plutons and the later Galway Batholith (~410–380 Ma). The latter comprises the Carna and Kilkieran Plutons. Zircon U-Pb and Molybdenite Re-Os chronometry of the component granites indicates that the formation of the GGC reflects four main stages of magma emplacement into the Grampian Connemara Metamorphic Complex and the Lower Ordovician South Connemara Group ranging from ~424 Ma to at least ~380 Ma. The five stages are: stage (1) The emplacement at ~424 Ma of the Omey, Roundstone and Inish Plutons and possibly the Letterfrack Pluton–collectively termed the Earlier Plutons; stage (2) emplacement of the Carna Pluton at ~410 Ma followed by stage (3) emplacement of the Kilkieran Pluton at ~400 Ma and stage (4) later granite intrusions within the Kilkieran Pluton e.g., the Shannapheasteen and Costelloe Murvey Granites, the latter was emplaced at ~380 Ma. A regional composite dolerite–rhyolite diking is also recorded and represents an Upper Devonian event in the GGC. These magmatic stages span the changing tectonic regimes i.e., from regional transpression during emplacement of the Earlier Plutons and the Carna Pluton to regional transtension during emplacement of the Kilkieran Pluton. The ~380 Ma Costelloe Murvey Granite is ~20 Ma younger than all other granite intrusions of the GGC and postdates Caledonian tectonism. This prolonged and episodic history of granite emplacement occurs in tandem with magmatic molybdenite and chalcopyrite mineralisation in the GGC and forms an integral part of the granite related molybdenite mineralisation corridor located along the Caledonian orogen of the North Atlantic Massif. Regional scale vein fluorite with base metal sulphides in the GGC is correlated with Triassic-Jurassic hydrothermal vein mineralization throughout Europe and the North Atlantic margins.