TWO-STAGE DIKING AT THE DYING OF THE GALWAY BATHOLITH

It could be said that the Galway batholith died more than once. The main plutonic episode of granodioritic magmatism extended between 420 and 400 Ma. It was at the end of this history that the first of two very different suites of dikes was intruded. Discrete sets of highly phyric dacite ('porphyry') dikes cut NNE-SSW across the batholith, and can be followed for several kilometres into its gneissic envelope. The trend is perpendicular to the long-axis of the batholith and expresses a relaxation phase in the structural history of the region. Dacite chemistry places it as a hypabyssal equivalent of the K-feldspar megacrystic granodiorites comprising the bulk of the batholith. Superimposed on this chemistry is a small yet regular increase in maficity with distance from the batholith axis, implying a mixing relationship that is endorsed in the mineralogy. The crustal stress-field was changing at the end of dacite diking with initiation of ENE-WSW trending dextral oblique-slip faults. This field held when the second suite of dikes was intruded ca. 375 Ma ago, a bimodal suite comprising dolerite and intrusive rhyolite members that manifest a spectrum of mingling and mixing relationships. For one of these dikes, the ratio mingling/mixing grades smoothly from near zero at one end of the dike to extremely high at the other along a 5-km length. The bimodal suite, though centred on the batholith extends well outside it, and according to location follows one of three structural controls. A MORB source supplied the dolerite magma, while rhyolite magma was melted from severely depleted crustal rocks.