FIELD AND PETROGRAPHIC EVIDENCE FOR MAGMA MIXING AND MINGLING IN GRANITOIDS: EXAMPLES FROM THE GALWAY GRANITE, CONNEMARA, IRELAND

Many granitoid intrusions display textural evidence for the interaction of mafic and silicic magmas during their genesis. The ~400Ma Galway Granite in the west of Ireland provides an outstanding natural laboratory in which to study evidence for magma mixing and mingling both at outcrop/map scale (magma mingling and mixing zones; mafic microgranular enclaves(MME)), and at thin-section/crystal scale (mixing textures). Within the uplifted Central Block of the batholith, two zones of magma mixing and mingling are beautifully exposed, especially along the coast. In these zones, mafic sheets and enclaves of varying sizes (up to 1.2m), shapes (axial ratios vary from 1:1 to 20:1) and compositions (diorite - quartz diorite - granodiorite) are hosted by a heterogeneous hybrid granodiorite. In addition, MME are a relatively common constituent of many of the other lithologies in the batholith, where they sometimes occur in ‘swarms’, suggesting they are derived from disrupted synplutonic dikes.

Petrographic studies of both the MME and their host rocks have revealed a range of textures attributable to mafic-felsic magma interaction: quartz ocelli, rapakivi feldspars, acicular and mixed apatite morphologies, inclusion zones in feldspars, anorthite ‘spikes’ in plagioclase, sphene ocelli, K-feldspar megacrysts in MME, and mafic clots. Variations in the abundance of each texture in different hybrids, even those hybrids which have a close spatial relationship, suggest mixing occurred under different conditions and at different times: ie. there is strong evidence for multiple mixing events. Furthermore, textures from this assemblage have been recorded throughout the Galway batholith, even in relatively felsic lithologies, indicating that mafic-felsic magma interaction played a key role during the evolution of the batholith.

Finally, the ubiquity of mixing textures, even in the most mafic of MME, prompts the question – do any true mafic end member lithologies remain at the present exposure level of the batholith? – a question that must be addressed when choosing end member lithologies for magma mixing models.