Field and geochemical studies of the Galway granite batholith - the Kilkieran sector, Connemara, western Ireland
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The late-Caledonian granites of south Connemara occupy a key location in the western Irish Caledonides. The granites comprise the Galway Granite Batholith and its satellite plutons of Roundstone, Inish, Omey and Letterfrack. The granite's 80 km long, WNW-trending axis reflects a stitching relationship between the granite and the EW-trending Skird Rocks Fault which is one of a number of major strike-slip faults that parallel the Iapetus suture in the British and Irish Caledonides. The NNE-SSW-trending Shannawona and the NNW-SSE-trending Barna Faults, divide the batholith into three blocks i.e. the Western, Central and Eastern Blocks. This study was focussed on a ~34 km2 area of the Western Block i.e. the Kilkieran Sector located ~60 km west of Galway city. Detailed geological mapping has identified five main granite types i.e. the Microgranite, the Glenaruid Layered Granite, the Megacrystic Porphyritic Granite, the Scainnimh Coarse Grained Granite and the Northern Coarse Grained Granite. A suite of NNE-SSW trending dikes were emplaced during final consolidation of the granite. Locally these display margins marked by brecciation and intricate injectional patterns with the host granite. Digital datasets were used to generate a 3D geological model for the area. Petrographic and whole rock geochemical studies of 45 samples show that the granites range from Monzo-Granite to Syeno-Granites while the Mafic Microgranular Enclaves plot in the Diorite field. Major and trace element bivariate and trivariate plots define linear differentiation trends from the less silicic coarse grained granites to the more silicic microgranites data. Rare Earth Element (REE) plots for the granites display LREE enrichment, negative Eu anomalies and HREE depletion similar to those displayed by granites elsewhere in the batholith. Hornblende-plagioclase geothermobarometry, from eight new locations in the batholith, place pressure constraints on crystallisation that range from 3.16 ±0.6 kb to 4.13 ±0.8 kb. Estimated crystallisation temperatures range from ~690oC to ~730oC. U-Pb zircon geochronometry was carried out on five samples i.e. the study area (3), the Carna Granite (1) and the Lough Fadda Granodiorite (1). All three samples from the study area yield an age of ~402 Ma, The Carna Granite sample yields an age of ~412 Ma and the Lough Fadda Granodiorite sample yields an age of 393 ±3.4 Ma. Re-Os molybenite geochronometry on a sample of An Maoileann Microgranite yields an age of 402.2 ±1.1 Ma. The Re-Os and U-Pb age determinations for this microgranite provide a maximum age for timing of emplacement of the dacite dike suite and by inference a maximum age for their host granites. The geobarometric and geochronometric data for all of the batholith are integrated with the spatial distribution and structural relationships of the granites to develop a Pressure-time model for the emplacement of the Galway Batholith.
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