ABSTRACT

Re-Os ages were determined for molybdenites from the late-Caledonian Galway Granite in the west of Ireland. Molybdenites from three localities within the batholith, Mace Head, Murvey, and Travore, yield respective Re-Os ages of 425.2 ± 3.1 Ma, 383.2 ± 8.1 and 426.1 ± 5.5 Ma, and 338.9 ± 5.6 and 464 ± 28 Ma. The molybdenites from the three localities are believed to be contemporaneous. However, the Re-Os ages obtained for each locality are not only in disagreement with the ages of the samples from the other localities but also are inconsistent within a single locality. Replicate analyses of a molybdenite standard (HLP-5) characterized by AIRIE using our method (Suzuki et al., 1992) yields reproducible Re-Os ages, which are in agreement with the results of Markey et al. (1998). Therefore, the age discrepancies in Galway molybdenites in this study should not be attributed to analytical errors. Suzuki et al. (2000) revealed experimentally that the Re-Os system in molybdenite can behave as an open-system when immersed in warm aqueous NaCl or NaHCO₃ bearing fluid. Detailed fluid inclusion studies of molybdenite vein quartz and granite quartz (Gallagher et al., 1992; O'Reilly et al., 1997) have shown that the Galway Granite was the locus for repeated fluid events, i.e., 1. Expulsion of aqueous carbonic magmatic fluids during the final stages of crystallization, 2. Meteoric fluid convection probably driven by waning magmatic heat and 3. Incursion of basinal brines associated with fluorite mineralization at least 100 Ma after granite consolidation. We suggest that younger hydrothermal fluids perhaps associated with fluorite mineralization may have perturbed the Re-Os systematics of molybdenites from the Galway Granite.