FLUID INCLUSION SIGNATURES FROM IRISH CALEDONIAN GRANITES ON EITHER SIDE OF THE IAPETAN LAURENTIAN-AVALONIAN DIVIDE

Granites are a major constituent of the Appalachian-Caledonian crust. Fluid inclusion signatures in granite quartz can provide direct compositional evidence for syn- and post magmatic fluids. The latter provide evidence of major hydrothermal convection and tectonic related events in the upper crust. The Irish onshore massif facilitates the study of fluid inclusion signatures in granite quartz from a range of Caledonian granites types located in close proximity to each other. Numerous Caledonian granites ranging in age from ~460 Ma to 380Ma are well exposed throughout the island and occur on either side of the Iapetan Laurentian-Avalonian divide, providing the opportunity to study fluids from two different tectonic plate settings. We report on the results of a systematic fluid inclusion study of Caledonian granite quartz from the Irish massif which display a variety of fluid signatures indicative of magmatic, meteoric and low-temperature fluid trapping events during the evolution of Irish Caledonian crust.

The granite quartz contains three fluid inclusion types. The earliest fluid was a H₂O-CO₂-NaCl ± CH₄ fluid of moderate salinity (3-16 eq. wt% NaCl) and high T_H (~300-450° C), and are present in the Galway, Leinster, Newry, Thorr and Kentstown granites. An origin wholly within the granite is implied for this fluid. A later fluid influx is recorded as H₂0-NaCl ± KCl inclusions with a wide range of T_H (160-340° C) and salinities (0-12 eq. wt% NaCl) and is observed in all late Caledonian granites. These inclusions have been interpretated as representing either mixing between late-magmatic and meteoric water or as wholly meteoric in origin. H₂0-NaCl-CaCl₂ ± KCl fluids have been identified as secondary inclusions in the Galway, Kentstown, Leinster, Oughterard and Newry granites, and their low T_H (50-200°C) and high salinity (8-28 eq. wt% NaCl) indicate they originated as basinal brines. They are similar to cool high-salinity CaCl₂-rich inclusions associated with Pb-Zn deposits in the Carboniferous rocks of the Irish Midlands. Genetic models indicate circulation of fluids within the fractured basement rocks, including the Caledonian granites, was important in the formation of these deposits, and therefore these fluids are believed to be Carboniferous in age. However a younger age cannot be discounted.