The density and composition of single generation fluids in quartz veins from two similar host lithologies located either side of the Irish Variscan front were investigated using fluid inclusion microthermometry and crush leach analysis. The front marks the northern boundary of a distinctive cleavage developed under sub-greenschist facies conditions and coincides with the margin of a fault-controlled inverted half graben, the Late Devonian Munster Basin. Trapped fluids in Variscan veins north of this basin have medium salinities (4–14 wt% NaCl eq.) and densities generally compatible with the expected P—T conditions of deformation in the area. Fluids in veins south of the half graben margin can be grouped into those in (a) syn-compressional veins with moderate salinities (8–16 wt%) and (b) late stage veins associated with post-orogenic extension which have trapped high salinity fluids (22–27 wt%). Fluid densities for both vein types are broadly compatible with estimated P–T conditions of both trapping events. Crush-leach analyses reveal that all the fluids analysed have Br/Cl ratios close to SMOW. Variations in I/Cl ratios and cation chemistry indicate significant water/rock interaction for medium salinity fluids on either side of the front.

The marked internal consistency in Br/Cl ratios from northern and southern fluids coupled with the broad ranges of cation to chloride ratios suggests that an early homogenous marine brine was subsequently modified by differing migration histories during the Late Palaeozoic tectono-sedimentary evolution of the area. It is contended that on a megascopic scale, gravity driven fluid flow controlled by megascopic fold geometries was the dominant mechanism of fluid migration associated with the buckling phase of the orogen, while expulsion of supra-hydrostatically pressurised fluids is associated with late stage orogenic extension.

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