Sub-greenschist facies metamorphism from the Variscides of SW Ireland: an early syn-extensional peak thermal event

Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25°Δ2θ indicating a metamorphic grade in the upper anchizone-lower epizone (c.275-325°C). Chlorite geothermometry yields a similar metamorphic temperature range. Combining these data with known overburden estimates for the area implies late Palaeozoic geothermal gradients in excess of 60°C km^-1. Fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0.7-0.93 g cm^-3 (Group 1) and 0.9-1.0 g cm^-3 (Group 2) respectively. This variation in fluid density is thought to be dominantly controlled by temperature, with Group 1 veins linked to lake Palaeozoic extension and high geothermal gradients and Group 2 veins associated with subsequent Variscan deformation and lower temperature conditions. -Author