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Journal of the Geological Society; 1995; v. 152; issue.3; p. 511-521;
DOI: 10.1144/gsjgs.152.3.0511
© 1995 Geological Society of London

Article

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

P. A. MEERE

Department of Geology, University College, Cork, Ireland

Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25°{Delta}2{theta} indicating a metamorphic grade in the upper anchizone-lower epizone (c. 275–325°C). Chlorite geothermometry yields a similar metamorphic temperature range (280–315°C). Combining these data with known overburden estimates for the area (c.5km) implies late Palaeozoic geothermal gradients in excess of 60°Ckm–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 0.9–1.0gcm–3 (Group 2) respectively. This variation in fluid density is thought to be dominantly controlled by temperature, with Group 1 veins linked to late Palaeozoic extension and high geothermal gradients and Group 2 veins associated with subsequent Variscan deformation and lower temperature conditions.


Keywords: Ireland, Varicides, low-grade metamorphsim, crystallinity, fluid inclusions.




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