THE VARISCAN THERMAL HISTORY OF WEST CLARE, IRELAND

Vitrinite reflectance data from Namurian rocks in west Clare suggest t hat high maturation levels, corresponding to palaeotemperatures of 340 -370-degrees-C, were attained prior to Variscan deformation. Fluid inc lusions in syntectonic quartz veins homogenize between 330-degrees-C a nd 50-degrees-C with an accompanying decrease in salinity from 27 to 5 eq. wt% NaCl. Maximum fluid inclusion entrapment temperatures ranged from more than 300-degrees-C to 250-degrees-C during Variscan folding in County Clare. The observed maturation levels (c. 6-7.5% R(max)) far exceed values for simple burial maturation based on the estimated bur ial history and 'normal' geothermal gradients, and do not increase wit h depth in the Doonbeg No. 1 exploration well. Fluid advective heating is suggested as the most likely mechanism consistent with the Clare r eflectance and thermometric data. Vein and shear zone dimensions precl ude rapid vertical movements of hot fluids through the section, and ex tensive lateral fluid migration from sedimentary basins undergoing tec tonically driven dewatering to the south or west is therefore proposed .