The Variscan thermal history of west Clare, Ireland

Abstract

Vitrinite reflectance data from Namurian rocks in west Clare suggest that high maturation levels, corresponding to palaeotemperatures of 340–370 °C, were attained prior to Variscan deformation. Fluid inclusions in syntectonic quartz veins homogenize between 330 °C and 50 °C with an accompanying decrease in salinity from 27 to 5 eq. wt % NaCl. Maximum fluid inclusion entrapment temperatures ranged from more than 300 °C to 250 °C during Variscan folding in County Clare. The observed maturation levels (c. 7.5% Rmax) far exceed values for simple burial maturation based on the estimated burial history and ‘normal’ geothermal gradients, and do not increase with depth in the Doonbeg No. 1 exploration well. Fluid advective heating is suggested as the most likely mechanism consistent with the Clare reflectance and thermometric data. Vein and shear zone dimensions preclude rapid vertical movements of hot fluids through the section, and extensive lateral fluid migration from sedimentary basins undergoing tectonically driven dewatering to the south or west is therefore proposed.

Variable character and diverse origin of hybrid event beds in a sandy submarine fan system, Pennsylvanian Ross Sandstone Formation, western Ireland

“…Sediment classification terminology for sand–silt–clay mixtures follows Folk (). High vitrinite reflectance values (>6 R max ) and fluid‐inclusion measurements reveal that the Ross Formation was exposed to high temperatures prior to and during end‐Carboniferous Variscan deformation (Fitzgerald et al ., ), so the detrital clay is recrystallized and dominated by illite. Core plugs in sandstones from the ‐01 borehole have helium porosities of ca 0·5%.…”

“…A re‐assessment of the bed‐level organization in the Ross Formation has been made possible by a recent programme of behind‐outcrop drilling that has provided over 1300 m of split drill core. The Ross Formation is overprinted by Variscan deformation (Fitzgerald et al ., ) and the near‐surface sandstones carry a widespread fracture cleavage with a more penetrative foliation developed in the finer‐grained facies. Consequently, details of the bed structure and subtle changes in clay content are not easily recognized in many of the outcrops.…”

Carboniferous dykes as monitors of post-Caledonian fluid events in West Connacht, Ireland

“…If the major chemical and isotopic alteration of these dykes did take place at the time of intrusion, then an additional source of hot fluid is required. One possibility would be contemporaneous metamorphic fluid at ~35O C from the Namurian rocks of Co. Clare, only 70 km to the south (Fitzgerald et al 1994 Jochum et al (1983). K contents for continental alkali-basalts are for Scottish Lower Carboniferous continental alkali basalts (Smedley 1986).…”

Mid-Dinantian brachiopod biofacies from western Ireland

“…However, the overlying Brigantian Slievenaglasha Formation includes over 100 m of coarse bioclastic limestones, dominated by crinoid debris and extensive chert horizons (Drew 1989;Gallagher 1994). Data for the younger Carboniferous strata in northwest Clare are noted in Fitzgerald et al (1994).…”