REGIONAL DOLOMITIZATION OF THE WAULSORTIAN LIMESTONE IN SOUTHEASTERN IRELAND : EVIDENCE OF LARGE-SCALE FLUID FLOW DRIVEN BY THE HERCYNIAN OROGENY

The Lower Carboniferous Waulsortian limestone has been affected by late diagenetic, regional burial dolomitization over an area of ∼7000 km 2 extending 170 km northeastward from the Hercynian front. The dolostone is composed of two components: (1) a very fine crystalline replacive component, and (2) a coarse-crystalline, baroque component that fills vuggy porosity developed within the replacive dolostone. Fluid inclusions within the coarse-crystalline component of the regional dolostone indicate that the dolomitizing solutions were warm (∼100 °C) and moderately saline (10–13 wt% NaCl equivalent). The replacive dolomite displays systematic decreases in crystal size and degree of xenotopic texture northward. Both components of the regional dolostone display a regular northward increase in mean δ 18 O values; the replacive dolomite has 87 Sr/ 86 Sr values that decrease northward. These laterally variable characteristics suggest that regional dolomitization resulted from large-scale, north-directed fluid flow of heated brines developed in response to topographic uplift associated with collapse of the continental margin south of Ireland during the Hercynian (Variscan) orogeny.