LOW-TEMPERATURE THERMAL HISTORY OF EASTERN IRELAND - EFFECTS OF FLUID-FLOW

A reconnaissance apatite fission track study was undertaken in eastern Ireland to evaluate the low temperature thermal history of Palaeozoic basement rocks outcropping adjacent to the Irish Sea basin. On a samp ling transect across the lapetus suture zone the distributions of sing le grain ages and track lengths indicate cooling after maximum palaeot emperatures of between 80 and 100-110-degrees-C at various times durin g the Early Tertiary (about 30-45 Ma). Although a significant componen t of Tertiary uplift and erosion is inferred in eastern Ireland from g eological observations, maximum palaeotemperatures are interpreted as principally due to heat advection by gravity-driven groundwater flow f ollowing Early Tertiary uplift at about 60 Ma. This phase of fluid flo w produced the localized remagnetization of the Palaeozoic basement an d overlying Carboniferous limestones in north Co. Dublin. Limited samp ling in eastern Ireland indicates extreme local heterogeneity in apati te age, also characteristic of apatite fission track results from nort hern England. Thus heterogeneity requires an annealing control, indepe ndent of the sample's thermal history, suggested to be the individual crystal uranium content and indirectly the effects of fluid flow.