FLUID INCLUSION STUDIES ON THE LATE STRUCTURAL HISTORY OF THE CONNEMARA DALRADIAN, WESTERN IRELAND

The metamorphic inlier of Connemara has a well established stratigraphic correlation with the Dalradian of Scotland. Sediments thought to have been deposited on the rifting continental margin of Laurentia, 700Ma to 600Ma, were subjected to at least four phases of deformation, two phases of metamorphism and episodes of basic and acidic intrusion. This was due to arc-continent collision, 490-470Ma, prior to final closure of the Iapetus Ocean.

Dalradian rocks are confined to the regions south of the Great Glen fault and north of the Highland Boundary fault in Scotland and Ireland. Connemara is the exception to this. The Connemara Dalradian is 60 -100 km south of the Highland Boundary fault continuation in Ireland known as the Fairhead-Clew Bay line. The nature of the Dalradian and Ordovician contact is unknown as it is overlain by the later Silurian of South Mayo.

Fluid inclusion studies of quartz veins have been used to seperate pre and post Silurian veins. Pre Silurian veins in the Dalradian, showing D3 deformation, have CO2 rich inclusions which give a temperature of 350C and a pressure of 1kbar. The geothermal gradient is unnaturally raised due to 480Ma Gabbroic intrusions. These results give uplift rates of 7km per 10Ma from 490-480Ma.

Post Silurian veins show at least two populations of aqueous rich inclusions giving lower temperatures and pressures. Post Silurian veins cut mildly deformation Silurian sedimentary rocks and give homogenisation temperature results the same as similar style veins cutting the Dalradian pile. In the Dalradian the veins are thought to be late stage extensional veins.