The British Tertiary Igneous Province: Young Rb-Sr Ages for the Mourne Mountains Granites

Abstract

Radiometric, palaeomagnetic and palaeontological studies continue to refine information on the precise sequences of events in the various parts of the North Atlantic Tertiary Igneous Province before and after the initiation of seafloor spreading. On timing models of approximately 53 or 54 Ma for the main opening of the NE Atlantic Ocean, major intrusive activity in the British Isles mostly preceded the latter at around 59 Ma (Macintyre, McMenamin and Preston 1975, 1979; Mussett 1980). By contrast in East Greenland magmatism was closely associated in time with this major event (Soper, Higgins et al. 1976; Soper, Downie et al. 1976; Myers 1980) and continued intermittently for more than 15 Ma after it adjacent to the rifted continental margin (Brown et al. 1977; Gleadow and Brooks 1979). This contribution complements the data available for Scottish Tertiary igneous rocks, presenting the results of Rb–Sr age determinations on each of the high level Mourne Mountains granite intrusions (G1–G5), the first by this method on Irish Tertiary igneous rocks. These imply that there were major intrusive events at approximately 56 Ma and 52 Ma in NE Ireland, with G5, the latest ring dyke exposed in the complex, appearing to be significantly younger than its predecessors G1–G4 and contemporaneous with some of the earlier intrusions in East Greenland. The Mourne Mountains central intrusive complex comprises five metaluminous-peraluminous (biotite ± calcic amphibole) granite intrusions, three (G1, G2 and G3) in the eastern centre (Richey 1928) and two (G4 and G5) in the younger, contiguous western centre . . .