Age of metamorphism in the Rosslare Complex, S.E. Ireland

Abstract

40 Ar/ 39 Ar step heating ages have been determined on amphiboles from rocks of the Precambrian Rosslare Complex in southeasternmost County Wexford, Ireland. Four tectonothermal events are recognized in the complex. Data from Dark Gneiss of the banded gneisses to the north of St. Helens in the eastern part of the complex date the second metamorphic event at 626±6 Ma while the St. Helen's Gabbro, which intruded these gneisses during the second event, yielded a similar age of 618±5 Ma. These ages date a Cadomian or Monian event in the Rosslare Complex. In the southwestern end of the complex, near Kilmore Quay, amphiboles yielded 480±5 Ma ages from both Dark Gneiss and an older basic dyke that cross-cuts the gneisses and their deformational fabrics. Because there is evidence for a significant time gap between the formation of the Dark Gneiss and the Older Basic Dykes, these identical ages suggest an important heating event which degassed the argon system in the Dark Gneiss and may be related to the intrusion of the dykes. Evidence of this heating event is not seen in the St. Helens section, which is 18 km to the northeast.