Sm-Nd Mineral Ages and P-T Constraints on the Pre-Grampian High Grade Metamorphism of the Slishwood Division, Northwest Ireland

Abstract:Granulite-facies metamorphism affecting the Slishwood Division was extreme. Three samples yielded P-T conditions of 15.8, 14, 14.9kbar at 810, 750 and 880°C, respectively. Four Sm-Nd mineral isochrons, defined by granulite-facies basic and pelitic metamorphic assemblages, yield ages of 544 ± 52 Ma, 539 ± 11 Ma, 596 ± 68 Ma and 540 ± 50 Ma. These ages confirm that granulite- and earlier eclogitefacies metamorphism took place before the c. 470Ma Grampian Orogeny. Detailed chronological interpretation is inhibited by microscopic inclusions within, and isotope disequillbnum between, the dated minerals It is possible that the ages record crystallisation of either the granulite or eclogite-facies assemblages. However, it is more likely that they record post-metamorphic cooling. Relict pre-granulite-facies igneous minerals from a metagabbro body possibly date its intrusion at 580 ± 36 Ma. Extreme metamorphism in the late Neoproterozoic to Early Cambrian suggests that the Slishwood Division is exotic to Laurentia.