Distinguishing dolerite dike populations in post-grampian Connemara

Abstract

Abstract:The discovery of a dolerite dike exposed near Ross, Ballynakill Harbour, in north-western Connemara has re-opened investigation into two contrasting dolerite dike populations in Connemara, of respective mid-Paleozoic and Paleocene age. Initial examination of the Ross dike found it to share characteristics with both populations. Its mineralogical alteration and structural parameters appear to match those of the mid-Paleozoic dikes, whereas its geochemistry shows similarities with the Paleocene dikes; however, trace element and rare-earth element data assign it convincingly with the latter, despite differences that are explicable. A review of incompatible trace element parameters for the two suites confirms that the mid-Paleozoic magmas were volcanic-arc type basalts and the Paleocene magmas a singular species of within-plate type basalts. Initial 87Sr/86Sr ratios for the Paleocene dolerites fall in a tight cluster around 0.704. A wider range of 0.704–0.708 for the mid-Paleozoic dikes is attributed to a regionally imposed retrograde hydrothermal metamorphism. This is confirmed from an additional 39Ar-40Ar analysis to have a late Triassic age. The mid-Paleozoic dikes are further constrained to be of late Devonian age.