Faunal Evidence bearing on the Age of Late Cambrian – Early Ordovician Metamorphism in Britain and Norway

Abstract

IT can no longer be stated categorically that metamorphism of the Dalradian Series in Connemara occurred before the Didymograptus extensus zone of the Arenig Series, because those sediments generally accepted as the earliest post-tectonic sediments in the area have provided a Didymograptus bifidus zone (Lower Llanvirn) graptolite fauna. This means that the only stratigraphical younger age limit which can be placed on the Dalradian metamorphism in Connemara is some time plane at or near the base of the bifidus zone. This upward extension of the younger, minimum stratigraphical limit of the Dalradian metamorphism necessarily extends considerably that interval in Late Cambrian–Early Ordovician times during which an important phase of orogeny could have taken place. Furthermore, the statement by Leggo et al.¹ that “the base of the Arenig would seem to be younger than 510 ± 35 m.y.”, on the evidence of the rubidium/strontium age given by the post-metamorphic and (hitherto) pre-Arenig Oughterard granite, should be amended to refer to the base of the bifidus zone.