Using a ‘speedy’ unspiked K–Ar methodology to investigate age patterns in giant mafic dyke swarms

Abstract We present the results of a geochronological investigation that was conducted on mafic rocks (dolerites) that form two giant dyke swarms in NE Brazil, whose intrusion was correlated to the break-up of the West Gondwana supercontinent and the opening of the Atlantic Ocean. Despite their impressive dimensions, these swarms lack any geological information, which motivated us to develop a speedy, low-cost analytical protocol, modified from the Cassignol unspiked K–Ar technique, to define age patterns. The results were interpreted in light of basic statistical treatments and, although some limitations were mostly related to grain-size heterogeneities, they successfully matched other 40K-based ages (conventional K–Ar and 40Ar/39Ar) reported in the literature and showed that the dyke swarms share two main age intervals of rock formation identified as the Early Cretaceous and Late Jurassic.