40Ar/39Ar geochronology of the Drakensberg continental flood basalts: Understanding large argon isotopic variations in mafic groundmass and plagioclase size fractions

The extrusive component of the Karoo Large Igneous Province, Drakensberg Continental Flood basalts South Africa and Lesotho, is mainly composed altered tholeiitic without uraniferous mineral phases that are suitable for high-precision dating (e.g. baddeleyite or zircon), thus restricted to 40Ar/39Ar groundmass plagioclase. Previous dates plagioclase span from 178.9 ± 4.2 Ma 189.2 3.3 Ma, while most range between 180 183 (Jourdan et al., 2007b; Moulin 2011; 2017). Dating mafic rocks can be hindered by presence xenocrysts (inherited 40Ar), ubiquitous alteration phases. Separation protocols acid leaching used minimize effect these issues, although previous studies Drakensburg lavas did not interpret their within a petrochronological context, they open inaccuracy. We have cathodoluminescence imaging, QEMSCAN analysis optical petrology thin sections show two distinct generations occur lavas, at least different K-rich (adularia clays) Ca- occurred. been obtained some acid-leached unleached aliquots both generations. Our results i) large, cluster-forming yield older interpreted xenocrystic, ii) apparently unaltered in hosts secondary Ar reservoirs related alteration, iii) enhance effects 37,39Ar recoil conclude accurate date applies entire thickness lava stacks 184.4 1.2 which was matrix-forming contains highest proportion non-inherited radiogenic 40Ar, affected suffer isotope recoil.