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

卡鲁大火成岩省的喷出成分，南非和莱索托的德拉肯斯贝格大陆洪水玄武岩，主要由蚀变的拉斑玄武岩组成，不含适合高精度测年的含铀矿相（如斜锰矿或锆石），因此可以测年限于⁴⁰ Ar/ ³⁹ Ar 基体和斜长石。过去⁴⁰ Ar/ ³⁹ Ar 斜长石的年代跨度从 178.9 ± 4.2 Ma 到 189.2 ± 3.3 Ma，而大多数日期范围在 180 到 183 Ma 之间（Jourdan 等人，2007b；Moulin 等人，2011；Moulin 等人， 2017）。斜长石异晶的存在会阻碍镁铁质岩石的测年（继承⁴⁰Ar），以及普遍存在的变化阶段。^{尽管之前对德拉肯斯堡熔岩的40} Ar/ ³⁹ Ar 研究并未在岩石年代学背景下解释它们的日期，但使用分离协议和酸浸出将这些问题的影响降至最低，因此它们可能不准确。我们使用阴极发光成像、QEMSCAN 分析和薄片光学岩石学来证明熔岩中出现了两代不同的斜长石，并且至少有两个不同的富钾（adularia 和富钾粘土）和钙化阶段具有发生了。⁴⁰氩/ ³⁹已经从一些酸浸出和未浸出的等分试样和两代斜长石中获得了 Ar 日期。我们的研究结果表明，i) 大型簇状斜长石产生较早的⁴⁰ Ar/ ³⁹ Ar 日期，并被解释为异晶，ii) 薄片中明显未改变的斜长石包含与蚀变相关的次生 Ar 储层，以及 iii) 蚀变和浸出可以增强斜长石中^37,39 Ar 的反冲效果。我们得出结论，适用于整个熔岩堆厚度的德拉肯斯堡熔岩的最准确日期是 184.4 ± 1.2 Ma，这是从包含最高比例的非遗传放射成因^{40的基质形成斜长石中获得的}Ar 受二次改变的影响最小，并且明显没有受到 Ar 同位素反冲的影响。