40Ar/39Ar ages of alkali feldspar xenocrysts constrain the timing of intraplate basaltic volcanism

Abstract

Abstract Intraplate continental basaltic volcanic provinces (ICBVPs) occur on all continents and represent some of the most enigmatic volcanic systems. Constraints on the origin(s) and evolution of ICBVPs are predicated on detailed knowledge of eruption histories, which are often poorly quantified. Although the 40Ar/39Ar geochronology method has been applied successfully to lava flows, age determinations on scoria cones and maars are more challenging. In this study, we test the potential of entrained anorthoclase megacrysts to yield accurate 40Ar/39Ar ages for scoria cones and maars from the Pliocene-Holocene Newer Volcanic Province (NVP) of south-eastern Australia. The NVP is an ICBVP containing >400 eruption centres with ages spanning 4.6 Ma – 5 ka. K-Ar and 40Ar/39Ar age data exist for a large number of lava flows, but age constraints for scoria cones and maars that lack associated lavas are rare. High-precision 40Ar/39Ar step-heating data were measured on anorthoclase megacrysts from five eruption centres in the NVP. From youngest to oldest, anorthoclase xenocrysts from Mount Noorat (n = 3), Mount Leura (n = 2), Mount Shadwell (n = 3), and The Anakies East Cone (n = 2) yielded reproducible mean inverse isochron ages of 101.75 ± 0.96 ka (2σ), 165.4 ± 1.6 ka (2σ), 353.8 ± 1.9 ka (95% CI), and 2.178 ± 0.005 Ma (95% CI), respectively. In contrast, Lake Keilambete anorthoclase (n = 2) produced discordant age spectra and incongruous isochron/mini-isochron ages of 433.5 ± 7.2 ka (2σ) and 413.4 ± 4.5 ka (2σ). A single anorthoclase megacryst from Mount Franklin gave an age of 126.3 ± 7.2 ka (95% CI). By directly comparing megacryst age results with recently published, high-precision (