| Abstrakt: |
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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 (<1%) ages for basalt groundmass samples from two of the eruption centres, we show that megacryst ages are indistinguishable from, or very similar to (within a few thousand years) corresponding alkali basalt ages (Mount Shadwell and Mount Leura). In another case, distinctly different ages for two megacrysts from the Lake Keilambete eruption centre suggest that some megacryst formation events may pre-date basalt magmatism by ca. 20 ka or more. Overall, high-temperature alkali feldspar megacrysts (and potentially other K-bearing megacrysts), found in ICBVPs worldwide, appear to be valuable tools for approximating host basalt eruption ages, or at the very least, providing maximum eruption age constraints. The new data also reveal a previously unidentified increase in volcanic activity in the NVP at ca. 200–110 ka. [ABSTRACT FROM AUTHOR] |
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