Potassium-argon dating of Mt Somers Volcanics, South Island, New Zealand: Limitations in dating Mesozoic volcanic rocks

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Abstract
Potassium-argon total rock, sanidine, and plagioclase ages are presented for 24 rhyolite, dacite, andesite, and basalt rocks of the Upper Cretaceous, Mt Somers Volcanics of mid Canterbury. Ages are grouped as follows: (1) total-rock ages of andesites and dacites: 45–94 Ma, but with a significant concentration at 82–91 Ma; (2) plagioclase ages of andesites and dacites: 87–93 Ma; (3) sanidine ages of pitchstone flows and rhyolitic intrusives: 88–99 Ma. The wide range of andesite total-rock ages, which do not correlate with co-existing plagioclase ages, indicate that substantial argon loss (up to 50%) may, in a few cases, have occurred from a second potassic phase, most probably volcanic glass. The majority of total-rock/plagioclase pairs show concordant ages, which in turn are similar to sanidine ages of closely associated rhyolitic rocks. Detailed stratigraphic evidence and potassium variations in the feldspars show that argon loss (about 5%) has occurred in most plagioclases (K < 0·8%) and some sanidines (K < 7%), causing younger ages than anticipated. In addition, despite favourable potassium concentrations (7% < K < 0·9%) of their feldspars, andesites known to predate rhyolites none the less yield younger ages. To some extent this pattern is explained by slight differences in the andesite-rhyolite sequences at the three main eruptive centres at Mt Somers, Malvern Hills, and Rangitata Gorge. Both andesite and rhyolite volcanism at Mt Somers (94–98 Ma) may predate rhyolites in the Malvern Hills area, 91–94 Ma. Mt Somers Volcanics extend from about 98 Ma (upper Albian) to at least 92 Ma (Turonian). Owing to the slight argon loss problem, the younger age limit for volcanism cannot be assessed. A basalt sample from Malvern Hills, dated as 37 Ma, is probably related to widespread tholeiitic Oligocene volcanism in North Canterbury, rather than to Mt Somers Volcanics. Mt Somers Volcanics show age similarities with more basic alkaline volcanics of inland Marlborough (and may form part of an Upper Cretaceous volcanic arc in the South Island). More distant age correlations can be made with rhyolites on Lord Howe Rise (Tasman Sea) and in central Marie Byrd Land, West Antarctica.