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

摘要: Abstract Potassium-argon total rock, sanidine, and plagioclase ages are presented for 24 rhyolite, dacite, andesite, basalt rocks of the Upper Cretaceous, Mt Somers Volcanics mid Canterbury. Ages grouped as follows: (1) total-rock andesites dacites: 45–94 Ma, but with a significant concentration at 82–91 Ma; (2) 87–93 (3) sanidine pitchstone flows rhyolitic intrusives: 88–99 Ma. The wide range andesite ages, which do not correlate co-existing indicate that substantial argon loss (up to 50%) may, in few cases, have occurred from second potassic phase, most probably volcanic glass. majority total-rock/plagioclase pairs show concordant turn similar closely associated rocks. Detailed stratigraphic evidence potassium variations feldspars (about 5%) has plagioclases (K < 0·8%) some s...