Age, petrology and geochemistry of Wairakau Andesites, Whangaroa, Northland

Abstract

Abstract The Wairakau Andesites of Whangaroa, New Zealand, are predominantly andesitic to rhyolitic volcanogenic sediments, with some intrusives and possibly lava flows. K-Ar radiometric age determinations indicate an Early Miocene age, rather than Pliocene-Pleistocene as previously reported. The most abundant rock type is andesite, which contains phenocrysts of plagioclase, clinopyroxene, and lesser orthopyroxene, hornblende and olivine. In some of the andesites, data from electron microprobe analyses record abrupt and commonly reverse compositional zoning in phenocrysts. This and textural evidence for disequilibrium are consistent with mixing and/or assimilation processes. Al-rich pyroxenes and amphiboles present in many samples may represent lower crustal xenocrysts, though garnet, which occurs in other Northland andesites, was not found. Major and trace element analyses show that the Wairakau Andesites are a moderate to high-K calcalkaline suite, with relatively high Rb, Th, Zr and Ni contents. Geoche...