Zircon megacryst ages and chemistry, from a placer, Dunedin volcanic area, eastern Otago, New Zealand

Abstract

Zircon megacrysts from a sapphire‐bearing placer near Glenore, South Otago, New Zealand, gave U‐Pb ages linked to the Miocene Waipiata Volcanics. Yellow and brown (zoned) zircons gave ages almost within error of a mean age of 19.1 ± 0.2 Ma. Laser‐ablation inductively coupled plasma mass spectrometry analysis suggests these two zircons crystallised from separate magmas. The Ti‐in‐zircon thermometer gave different formation temperatures, between 550–685°C for yellow zircon and 700–730°C for brown zircon. The zircons have typical felsic chemistry, with positive Ce anomalies, while negligible Eu depletion indicates little plagioclase fractionation in their source magmas. Incompatible element values are enriched in the brown zircon. Such zircon megacrysts are rare in New Zealand, but they resemble east Australian zircons from basalt fields and may come from metasomatised mantle melts or fractionated alkali basalt magmas.