U-Pb Dating of Subsurface Pyroclastic Deposits (Tahorakuri Formation) at Ngatamariki and Rotokawa Geothermal Fields

Abstract

We present secondary ion mass spectrometry (SIMS) U-Pb age data and crystallisation-age spectra for zircons from altered pyroclastic rocks penetrated by drillholes at the Ngatamariki and Rotokawa geothermal fields, Taupo Volcanic Zone (TVZ), New Zealand. These rocks form part of the Tahorakuri Formation, which includes the oldest silicic deposits in the TVZ and therefore provides valuable insights into early TVZ history. The age spectra are used to estimate eruption ages, with the oldest two estimated at 1.89 ¡Ó 0.02 Ma (95% confidence limits) and 1.88 ¡Ó 0.03 Ma; 200-300 ka older than the oldest exposed silicic deposits from the TVZ. Samples at 0.915 ¡Ó 0.055 Ma and 0.873 ¡Ó 0.031 Ma most likely correlate to the Akatarewa ignimbrite, previously recorded at nearby geothermal fields. The youngest units at 0.806 ¡Ó 0.032 Ma and 0.701 ¡Ó 0.039 Ma, and were followed by a long period with no large-scale explosive volcanism until the Whakamaru-group ignimbrites at ƒî350 ka. A large buried andesite volcano at Rotokawa is >1.89 Ma in age, indicating that andesitic volcanism of this age was not restricted to the western margin of the TVZ as has previously been suggested. Rifting within TVZ appears to have begun prior to ƒî1.9 Ma, as shown by significant basin development that occurred prior to the emplacement of the oldest silicic deposits. These results indicate that silicic volcanism within the TVZ may have begun earlier than previously thought.