Thermoluminescence Dating of Ignimbrites and Hydrothemally Altered Rocks in the Taupo Volcanic Zone

Abstract

The central Taupo Volcanic Zone (TVZ) of New Zealand is one of the most active areas of rhyolite volcanism and geothermal activity in the world. The absolute ages of many rhyolitic eruptives have been measured, but as yet the chronology of the geothermal systems is unknown. We report prelimhary data from a pilot study using the thermoluminescence (TL) method to date both fresh and hydrothermally altered ignimbrites in the TVZ. Six unaltered, welded ignimbrites from the ubiquitous whakamaru-group (4 from whakamaru Ignimbite and 2 from Te Whaiti Ignimbrite) range from 297 ka to 333 ka, in agreement with published age data. The TL age of 194 ka for fresh pumice tuff from the Ohakuri Ignimbrite is consistent with its stratigraphic position. Two hydrothermally altered Ohakuri Ignimbrite samples gave 68 ka and 113 ka ages, which, as would be expected, are younger than for the fresh rock. We established a method of analyzing quartz grain size to use in the evaluation of beta ray contribution in the rock, and cosmic ray contribution in annual dose evaluation process. This pilot study of TL dating in the TVZ has been successful, and has encouraged us to undertake a comprehensive age mapping programme of both fresh and hydrothermally altered rocks in the Taupo Volcanic Zone.