U-Pb Dating of Silicic Volcanic Rocks of the Eastern Coromandel Peninsula

Abstract

The eastern Coromandel Volcanic Zone (CVZ) is an area of predominantly andesite-dacite-rhyolite volcanism which was erupted and subsequently hydrothermally altered in parts during the Late Miocene to Early Pleistocene. Many of the andesitic host rocks and products of hydrothermal alteration have been dated, but there are few ages on the rhyolites. This study was undertaken to help better constrain the age of selected rhyolite lavas and ignimbrites of the onshore eastern CVZ. 21 units were sampled for U-Pb dating of zircon using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This method was chosen because most CVZ rhyolites are known to contain zircon. In this study, 18 of the 21 samples contained sufficient zircons for dating, and 18 new zircon U-Pb ages are presented. These ages are the age of crystallisation and therefore represent the maximum eruption age. This method has not previously been used in the CVZ, so was also chosen to assess if it is an appropriate method for dating CVZ rhyolites. This was determined by comparing the new ages generated in this study with any previous ages. They mostly agree, which supports the validity of the method, and also provides new age data of several units which had not previously been dated.

In order to set the new ages into a geological context, it was necessary to describe the units sampled. The petrography, mineralogy and petrochemistry were assessed by petrographic microscope, electron microprobe and whole rock X-ray diffraction and fluorescence. 21 new whole rock major and trace element XRF analyses are presented.