Stratigraphy and structure of the Ngatamariki geothermal system from new zircon U-Pb geochronology: Implications for Taupo Volcanic Zone evolution

Abstract

Recent drilling at the Ngatamariki Geothermal Field, Taupo Volcanic Zone, New Zealand has provided new constraints on the stratigraphy and volcanic evolution of the region. Over 2800m thickness of volcanic products are present at Ngatamariki, mainly comprised of rhyolitic ignimbrites linked to large caldera-forming events at sources outside the field area, but locally sourced andesite and rhyolite lavas and domes are also encountered. Most of the rocks are allocated to the pre-0.35Ma Tahorakuri Formation. Crystallisation age spectra (and consequent best estimates of eruption age) have been obtained by U-Pb dating on zircons from otherwise severely hydrothermally altered magmatic rocks by Secondary Ion Mass Spectrometry techniques using a SHRIMP-RG instrument. The oldest rock dated is an ignimbrite, which yields an eruption age estimate of 1.85±0.06Ma. This ignimbrite, plus comparable-aged units dated at the adjacent Rotokawa and Ohaaki geothermal fields, are interpreted to represent the oldest silicic volcanic deposits in the area, and onlap the basal andesite lava pile that is best developed at Rotokawa to the south. Other pyroclastic units and associated volcaniclastic sediments (with another intercalated andesite lava unit) return age estimates between 1.85±0.06 and 0.701±0.039Ma. Between ~0.7 and 0.35Ma, contemporaneous surface lithologies in the Ngatamariki area are dominated by sediments, with subordinate lava domes. Between 0.716±0.017 and 0.655±0.016Ma at least three shallow (to<2km depth) intrusions were emplaced under the northern part of the field: a diorite, microdiorite and a large tonalite body totalling >5km3. The intrusions generated a large alteration halo (~25km3 minimum) and intense silicification of the wall rocks. At 0.35 and 0.34Ma the area was buried by two ignimbrite packages of the Whakamaru Group, erupted from sources just west and well north, respectively, of the field. Ignimbrites of the Waiora Formation and several rhyolite lava domes were then emplaced over a period bracketed by domes dated at 0.282±0.011 and 0.257±0.011Ma, coeval with more extensive volcanic activity in the Maroa dome complex west of the field. Sediments of the Huka Falls Formation and deposits of the 25.4±0.2ka Oruanui eruption then cap and seal the system. The new U-Pb data coupled with detailed petrographical studies allow us to build the history of the area encompassed by the Ngatamariki Geothermal Field. The field, despite >2.8km of subsidence, does not lie in a caldera and is the only one known to date to have a plutonic intrusive complex of Quaternary age. Two chemically and temporally distinct hydrothermal events are located at the Ngatamariki field, with no evidence of continuity between the two.