Geology of the Tāheke Geothermal Field: insights from U-Pb dating

Situated northwest of Lake Rotoiti in the Rotorua Lakes District, the Tāheke Geothermal Field offers insights into the evolution of volcanism in the northwest Taupō Volcanic Zone (TVZ) and thereby the transition of the volcanic arc into the current configuration. Over 2800 m of drillhole cuttings and ~2000 m of drill cores were sampled in 32 places for lithological analysis, aided by petrographic slides and images from the exploration drilling. From these materials, zircons from 15 samples (14 core, 1 cuttings) were dated by Secondary Ion Mass Spectrometry (SHRIMP) to yield crystallisation age estimates, close to (<100 kyr) but older than eruption ages. From the drillcore, a combination of the age data and petrographic/lithologic analysis served to identify 21 separate units (labelled A-U from oldest to youngest) that were grouped into two suites above and below a locally dominant ~400 m-thick ignimbrite (Unit L). The older suite (units A-K) contains several local units plus other deposits reported at the surface from the Tauranga Volcanic Centre (TgaVC) and from buried deposits at the Kawerau Geothermal Field and inferred to be erupted from the former. Pyroclastic deposits at Tāheke correlated with units from the TgaVC include the Lower Pāpāmoa Formation and the Aongatete Formation, the latter of which at Tāheke rests directly on greywacke basement at -875 m relative to sea level (mRL). A mean crystallisation age of 3.87 ± 0.10 Ma (2 s.d.) from the Aongatete Formation just above greywacke highlights the exposure of the metasedimentary basement at the surface at Tāheke before this time. The following 300 m of older suite deposits yielded mean crystallisation ages from 2.40 ± 0.04 Ma to 2.13 ± 0.08 Ma, which necessitates an average subsidence rate of ~3 mm/yr for this period. Several andesite cones are intercalated in the older suite, and from bracketing ages these are broadly contemporaneous with andesite activity throughout the central North Island during this period, including the Otawa dome, Karioi and Pirongia cones, and the buried Rotokawa andesite. A hiatus of 0.5 Myr separates the older suite from the thick Unit L ignimbrite above (crystallisation age of 1.53 ± 0.03 Ma) that is correlated with the lower ignimbrite of the Te Teko Formation in the Kawerau Geothermal Field. A large fault offsetting consecutive units across the older suite between well T8C-01 and other wells is inferred to have formed during the Unit L eruption as a caldera- related fault. The reasoning for this is two-fold: first, overthickening of this ignimbrite is characteristic of caldera infill deposits and second, the fault appears not to extend into units above and would accommodate the ~400 m thickness seen here. The younger suite includes two 1 m thin pyroclastic deposits (units M, N) with ages and lithologies that imply correlations with the Whakamaru Group and Paeroa Subgroup pyroclastics, respectively. Other deposits identified include those of the Ōkataina Volcanic Centre (Ōnuku Pyroclastic Subgroup) Rotorua Volcanic Centre (Māmaku Plateau Formation ignimbrite) and the Kapenga Volcanic Centre (Chimpanzee and Pokai ignimbrites). The succession of volcanic deposits from the Tāheke Geothermal Field may represent buried volcanic centres that further illuminate the eastward migration arc volcanism on the central North Island, New Zealand.