U–Pb dating of zircon in subsurface, hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift: Taupo Volcanic Zone, New Zealand

Abstract

Recognising and correlating hydrothermally altered rock units within buried volcanic sequences in the Taupo Volcanic Zone (TVZ) in New Zealand is difficult. This is because of broad similarities in the lithologies of many major ignimbrite units, and the destruction by hydrothermal alteration of distinctive chemical and mineralogical characteristics. However, magmatic zircons are commonly present, are highly resistant to hydrothermal alteration and yield crystallisation ages in intensely altered rocks. Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons extracted from cores from altered ignimbrites penetrated by drillholes at the Waiotapu, Te Kopia and Orakei Korako geothermal fields in the central TVZ. At Waiotapu, the thick (up to 350 m) densely welded Waiotapu Ignimbrite returned a zircon age spectrum with a probability density function (pdf) peak of 0.79 Ma, consistent with an eruption age (from 40Ar/ 39Ar techniques) of 0.71 ± 0.06 (1 s.d.) Ma. Three older ignimbrite sheets yielded age spectra that were consistent stratigraphically. The shallowest of the three yielded sparse zircons that gave a pdf peak of 1.24 Ma and it may correlate with the 1.18 ± 0.02 Ma Ahuroa ignimbrite. The middle sheet, although 220 m thick, yielded an age spectrum identical to that obtained from pumice in the widespread 1.21 ± 0.04 Ma Ongatiti ignimbrite, extending earlier estimates of the likely volume of this large deposit. The deepest sheet has a spectrum consistent with an eruption age of 1.45 ± 0.05 Ma; it has no surficial correlative, but its likely coeruptive ash forms part of a concentrated group of primary or secondary tephra in sediments on the ocean floor east of New Zealand and in sedimentary basins across the North Island. These three ignimbrites were previously correlated with either major ignimbrites exposed on the Paeroa Fault scarp, 10 km to the west, or the Akatarewa Ignimbrite that occurs in drillholes at Te Kopia and Orakei Korako, but these correlations are disproved from our age data. Drillholes at Te Kopia (TK) and Orakei Korako (OK), reach, but do not penetrate the base of, one or more deposits collectively termed the Akatarewa Ignimbrite. Samples from 62 (TK) and 165 m (OK) below the top contact of this unit yielded closely similar age spectra and pdf maxima of 1.00 (TK) and 1.02 Ma (OK), implying that the same deposit was penetrated at both fields. This ignimbrite is not correlated with a specific surface equivalent; several candidate deposits exist, but all are areally restricted. The Akatarewa Ignimbrite at TK and OK is, however, not the same deposit as those earlier correlated with it at Waiotapu. The age data are consistent with there being two zones of subsidence on both sides of a horst aligned with the three geothermal fields sampled. The eastern zone is the Reporoa Basin, which has high heat flow but little surface faulting, and the western one is the axis of the Taupo Rift, currently with low heat flow but abundant surface faulting. Overall subsidence rates at the three geothermal fields studied vary between 2 and < 0.2 mm/yr and imply that subsidence is uneven in time and with position along and across the strike of the TVZ. Such complex patterns will complicate distinctions between background regional subsidence and more localised effects due to exploitation in the TVZ.