July 18, 2023

Team Profile:
It’s a wrap! The latest word on the New Zealand basement

Nick Mortimer

photo credit:

The Xenolith Hunters and Analysing Xenoliths posts explained what xenoliths were, why basement rocks are important to New Zealand deep geothermal investigations, and how the xenoliths were analysed. This, the last of three posts about the xenoliths, summarises the analytical results, conclusions and implications of this part of the GNG Programme.  

Cartoon-like crustal sections across the Taupō Volcanic Zone (TVZ) showing the two end-member options for the dip of the terrane contact. Orange areas are inferred plutonic rocks of TVZ.

At the start of this project, three years ago, there were two different hypotheses for the dip of contact between the old Waipapa and Torlesse crustal basement terranes, hidden under the young rocks of the Taupō Volcanic Zone. In one, the terrane boundary would dip vertically, and in the other it would dip gently (see diagram above). Knowing which is correct matters because terrane contacts are typically very faulted, fractured and permeable zones which are easily re-activated, and which may exert a strong influence on fluid flow and tectonic deformation. It would be important to know if potential supercritical geothermal target regions in the eastern TVZ would be in or near the terrane contact. 

Interpreting the analytical data from the xenoliths. Photo credit: Greer Gilmer.

The Taupō Volcanic Zone xenoliths are 1-20 cm sized pieces of rock that are now found in lava flows and ignimbrites, but which originally came from under the volcanoes. They allow us to directly sample and analyse usually inaccessible pieces of crust from under the volcanoes. Using geochemical and geochronological methods, the aim was to try and correlate xenoliths from different volcanoes with Torlesse Terrane, Waipapa Terrane or another geological unit. We are fortunate in New Zealand to have excellent reference collections and analytical datasets of basement rocks in GNS Science’s Petlab database. Of the 112 newly-collected xenoliths, 30 were analysed for concentrations of 51 elements and for isotope ratios of oxygen, strontium, neodymium and lead. Two samples contained grains of the mineral zircon and these were dated by U-Pb methods. Interpretation of the element and isotopic data was not easy because intense heat from the lava flows had shifted many elements out of their normal minerals, creating noise rather than signals.  

Schist sample from Lichfield quarry. The ages of its zircon grains match those of a typical surface Torlesse Terrane reference rock.   

The single most useful - and the largest - sample in our new xenolith dataset was an unassuming lumpy rock from Lichfield quarry near Tokoroa. This turned out to be a schist (see thin section image in previous blog post). The ages of the zircon grains in the P92622 sample matched those from the Torlesse Terrane which, at the surface, is found east of TVZ. All other geochemical measurements and isotope ratios of P92622 confirmed a correlation with Torlesse Terrane, rather than Waipapa Terrane: this sample had good signals and not much noise. 

Drawn-to-scale section across Taupō Volcanic Zone. The new xenolith data show that the gently-dipping terrane contact (first diagram) is correct. Potential supercritical geothermal sites under eastern TVZ will not be influenced by terrane contact structures. 

The P92622 Torlesse schist was erupted from Mangakino Caldera, on the west side of TVZ. The presence of Torlesse Terrane in the crust so far west forces the dip on the terrane boundary to be shallow (diagram above).  

In conclusion, we have been able to successfully advance the state of knowledge of TVZ crustal structure beyond what we knew three years ago. We can decide between the vertical and dipping terrane contact options. Two main results are visible in the last diagram:

  1. most of the hot, deep crust under Taupō Volcanic Zone – crust that is absorbed by ascending magmas – is probably floored by Torlesse Terrane, not Waipapa Terrane; and
  2. supercritical geothermal target zones in eastern TVZ will be hosted in Torlesse Terrane and will be unaffected by complications related to terrane contact faults and fractures. 

The scientific paper of the results was published in the Journal of Volcanology and Geothermal Research:

Nick Mortimer, Bruce LA Charlier, Shane M Rooyakkers, Rose E Turnbull, Colin JN Wilson, Marianne Negrini, Stephen Bannister, Sarah D Milicich, Isabelle Chambefort, Craig A Miller and Geoff Kilgour (2023) Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: context for hydrothermal and magmatic processes. Journal of Volcanology and Geothermal Research, 107855. 

We thank representatives of Ngāti Rangi, Ngāti Tūwharetoa, and the Department of Conservation for permission to carry out fieldwork on Mts Ruapehu and Tongariro. This work was funded by MBIE Endeavour grant C05X1904.

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