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Post-caldera recovery of a peralkaline magmatic system: Tūhua, New Zealand

Pip F Tildesley 1, Colin J N Wilson1, Bruce L A Charlier1, Graham S Leonard2, Jacqueline Grech Licari1, Simon J Barker1, Steve D Weaver3, Bruce F Houghton4, Andy Calvert5

  • Affiliations:  1School of Geography, Environment and Earth Sciences, Victoria University of Wellington, New Zealand; 2GNS Science, Lower Hutt, New Zealand; 3School of Earth and Environment, University of Canterbury, Christchurch, New Zealand; 4SOEST, University of Hawaiʻi at Mānoa; 5US Geological Survey, Moffett Field, CA USA 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 165

  • Programme No: 3.4.39

  • Theme 3 > Session 4

Abstract

Tūhua (Mayor Island) is an isolated peralkaline rhyolite volcano situated ~80 km behind the active calc-alkaline volcanism of the Taupo Volcanic Zone, New Zealand. The largest, most recent caldera-forming eruption occurred ~7.2 ka (the 'Tūhua eruption') with tephra reaching up to 300 km from source. This eruption was followed by overwhelmingly effusive activity generating a ~0.4 km3 composite pile of intra-caldera lava lobes that bury ~80 % of the caldera floor. Current interest focuses on recent, post-caldera activity and the potential for future unrest or eruption. Here we consider the eruptive record of post-caldera activity to assess the young evolution of the magmatic system. Tūhua eruption products show wide geochemical variations (Zr = 1010-1369 ppm) and are relatively crystal poor (<5 %). In contrast, the younger lava lobes have a narrower compositional range, with elevated Zr (1454-1693 ppm) and higher crystal contents (25-35 %). In addition, there is a post-7.2 ka fall deposit found on the caldera floor that is missing from the lava domes. Pumice in this deposit is crystal poor (like 7.2 ka pumices) but has elevated Zr values (1665-1698 ppm) like the young lavas. This explosive phase may correlate with a ~3-4 ka tephra in a nearby marine core, while the youngest lava activity is constrained by a 2.2 ka charcoal spike in caldera floor peat deposits. The absence of significant geothermal activity and the high crystal content of the latest eruption products suggests that the magmatic system is currently moribund.