A high resolution 40Ar/39Ar lava chronology and edifice construction history for Tongariro volcano, New Zealand

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Detailed mapping and geochronological investigations of edifice-forming materials reconstruct the growth history of Tongariro volcano, New Zealand, and subdivide the edifice into thirty six distinct units which are organised into twelve formations and constituent members. Twenty nine new 40Ar/39Ar age determinations, along with published K/Ar ages combined with volume estimates, petrographic observations and rock chemistry provide an integrated history of the volcano's growth through edifice-forming lavas and pyroclastic deposits. The oldest lava (512 ± 59 ka, 2 s.d.) is a small inlier of basaltic-andesite on Tongariro's NW sector that may reflect a nearly buried independent volcano. The next oldest material that can be confidently attributed to a Tongariro source is 304 ± 11 ka andesite, incorporated as boulders in late Pleistocene ejecta from the Tama Lakes area. In-situ lavas at Tongariro date from 230 ka to present, including numerous flows erupted during glacial periods and building the edifice unevenly due to emplacement against valley-filling ice bodies. Tongariro has a total edifice volume of ~90 km3, 19 km3 of which is represented by exposed map units, with glacial deposits amounting to <1 km3. The ring plain volume immediately adjacent to the volcano contains ~60 km3 of material.Sequential eruptive records, from 230 ka to present, reveal an irregular cyclicity in MgO concentrations over ~10–70 kyr intervals. During these cycles, rapid (≤10 kyr) increases in MgO concentrations to ≥5–9 wt% are inferred to reflect episodes of enhanced mafic magma replenishment, with maxima at ~230, ~160, ~117, ~88, ~56, ~35, ~17.5 ka and during the Holocene, which are each followed by gradual declines to ~2–5 wt%. Field evidence, including extensive moraines and U-shaped valleys, and lava textures, implies repeated occupation of valleys on Tongariro by major glaciers and possibly ice caps. During periods of major ice coverage, which generally correlate with global cold climate/glacial Marine Isotope Stages, edifice-building rates on Tongariro were only 17–26% of those during warmer climatic periods. Because the changes in edifice-building rates do not coincide with changes in the magmatic system, these contrasts are inferred to reflect a preservation bias whereby materials erupted onto ice were contemporaneously (or subsequently, as ice masses melted) conveyed to the ring plain as debris rather than building the edifice. Although the Tongariro edifice is smaller than that of neighbouring Ruapehu (~150 km3), the exposed edifice materials on Tongariro record a longer and more complex growth history. The wider geographic distribution of <50 ka vent locations at Tongariro reflects greater rifting rates than at Ruapehu.