The petrology, geochronology and geochemistry of Hauhungatahi volcano, SW Taupo Volcanic Zone

Hauhungatahi volcano is an eroded andesitic edifice 10 km west of Ruapehu volcano constructed on an upfaulted block of Mesozoic marine sediments. Hauhungatahi andesites are distinctively clinopyroxenephyric with high ratios of clinopyroxene:plagioclase. This contrasts with the plagioclase-phyric dominated assemblages in the stratovolcanoes such as Ruapehu and Tongariro. Hauhungatahi andesites show high MgO (>8.0% wt), Sr (>400 ppm), Ni (>100 ppm) and Cr (>400 ppm) and low Si, Rb, Ba and Zr relative to andesites from Ruapehu volcano, with fractionated LREE (Ce/Sm)(n)similar to 2 and flat HREE (Dy/Yb)(n)similar to 1. Sr-isotope ratios are lower at equivalent Nd isotope ratios than Ruapehu andesites of the (oldest) Te Herenga Formation. 40Ar/39Ar step-heating experiments of groundmass concentrates of 4 samples have yielded ages between 88 1 +/- 83 ka and 961 +/- 59 ka, with a weighted mean age of 933 +/- 46 ka, indicating that Hauhungatahi is significantly older than Ruapehu Volcano where volcanism commenced similar to 250-300 ka. Hauhungatahi is therefore similar in age to the older andesitic edifices that are marginal to the TVZ (eg Titiraupenga, Pureora and Rolles Peak). We suggest that these high-Mg andesites hold clues to the early conditioning of the lithosphere beneath TVZ, prior to the establishment of the large andesite stratovolcanoes that presently dominate the skyline at the southern end of TVZ. The relatively low abundance of plagioclase in the phenocryst assemblages hints at high PH2O and possible involvement of amphibole in the source. (C) 2009 Elsevier B.V. All rights reserved. Hauhungatahi volcano is an eroded andesitic edifice 10 km west of Ruapehu volcano constructed on an upfaulted block of Mesozoic marine sediments. Hauhungatahi andesites are distinctively clinopyroxenephyric with high ratios of clinopyroxene:plagioclase. This contrasts with the plagioclase-phyric dominated assemblages in the stratovolcanoes such as Ruapehu and Tongariro. Hauhungatahi andesites show high MgO (>8.0% wt), Sr (>400 ppm), Ni (>100 ppm) and Cr (>400 ppm) and low Si, Rb, Ba and Zr relative to andesites from Ruapehu volcano, with fractionated LREE (Ce/Sm)(n)similar to 2 and flat HREE (Dy/Yb)(n)similar to 1. Sr-isotope ratios are lower at equivalent Nd isotope ratios than Ruapehu andesites of the (oldest) Te Herenga Formation. 40Ar/39Ar step-heating experiments of groundmass concentrates of 4 samples have yielded ages between 88 1 +/- 83 ka and 961 +/- 59 ka, with a weighted mean age of 933 +/- 46 ka, indicating that Hauhungatahi is significantly older than Ruapehu Volcano where volcanism commenced similar to 250-300 ka. Hauhungatahi is therefore similar in age to the older andesitic edifices that are marginal to the TVZ (eg Titiraupenga, Pureora and Rolles Peak). We suggest that these high-Mg andesites hold clues to the early conditioning of the lithosphere beneath TVZ, prior to the establishment of the large andesite stratovolcanoes that presently dominate the skyline at the southern end of TVZ. The relatively low abundance of plagioclase in the phenocryst assemblages hints at high PH2O and possible involvement of amphibole in the source. (C) 2009 Elsevier B.V. All rights reserved.