Meta-igneous granulite xenoliths from Mount Ruapehu, New Zealand: Fragments of altered oceanic crust?

Abstract

Meta-igneous granulite (MIG) xenoliths in lavas from Mount Ruapehu, Taupo Volcanic Zone, New Zealand, have variable but relatively high δ18O (+8.2 to +11.7) and 87Sr/86Sr (0.70506 to 0.70872), and eNd ranging from +1.5 to +6.2. They show a strong positive correlation between 87Sr/86Sr and δ18O, both of which are also broadly correlated with Mg number, but lack any correlation between 87Sr/86Sr and eNd. The xenoliths have been mineralogically re-equilibrated at lower-crustal temperatures (800–930°C) and pressures (7–10 kbar). Geochemical and isotopic evidence suggests they are fragments of oceanic crust that have been altered previously in an ocean floor hydrothermal system. Alternatively, they may be igneous rocks of unknown origin hydrothermally altered in the lower crust. Irrespective of this uncertainty, the xenoliths provide rare samples of the lower crustal basement beneath Taupo Volcanic Zone and represent a potential source rock for the voluminous rhyolites and ignimbrites that dominate the zone.