High- to ultrahigh-temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand

Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high-temperature (HT) to ultrahigh-temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c.110-105Ma. P-T calculations for garnet-orthopyroxene-bearing felsic granulite xenoliths indicate equilibration at similar to 815 to 910 degrees C and 0.7 to 0.8GPa, with garnet-bearing mafic granulite xenoliths yielding at least 900 degrees C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re-integration of exsolution in feldspar (similar to 900-950 degrees C at 0.8GPa), Ti-in-zircon thermometry on Y-depleted overgrowths on detrital zircon grains (932 degrees C +/- 24 degrees C at aTiO(2)=0.8 +/- 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (850 degrees C at 0.7 to 0.8GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U-Pb pooled age of 91.7 +/- 2.0Ma. The cause of Late Cretaceous HT-UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore-running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT-UHT conditions in the lower crust with little or no sign of this event at the surface.