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

Abstract

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. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO2 = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. 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.

Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist, New Zealand

“…If this region of widespread extension encompassed the Alpine Schist, it is possible that associated rise of the hot upper mantle underneath the accretionary complex could have driven diachronous garnet growth in the Alpine Schist. This explanation was proposed by Jacob et al (2017) for the HT to UHT metamorphism observed in lower crustal xenoliths from the base of the accretionary complex in the Otago Schist region.…”

“…The Lu–Hf garnet ages from WC14‐01 (97.3 ± 0.3 Ma) and WC14‐38 (91.8 ± 0.3 Ma) overlap with the Sm–Nd age of 100 ± 12 Ma from a nearby garnet amphibolite at Haast River (Mortimer & Cooper, ). Additionally, the WC14‐38 garnet age is consistent with the timing of high‐temperature (HT) to ultrahigh‐temperature (UHT) metamorphism of the lower crust beneath the Otago Schist, dated at 91.7 ± 2 Ma using zircon from granulite xenoliths contained within the Kakanui Mineral Breccia (Jacob, Scott, Turnbull, Tarling, & Sagar, ). Although these xenoliths are located 200 km from the location of WC14‐38, it is possible that high‐grade metamorphism in these regions was driven by the same regional heating event, such as conductive heating of the lower crust by hot upper mantle, as suggested by Jacob et al.…”

“…Although these xenoliths are located 200 km from the location of WC14‐38, it is possible that high‐grade metamorphism in these regions was driven by the same regional heating event, such as conductive heating of the lower crust by hot upper mantle, as suggested by Jacob et al. (). The garnet age from WC14‐31 (79.9 ± 2.3 Ma) overlaps with an 83 ± 1 Ma U–Pb monazite date from a nearby mylonitized schist at Fox Glacier (Figure ; Scott et al., ).…”

“…This explanation was proposed by Jacob et al. () for the HT to UHT metamorphism observed in lower crustal xenoliths from the base of the accretionary complex in the Otago Schist region.…”

Continent stabilisation by lateral accretion of subduction zone-processed depleted mantle residues; insights from Zealandia

“…2) (Mortimer, 2004). Unlike formerly contiguous Antarctica and Australia, neither surficial nor deeply exhumed crust (e.g., Beyssac et al 2015;Jacob et al, 2017) or zircon U-Pb-Hf isotope systematics in igneous rocks (e.g., Bolhar et al, 2008) provide evidence that Zealandia has an Archean or even Proterozoic crustal core.…”