The Pounamu terrane, a new Cretaceous exotic terrane within the Alpine Schist, New Zealand; tectonically emplaced, deformed and metamorphosed during collision of the LIP Hikurangi Plateau with Zealandia

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

“…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 AE 2 Ma using zircon from granulite xenoliths contained within the Kakanui Mineral Breccia (Jacob, Scott, Turnbull, Tarling, & Sagar, 2017). 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 (Scott et al, 2015) (Cooper and Ireland, 2013) (Vry et al, 2004) (Mortimer and Cooper, 2004) Thrust faulting A-type magmatism Lu-Hf Garnet, this study (Mortimer and Cooper, 2004;Cooper and Ireland, 2015) Zircon Monazite (Vry et al, 2004)…”

“…The Lu–Hf garnet ages reported here are consistently older than existing zircon rim and monazite ages from the PUB and the Mataketake Range regions (Figure ; Cooper & Ireland, , ; Mortimer & Cooper, ). A clear link between these zircon and monazite age data and the metamorphic conditions which they record has not been established, therefore the significance of these dates with respect to garnet growth remains unclear.…”

“…A cluster of Lu–Hf and Rb–Sr ages at 87–86 Ma (Vry et al., ) suggests that metamorphism persisted until the late Cretaceous. However, younger dates of 72–64 Ma from zircon rims (Cooper & Ireland, , ) and 83–71 Ma from monazite (Mortimer & Cooper, ; Scott et al., ) have also been reported; these are not yet reliably linked to any metamorphic stage, and their significance is yet unclear.…”

“…In addition, both Vry et al (2004) and Cooper and Ireland (2015) suggested that a potentially prolonged late Cretaceous convergent tectonic regime in the Alpine Schist region of the Pacific-Gondwana accretionary complex was caused by collision and partial subduction of the Hikurangi Plateau large igneous province (Figure 1; Reyners, Eberhart-Phillips, & Bannister, 2011). The exact timing of Hikurangi Plateau collision and duration of partial subduction is unclear, but must be younger than c. 122 Ma when the plateau was formed (Neal, Mahoney, Kroenke, Duncan, & Petterson, 1997).…”

“…Existing ages for the growth of prograde major and accessory metamorphic minerals in Alpine Schist have been obtained using a variety of methods and mineral chronometers (Sm–Nd and Lu–Hf garnet dating, U–Pb dating of monazite and zircon). The data cover a large range from 100 to 70 Ma for which interpretations differ (Cooper & Ireland, , ; Mortimer & Cooper, ; Scott et al., ; Vry et al., ). The continental arc regions of this subduction margin record extensional orogenic collapse from c .…”