Evidence for the transition from Cretaceous asthenospheric to Cenozoic lithospheric melting sources on the Chatham Islands, New Zealand

Late Cretaceous–Cenozoic diffuse intraplate volcanism is widespread across the continent of Zealandia, but only in the Chatham Islands have intraplate volcanoes erupted repeatedly for over 80 million years. Here we use new whole-rock major and trace element, and Sr-Nd-Pb-Hf isotope geochemical data from the Chatham Islands to characterize changes in the geochemical fingerprint of the melting sources over time to provide a better understanding of Zealandian intraplate volcanism. The first and most voluminous magmatic stage (∼85–75 Ma; Southern Volcanics) at the Chatham Islands has an isotopic signature nearly identical to the St. Helena HIMU-type end member. The following Red Bluff Tuff volcanic stage (∼65–50 Ma) has a distinct isotopic composition, influenced by an enriched mantle one-type, probably derived from the neighboring Hikurangi Plateau. The following magmatic stages, the Northern Volcanics (∼40–30 Ma) and Rangitihi Volcanics (< 10 Ma) extend from this compositional array to compositions with higher ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios at given ²⁰⁷Pb/²⁰⁴Pb ratio. This ‘Cenozoic array’ reflects radiogenic in-growth with high U/Pb and Th/U ratios derived from a metasomatized lithospheric mantle source. The recorded temporal geochemical evolution confirms a widespread Late Cretaceous HIMU melting event in Zealandia, which metasomatized the lithospheric mantle forming the Cenozoic HIMU-like melting source. This also reflects a change from asthenospheric to lithospheric melting sources. The nearly continuous volcanism during the Cenozoic in the Chathams and wider eastern Chatham Rise could have been triggered by asthenospheric upwelling causing melting of the base of the metasomatized lithosphere. Considering the fast-northward motion of Zealandia during the Cenozoic (∼2500 km), the upwelling is most plausibly explained by the change in lithospheric thickness caused by the partly subducted Hikurangi Plateau that lies directly north of the Chatham Islands.