The geochemical evolution of the Late Cretaceous intraplate volcanism on the Chatham Islands (NZ)

The Chatham Islands, situated on the eastern Chatham Rise ~800 km east of New Zealand, comprise two main (Chatham and Pitt) and many smaller islands. Collision of the oceanic Hikurangi Plateau with the Chatham Rise jammed the long-lived subduction system, causing Zealandia to separate from Gondwana. The breakup was accompanied by volcanism on the South Island of New Zealand, Hikurangi Plateau (Hikurangi seamounts) and Chatham Rise and islands, many of which were formed by the volcanism. The Late Cretaceous volcanism is characterized by HIMU (high time-integrated μ=²³⁸U/²⁰⁴Pb Mantle) endmember composition, which sensu stricto is defined by oceanic islands associated to deep-rooted mantle plumes. Consequently, an heritage of the HIMU source from the lower mantle transferred to the surface by a mantle plume is debated, but numerous alternative models have been proposed to explain the HIMU volcanism in Zealandia, including sub-continental lithospheric mantle modified by subduction or interaction with the partly subducted Hikurangi plateau. Our major and trace element analyses of 52 Cretaceous whole-rock samples from the Chatham Islands display characteristic HIMU trace element compositions, but the major element (MgO vs. CaO) and olivine composition indicate a predominantly pyroxenitic (or phlogopitic) rather than peridotitic source, which is usually associated with HIMU lavas. Further investigation on the composition of primitive melts, melting conditions and source lithologies will be used to evaluate the potential role of a mantle plume causing HIMU end member volcanism and separation of Zealandia from Antarctica in the Late Cretaceous.