85 Ma volcanism on the Chatham Islands – A window into the geodynamic and geochemical evolution of Zealandia
Final Report Abstract
The composition of Zealandian intraplate volcanism evolved from the St. Helena HIMU- type (high time-integrated μ=238U/204Pb) in the Cretaceous to HIMU-like volcanism with lower 207Pb/204Pb and variable 208Pb/204Pb ratios at a given 206 Pb/204Pb ratio in the Cenozoic. This temporal evolution has been established using rocks from many different localities across Zealandia, but only the Chatham Islands provide a record of this transition at a given locality. New mineral and whole rock (major and trace element and Sr-Nd-Pb-Hf isotope) geochemistry and Ar-Ar ages from the Chatham Islands were used to characterize the geochemical fingerprint, the involved mantle sources/lithologies, and temporal evolution over 85 Ma to provide a better understanding of Zealandian intraplate volcanism. The volcanic activity on the Chatham Islands and Eastern Chatham Rise seems to be nearly continuously since ~85 Ma. The first and main magmatic stage (~85-75 Ma) has an isotopic signature nearly identical to the St. Helena HIMU-type endmember, but derive rather from a pyroxenitic instead of a peridotitic dominated source lithology. The following volcanic phase (~70-60 Ma) has a distinct isotopic composition closer to that of the Hikurangi Plateau composition. The Cenozoic HIMU- like volcanic rocks (< 60 Ma) extend from this ‘Cretaceous array’ to compositions with higher 206Pb/204Pb and 208Pb/204Pb ratios at given 207 Pb/204Pb ratio. The ‘Cenozoic array’ reflect radiogenic in-growth in a HIMU-like source with high U/Pb and Th/U ratios, which is in accordance with a metasomatized lithospheric mantle source. In general, this temporal geochemical evolution supports a widespread Late Cretaceous HIMU melting event in Zealandia (i.e. mantle plume), which metasomatized the depleted lithospheric mantle and forms the Cenozoic melting source. The change from Late Cretaceous HIMU to Cenozoic HIMU-like volcanism reflects, therefore, the change from an asthenospheric to a lithospheric melting sources. The nearly continuous volcanism during the Cenozoic in the Chatham´s and 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 seems to be induced by the change in lithospheric thickness caused by the partly subducted Hikurangi Plateau that lies directly north of the Chatham´s.
Publications
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The Chatham Islands: A type locality for Zealandian HIMU-like intraplate volcanism [Poster] AGU 2023 in San Francisco
Homrighausen S., Hoernle K., Hauff F., Schenk J. & Campbell H.
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The Chatham Islands: A window into the geochemical evolution of Zealandia [Talk] GeoBerlin 2023 in Berlin
Homrighausen S., Hoernle K., Hauff F., Schenk J. & Campbell H.
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The Chatham Islands: Geochemical evolution of the Late Cretaceous intraplate volcanism on Zealandia [Poster] Oceanic volcanism workshop 2023 in Kiel
Schenk J., Homrighausen S., Campbell H. & Hoernle K.
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The geochemical evolution of the Late Cretaceous intraplate volcanism on the Chatham Islands (NZ) [Poster] GeoBerlin 2023 in Berlin
Schenk J., Homrighausen S., Campbell H. & Hoernle K.
