van der Meer, Quinten3; Waight, Tod Earle3; Scott, James2
1 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet2 Department of Geology, Otago University, Dunedin3 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet
The New Zealand microcontinent (Zealandia) formed as the active eastern margin of Gondwana. Upon cessation of subduction at ~110 Ma, extension led to opening of the Tasman Sea at 82 Ma, preceded by the formation of metamorphic core complexes, the opening and filling of halfgraben structures and the intrusion of mafic dikes (~88 to 68 Ma). Subsequently, Zealandia has been punctuated by volumetrically minor, intermittent yet widespread intraplate magmatism from ~100 Ma through to recent times. This magmatism has typical OIB-like trace element abundances and radiogenic isotope compositions that trend towards a HIMU (high time integrated U/Pb) end member mantle composition. Recent publications have argued that that the intraplate OIB-like magmatism is not related to a mantle plume but is rather formed by delamination of mantle lithosphere, replacement by asthenosphere and associated partial melting. The variably diluted HIMU signature is interpreted to be the result of mixing between depleted mantle bearing a HIMU component with an Enriched Mantle or continental crust component. New geochemical and isotopic analyses suggest the dike swarms also have an OIB-like chemistry. Initial Pb isotopic compositions of the dikes however are more similar to Pacific MORB. It is possible that the HIMU-like component in Zealandia is a result of trace element enrichment (with high U/Pb and Th/Pb) of a depleted mantle melt region in the Cretaceous. This source has remained stable beneath Zealandia while Pb ingrowth proceeded rapidly in U-Th rich domains. Repeated melt extraction from this source resulted in the intraplate magmatism with HIMU-like Pb isotopes. Isotopic data from peridotite xenoliths in the Cenozoic intraplate volcanoes reveal that the spinel facies lithospheric mantle beneath Zealandia also has high 206Pb/204Pb ratios. Lithospheric peridotite mantle is a potential source component for the intraplate basalts, which require an additional garnet rich source component.