ALBERT

Beschreibung: Highlights • Common HIMU end member in adjacent continental and oceanic volcanic provinces. • End member St. Helena HIMU derived from deep upwelling(s)/plume(s). • Plateau collision & plume interaction with Gondwana active margin causes breakup. • Hybrid volcanic-tectonic margins resulted from Zealandia – Antarctica breakup. Abstract Margins resulting from continental breakup are generally classified as volcanic (related to flood basalt volcanism from a starting plume head) or non-volcanic (caused by tectonic processes), but many margins (breakups) may actually be hybrids caused by a combination of volcanic and tectonic processes. It has been postulated that the collision of the Hikurangi Plateau with the Gondwana margin ∼110 Ma ago caused subduction to cease, followed by large-scale extension and ultimately breakoff of the Zealandia micro-continent from West Antarctica through seafloor spreading which started at ∼85 Ma. Here we report new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data for Late Cretaceous (99-69 Ma) volcanism from Zealandia, which include the calc-alkalic, subduction-related Mount Somers (99-96 Ma) and four intraplate igneous provinces: 1) Hikurangi Seamount Province (99-88 Ma), 2) Marlborough Igneous Province (98-94 Ma), 3) Westland Igneous Province (92-69 Ma), and 4) Eastern Chatham Igneous Province (86-79 Ma). Each of the intraplate provinces forms mixing arrays on incompatible-element and isotope ratio plots between HIMU (requiring long-term high U/204Pb) and either a depleted (MORB-source) upper mantle (DM) component or enriched continental (EM) type component (located in the crust and/or upper mantle) or a mixture of both. St. Helena end member HIMU could be the common component in all four provinces. Considering the uniformity in composition of the HIMU end member despite the type of lithosphere (continental, oceanic, oceanic plateau) beneath the igneous provinces, we attribute this component to a sublithospheric source, located beneath all volcanic provinces, and thus most likely a mantle plume. We propose that the plume material rose beneath the active Gondwana margin and flowed along the subducting lithosphere beneath the Hikurangi Plateau and neighboring seafloor and through slab tears/windows beneath the Gondwana (later to become Zealandia) continental lithosphere. We conclude that both plateau collision, resulting in subduction cessation, and the opening of slab tears/windows, allowing hot asthenosphere and/or plume material to upwell to shallow depths, were important in causing the breakup of Zealandia from West Antarctica. Combined tectonic-volcanic processes are also likely to be responsible for causing breakup and the formation of other hybrid type margins.