Seamount volcanism on large igneous provinces: New constraints from 40Ar/39Ar geochronology and geochemistry of the Tūranganui Knoll, Hikurangi Plateau (SW Pacific)

Seamount volcanism on oceanic large igneous provinces (LIPs) remains poorly understood. The early Cretaceous Hikurangi Plateau hosts many early to mid-Cretaceous seamounts, which provides a unique example to study the relation between oceanic plateau and seamount volcanism. New 40 Ar/ 39 Ar data from the Tūranganui Knoll lavas, drilled during IODP Expedition 375, gave an age of ~107 Ma, implying that Hikurangi seamount volcanism began ~8 Myr earlier than previously known and overlapping with basement volcanism. The Tūranganui Knoll is the oldest of a west-to-east younging chain of 107 to 89 Myr old volcanoes. This west-east younging trend is accompanied by a gradual shift from a HIMU (= high time integrated μ = high U/Pb) to an enriched mantle (EM) isotopic endmember composition. Together with elevated Zr/Hf and (Sm/Yb) N these collectively indicate the involvement of HIMU-type pyroxenitic or eclogitic recycled oceanic crust within an peridotitic EMI-type mantle plume. In addition, higher 207 Pb/ 204 Pb at intermediate 206 Pb/ 204 Pb in younger Hikurangi volcanic rocks furthermore requires a subduction-modified mantle component entrained after slab breakoff following Hikurangi Plateau - Chatham Rise collision. We infer that seamount construction was likely facilitated by lithospheric extension along pre-existing weak-zones reactivated during collision of the Hikurangi plateau with the Gondwana margin. Integrating our new results with literature data, we propose a new emplacement model to explain how LIP-related seamounts are generated with broader implications for seamount formation. • Large igneous province basement and seamount volcanism form a continuous process. • Tūranganui Knoll samples represent pyroxenitic HIMU-type plume endmember. • Chatham Rise–Hikurangi Plateau collision and rotation generates Hikurangi seamounts.