Wrangellia flood basalts in Alaska: A record of plume‐lithosphere interaction in a Late Triassic accreted oceanic plateau

Abstract

The Wrangellia flood basalts are part of one of the best exposed accreted oceanic plateaus on Earth. They provide important constraints on the construction of these vast submarine edifices and the source and temporal evolution of magmas for a plume head impinging beneath oceanic lithosphere. Wrangellia flood basalts (∼231–225 Ma) extend ∼450 km across southern Alaska (Wrangell Mountains and Alaska Range) where ∼3.5 km of mostly subaerial flows are bounded by late Paleozoic arc volcanics and Late Triassic limestone. The vast majority of the flood basalts are light rare earth element (LREE) ‐enriched high‐Ti basalt (1.6–2.4 wt % TiO2) with uniform ocean island basalt (OIB) ‐type Pacific mantle isotopic compositions (ɛHf(t) = +9.7 to +10.7; ɛNd(t) = +6.0 to +8.1; t = 230 Ma). However, the lowest ∼400 m of stratigraphy in the Alaska Range is LREE‐depleted low‐Ti basalt (0.4–1.2 wt % TiO2) with pronounced negative high field strength element (HFSE) anomalies and Hf isotopic compositions (ɛHf(t) = +13.7 to +18.4) that are decoupled from Nd (ɛNd(t) = +4.6 to +5.4) and displaced well above the OIB mantle array (ΔɛHf = +4 to +8). The radiogenic Hf of the low‐Ti basalts indicates involvement of a component that evolved with high Lu/Hf over time but not with a correspondingly high Sm/Nd. The radiogenic Hf and HFSE‐depleted signature of the low‐Ti basalts suggest pre‐existing arc lithosphere was involved in the formation of flood basalts that erupted early in construction of part of the Wrangellia plateau in Alaska. Thermal and mechanical erosion of the base of the lithosphere by the impinging plume head may have led to melting of arc lithosphere or interaction of plume‐derived melts and subduction‐modified mantle. The high‐Ti lavas dominate the main phase of construction of the plateau and were derived from a depleted mantle source distinct from the source of MORB and with compositional similarities to that of ocean islands (e.g., Hawaii) and plateaus (e.g., Ontong Java) in the Pacific Ocean.