WRANGELLIA FLOOD BASALTS: EXPLORING THE ARCHITECTURE AND COMPOSITION OF AN ACCRETED OCEANIC PLATEAU

Triassic Wrangellia flood basalts in Alaska, British Columbia, and Yukon form one of the largest and best preserved accreted oceanic plateaus on Earth, and offer an exceptional opportunity to explore the architecture, timescale of formation, and source of basaltic magmas of oceanic plateaus. Wrangellia basalts are >6 km thick on Vancouver Island and 3-4.5 km thick in southern Alaska (Wrangell Mountains and Alaska Range). The plateau formed atop crust of an old convergent margin at ca. 232-225 Ma. Triassic chert, minor limestone, and Daonella-bearing shale lie at the base of the flood basalts and thick limestones, grading from tidal to deep-water sequences, overlie the flood basalts. Karmutsen basalts on Vancouver Island consist of basal sediment-sill complexes overlain by pillow lava, minor pillow breccia, and massive flows. Exposures in the Wrangell Mountains and Yukon are a different variant of predominantly massive flows with a thin zone of conglomerate, pillow breccia, and pillowed lava (<70 m thick) along the base. In the eastern Alaska Range, well-exposed sediment-sill complexes similar to Vancouver Island are overlain by thick pillowed lavas and flows. The Wrangellia flood basalts are primarily tholeiitic in composition (6-10 wt% MgO, 49-53 wt% SiO₂, 2-4 wt% Na₂O+K₂O, 1.5-2.5 wt% TiO₂), with a few samples extending to mildly alkalic compositions. An important discovery is a succession of picritic pillow lavas (13-19 wt% MgO, 47-49 wt% SiO₂, 0.5-0.7 wt% TiO₂) on northern Vancouver Island that are depleted in LREE (La/Yb_CN 0.4-0.7) and HFSE compared to the majority of the flood basalts, which are LREE-enriched (La/Yb_CN 1.3-3.0). All the Vancouver Island samples form a well-defined linear array in a Lu-Hf isochron diagram corresponding to an age of 238 ± 15 Ma. The initial epsilon Hf (+10.3±2.2) lies at the low end of the depleted mantle range and is comparable to the values of the Ontong Java (εHf_120Maof +11.3±1.3) and Caribbean (εHf_90Ma of +10.8±3.2) Plateaus. The picrites and LREE-enriched basalts on Vancouver Island have uniform initial Hf isotopic compositions, which indicates that they both were likely derived from a common, depleted (but not MORB) mantle source. Pb, Nd, and Sr isotopic data provide a powerful tool to characterize the mantle source for the Wrangellia plateau.