WHAT DOES THE GEOCHEMISTRY OF THE ACCRETED TRIASSIC WRANGELLIA OCEANIC PLATEAU TELL US ABOUT THE PACIFIC MANTLE?

The Wrangellia oceanic plateau in the Pacific Northwest of North America is one of the best exposures of an accreted oceanic plateau on Earth and is providing important insights into the nature of the mantle source of oceanic plateaus in the Pacific. Here we highlight the prominent geochemical features of the Wrangellia oceanic plateau, and compare them to other oceanic plateaus, to refine models on the genesis of oceanic plateaus. The Wrangellia plateau (~231-225 Ma) extends ~2300 km as a discontinuous belt from central Alaska and western Yukon (Nikolai Formation; <3.5 km) to Vancouver Island (Karmutsen Formation; ~6 km). Wrangellia flood basalts are bounded by Middle to Late Triassic marine sediments and unconformably overlie Paleozoic arc volcanic sequences of different age.

The vast majority of the flood basalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% TiO₂; 6-8 wt% MgO) with relatively uniform isotopic compositions (ε_Hf(t) = +9.1 to +12.6; ε_Nd(t) = +6.0 to +8.7; t = 230 Ma). The lowest part of the stratigraphy in areas of Alaska in Yukon is LREE-depleted low-Ti basalt (0.4-1.2 wt% TiO₂) with pronounced negative-HFSE anomalies and Hf isotopic compositions (ε_Hf(t) = +13.7 to +18.4) that are decoupled from Nd (ε_Nd(t) = +4.6 to +5.4) and displaced well above the OIB mantle array (Dε_Hf = +4 to +8). Submarine stratigraphy on northern Vancouver Island contains picritic basalts (9-20 wt%) depleted in LREE (La/Yb_CN = 0.5 ± 0.2) with overlapping initial ε_Hf (+10.3 ± 2.1) and ε_Nd (+7.7 ± 1.3) to the high-Ti basalts. Sr-Nd-Hf-Pb isotopic compositions of the high-Ti basalts indicate a common, Pacific plume-type mantle source, with similarities to the source of basalts from the Ontong Java and Caribbean Plateaus and some local differences. The low-Ti basalts involved a HFSE-depleted, high ε_Hf component that is distinct from OIB and MORB and was only involved during the early phase of this major melting event. Intrusion or erosion of the lithosphere by an impinging plume head initially led to melting of subduction-modified mantle or interaction of plume-derived melts and arc material, whereafter melting occurred mostly within the plume to produce voluminous high-Ti basalts. The Pacific mantle has produced some oceanic plateaus and hotspot volcanoes with broadly similar isotopic compositions over the past 230 Myr.