Melting History and Magmatic Evolution of Basalts and Picrites from the Accreted Wrangellia Oceanic Plateau, Vancouver Island, Canada

The accreted Wrangellia oceanic plateau in the Pacific Northwest of North America provides important insights into the volcanic architecture of major oceanic plateaus, as well as the nature of their mantle source, conditions of melting and subsequent magmatic evolution. The 20 000 km(2) Karmutsen Formation flood basalts (Vancouver Island) were emplaced at c. 225230 Ma onto Middle Triassic marine sediments and Late Devonian to Early Permian island-arc volcanic and sedimentary sequences, and are overlain by Late Triassic platformal carbonates. The basalts form an emergent sequence consisting of a basal sedimentsill complex (600900 m thick), pillowed and massive submarine flows (25 km), pillow breccia and hyaloclastite (15 km), and massive subaerial flows (25 km). Although the Karmutsen Formation is predominantly composed of tholeiitic basalt, the submarine part of the stratigraphy on northern Vancouver Island also contains picritic basalts. These high-MgO (920 wt ) lavas are depleted in light rare earth elements (LREE; La/Yb-CN 05 02), whereas the tholeiitic lavas (68 wt MgO) are LREE-enriched (La/Yb-CN 22 03). Both lava groups have overlapping initial (Hf)(103 21) and (Nd)(77 13), indicating a common, plume-type Pacific mantle source with geochemical characteristics similar to the source of basalts from the Ontong Java and Caribbean plateaus. Major-element modeling results indicate that the picrites formed by extensive melting (2327) of anomalously hot mantle (1500C), which is consistent with a mantle plume initiation model for formation of the Karmutsen flood basalts on Vancouver Island. Trace element constraints indicate that the picrites require melting of a depleted spinel lherzolite source, whereas the tholeiitic basalts involved melting of garnet and spinel lherzolite. The tholeiitic basalts underwent significant fractional crystallization (50) and the fractionated residues may be represented by high-velocity rocks beneath Vancouver Island identified from seismic reflection studies.