ASSEMBLY OF THE WRANGELLIA OCEANIC PLATEAU: AN ACCRETED TRIASSIC LARGE IGNEOUS PROVINCE IN THE INSULAR BELT OF THE NORTH AMERICAN CORDILLERA

The accreted Wrangellia flood basalts and associated pre- and post-volcanic sedimentary rocks provide an unparalleled view of the architecture and compositional evolution of an oceanic plateau. Tectonic fragments of this Triassic large igneous province extend ~2300 km from central Alaska and western Yukon (Nikolai Formation) to Vancouver Island (Karmutsen Formation). Based on combined radiometric (U-Pb, ⁴⁰Ar/³⁹Ar, K-Ar), paleontological, and magnetostratigraphic age constraints, the Wrangellia flood basalts were emplaced during a single phase of tholeiitic volcanism at ca. 230-225 Ma, and possibly within as little as 2 Myr, onto pre-existing submerged arc crust. On Vancouver Island, ~6 km of high-Ti basalts, with minor amounts of picrites, record an emergent sequence of pillow basalt, pillow breccia and hyaloclastite, and subaerial flows that overlie Devonian to Mississippian (~380-355 Ma) island arc rocks and Mississippian to Permian marine sedimentary strata. In contrast, Alaska and Yukon contain sequences <3.5 km thick of mostly subaerial high-Ti basalts, with low-Ti basalt and submarine pillow basalts in the lowest parts of the stratigraphy, that overlie Pennsylvanian to Permian (312-280 Ma) volcanic and sedimentary rocks.

A mantle plume initiation model for the Wrangellia basalts is supported by major-element modeling, which indicates that the picrites formed by extensive melting (23-27%) of anomalously hot mantle (~1500°C). Thermal and mechanical erosion of the base of the lithosphere by the impinging plume head led to early melting of arc lithosphere (<5%) in Alaska and Yukon and formation of low-Ti tholeiites. High-Ti plume-derived magmas dominated the upper stratigraphy of the oceanic plateau as a result of increased decompression melting of the underlying mantle plume. High effusion rates resulted in the formation of extensive compound flow fields (submarine and subaerial) with limited repose time between flows (absence of weathering, erosion, sedimentation). Following rapid uplift, the plateau subsided during the final stages of construction and intervolcanic sedimentary lenses formed in shallow water in local areas as eruptive activity waned. After volcanism ceased, the plateau continued to subside for more than 25 Myr and was overlain by 100s to >1000 m of marine sedimentary rocks.