| Summary: | The Wrangellia flood basalts are parts of an oceanic plateau that formed in the
eastern Panthalassic Ocean (ca. 230-225 Ma). The volcanic stratigraphy presently extends
>2300 km in British Columbia, Yukon, and Alaska. The field relationships, age, and
geochemistry have been examined to provide constraints on the construction of oceanic
plateaus, duration of volcanism, source of magmas, and the conditions of melting and
magmatic evolution for the volcanic stratigraphy.
Wrangellia basalts on Vancouver Island (Karmutsen Formation) form an
emergent sequence consisting of basal sills, submarine flows (>3 km), pillow breccia and
hyaloclastite (<1 1cm), and subaerial flows (>1.5 km). Karmutsen stratigraphy overlies
Devonian to Permian volcanic arc (~380-355 Ma) and sedimentary sequences and is
overlain by Late Triassic limestone. The Karmutsen basalts are predominantly
homogeneous tholeiitic basalt (6-8 wt% MgO); however, the submarine part of the
stratigraphy, on northern Vancouver Island, contains picritic pillow basalts (9-20 wt%
MgO). Both lava groups have overlapping initial EHf and ENd, indicating a common, ocean
island basalt (OIB)-type Pacific mantle source similar to the source of basalts from the
Ontong Java and Caribbean Plateaus. The major-element chemistry of picrites indicates
extensive melting (23-27%) of anomalously hot mantle (~1500°C), which is consistent
with an origin from a mantle plume head.
Wrangellia basalts extend ~450 km across southern Alaska (Wrangell Mountains
and Alaska Range) and through southwest Yukon where <3.5 km of mostly subaerial
flows (Nikolai Formation) are bounded by Pennsylvanian to Permian volcanic (312-280
Ma) and sedimentary strata, and Late Triassic limestone. The vast majority of the Nikolai
basalts are LREE-enriched high-Ti basalt (1.6-2.4 wt% Ti0₂) with uniform plume-type
Pacific mantle isotopic compositions. However, the lowest ~400 m of stratigraphy in the
Alaska Range, and lower stratigraphy in Yukon, is light rare earth element (LREE)
depleted low-Ti basalt (0.4-1.2 wt% Ti0₂) with pronounced negative-HFSE anomalies
and high Elf values that are decoupled from Nd and displaced well above the OIB mantle
array. The low-Ti basalts indicate subduction-modified mantle was involved in the
formation of basalts exposed in Alaska and Yukon, possibly from mechanical and
thermal erosion of the base of the lithosphere from an impinging mantle plume head.
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