Wrangellia flood basalts in Alaska: A record of plume‐lithosphere interaction in a Late Triassic accreted oceanic plateau

Abstract

[1] The Wrangellia flood basalts are part of one of the best exposed accreted oceanic plateaus on Earth. They provide important constraints on the construction of these vast submarine edifices and the source and temporal evolution of magmas for a plume head impinging beneath oceanic lithosphere. Wrangellia flood basalts ($231-225 Ma) extend $450 km across southern Alaska (Wrangell Mountains and Alaska Range) where $3.5 km of mostly subaerial flows are bounded by late Paleozoic arc volcanics and Late Triassic limestone. The vast majority of the flood basalts are light rare earth element (LREE) -enriched high-Ti basalt (1.6-2.4 wt % TiO 2 ) with uniform ocean island basalt (OIB) -type Pacific mantle isotopic compositions (e Hf (t) = +9.7 to +10.7; e Nd (t) = +6.0 to +8.1; t = 230 Ma). However, the lowest $400 m of stratigraphy in the Alaska Range is LREE-depleted low-Ti basalt (0.4-1.2 wt % TiO 2 ) with pronounced negative high field strength element (HFSE) anomalies and Hf isotopic compositions (e Hf (t) = +13.7 to +18.4) that are decoupled from Nd (e Nd (t) = +4.6 to +5.4) and displaced well above the OIB mantle array (De Hf = +4 to +8). The radiogenic Hf of the low-Ti basalts indicates involvement of a component that evolved with high Lu/Hf over time but not with a correspondingly high Sm/Nd. The radiogenic Hf and HFSE-depleted signature of the low-Ti basalts suggest pre-existing arc lithosphere was involved in the formation of flood basalts that erupted early in construction of part of the Wrangellia plateau in Alaska. Thermal and mechanical erosion of the base of the lithosphere by the impinging plume head may have led to melting of arc lithosphere or interaction of plume-derived melts and subduction-modified mantle. The high-Ti lavas dominate the main phase of construction of the plateau and were derived from a depleted mantle source distinct from the source of MORB and with compositional similarities to that of ocean islands (e.g., Hawaii) and plateaus (e.g., Ontong Java) in the Pacific Ocean.

U-Pb geochronology and geochemistry of intrusive rocks from the Cougar Creek Complex, Wallowa arc terrane, Blue Mountains Province, Oregon-Idaho

“…The disputed correlation between the Wallowa and Wrangellia terranes of the northern U.S. and Canadian Cordilleran sectors is one example (Jones et al, 1977;Mullen and Sarewitz, 1983;Sarewitz, 1983;Scheffl er, 1983;Mortimer, 1986;Wernicke and Klepacki, 1988;Dickinson, 2004). Our new data combined with studies of the geochemistry (Barker et al, 1989;Greene et al, 2008Greene et al, , 2009aGreene et al, , 2009b, geochronology (Mortensen and Hulbert, 1991;Lassiter, 1995;Bittenbender et al, 2007;Schmidt and Rogers, 2007), and paleontology (Smith and MacKevett , 1970;Read and Monger, 1976;MacKevett, 1978;Israel et al, 2006) for Middle to Late Triassic rocks of Wrangellia help to clarify the question of Wallowa-Wrangellia correlation. Wrangellia and the Wallowa arc terrane share several common characteristics in terms of their general lithostratigraphy, including Late Paleozoic arc and marine rocks that are unconformably overlain by voluminous, predominantly mafi c volcanic units.…”

“…Richards et al (1991) interpreted the thick basaltic sequences of Wrangellia as an oceanic plateau and proposed a mantle plume initiation model for their genesis. Recent studies by Greene et al (2008Greene et al ( , 2009aGreene et al ( , 2009b agree with a plume origin and also interpret Wrangellia as an accreted oceanic plateau. However, alternative hypotheses do exist.…”

“…Our data synthesis provides the foundation for a new model of ridge subduction and slab window magmatism consistent with the Late Triassic evolution of the Wallowa arc terrane. Recent contributions to the geologic framework of the accreted Wrangellia oceanic plateau (Richards et al, 1991;Greene et al, 2008Greene et al, , 2009aGreene et al, , 2009b, a primary component of the Insular superterrane of Alaska and Canada, provide a more comprehensive basis for comparison with the Wallowa arc terrane. Based on lithostratigraphic, geochemical, geo chronologic, and paleontologic data, the Wallowa arc terrane may be correlated with Wrangellia; we explore the implications of this correlation through the larger-scale tectonic processes of ridge subduction and slab window magmatism.…”

The architecture of oceanic plateaus revealed by the volcanic stratigraphy of the accreted Wrangellia oceanic plateau

“…The volcanic stratigraphy on Vancouver Island consists of a tripartite succession of submarine fl ows (50%-60% of total thickness), volcaniclastics, and subaerial fl ows, whereas volcanic stratigraphy in Alaska and Yukon is predominantly subaerial fl ows (>90%; Table 5; Greene et al, 2008Greene et al, , 2009aGreene et al, , 2009b. In areas of central Vancouver Island, the stratigraphically lowest pillow basalts were emplaced on unconsolidated fi ne-grained sediments, and mafi c sills intrude marine strata with Daonella beds.…”

“…Based on the stratigraphic and temporal constraints outlined in this study, and the petrology and geochemistry of the Wrangellia basalts from Greene et al (2008Greene et al ( , 2009aGreene et al ( , 2009b, the main factors that controlled the construction of the Wrangellia oceanic plateau include high effusion rates and the formation of extensive compound fl ow fi elds from low-viscosity, hightemperature tholeiitic basalts, sill-dominated feeder systems, low volatile content, protracted eruption (perhaps >10 yr for individual fl ow fi elds), limited repose time between fl ows (absence of weathering, erosion, sedimentation), submarine versus subaerial emplacement, and relative water depth (e.g., pillow basaltvolcaniclastic transition). Combined, these factors contributed to the production of 4-6 km of basalt stratigraphy with limited magnetic reversals, a lack of fossil remains in associated sediments, and minimal evidence of eruptive explosivity.…”