Geochemistry of Triassic flood basalts from the Yukon (Canada) segment of the accreted Wrangellia oceanic plateau

Abstract

A large part of the accreted Middle to Late Triassic Wrangellia oceanic plateau is exposed as a linear belt (< 30 km × 300 km) in southwest Yukon. The first major- and trace-element, and isotopic compositions of the Nikolai Formation in Yukon are presented here, along with compositions for underlying Paleozoic arc rocks. The Nikolai Formation in Yukon is predominantly massive tholeiitic subaerial flows (~ 1000 m) with no intervening sediments and a thin zone of pillow breccia along the base (< 100 m). The Nikolai basalts unconformably overlie Late Paleozoic volcanic arc and marine sedimentary sequences and are overlain by Late Triassic limestone, which grades upwards into pelagic sediments. The Nikolai Formation is comprised of two distinct lava types: low-titanium basalts form most of the lower stratigraphy and high-titanium basalts form the upper parts of the volcanic stratigraphy. All of the low-titanium basalts (0.5–1.0 wt.% TiO 2; 5.6–11.3 wt.% MgO) have prominent negative HFSE anomalies, whereas the high-titanium basalts (1.4–2.3 wt.% TiO 2; 5.8–8.7 wt.% MgO) do not have HFSE anomalies and are more LREE-enriched. The low-titanium basalts are characterized by mostly higher initial ε Hf (+ 11.1 to + 15.8) and lower initial ε Nd (+ 2.3 to + 6.8) than the high-titanium basalts (initial ε Hf = + 10.4 to + 12.0; initial ε Nd = + 6.6 to + 9.0), and their Pb isotopic compositions overlap. Incongruent dynamic melting modeling of trace element compositions indicate the low-titanium basalts could have been derived from small degrees of melting (< 5%) of Paleozoic sub-arc lithospheric mantle that was HFSE-depleted and evolved with high 176Hf/ 177Hf. The high-titanium basalts formed from melting of Pacific plume-type mantle, similar to the source of the Caribbean Plateau. Plume-derived melts dominated the upper stratigraphy of the oceanic plateau as a result of increased decompression melting of the underlying mantle plume in response to thinning of the lithosphere.