INTEGRATED PROVENANCE ANALYSIS OF THE DEZADEASH FORMATION (JURA-CRETACEOUS), YUKON, CANADA: CONSTRAINING THE ACCRETIONARY TECTONIC HISTORY OF THE WRANGELLIA COMPOSITE TERRANE IN THE NORTHERN CORDILLERA OF NORTH AMERICA

The Wrangellia composite terrane (WCT) was the last major terrane accreted to the western margin of North America, which consisted of the Yukon composite terrane (YCT). Accretion was accompanied by the formation of Mesozoic magmatic arcs (Talkeetna, Chitna and Chisana), and the collapse of intervening ‘flysch’ basins (Gravina belt, Nutzotin Mountains sequence, Dezadeash Formation, and Kahiltna assemblage).

The 3000 m thick Dezadeash Fm, located north of the Denali fault system and closer to YCT, is interpreted as being deposited in a backarc basin, whereas the 3000 m thick Nutzotin Mountains sequence, located south of the fault system and farther from YCT, has recently been interpreted as forming in a retroarc foreland basin. As both units represent the same basin displaced by ~370 km of dextral movement on the Denali fault system in post-Early cretaceous time, evidence linking strata of the Nutzotin-Dezadeash basin with the YCT is critical to the retroarc model.

Integrated provenance analysis of the Dezadeash Fm reveals that conglomerates are dominated by clasts derived from WCT (basalt, limestone, chert, andesite) and roots of the Chitna arc (diorite, granodiorite); sandstones include litharenites and feldspathic litharenites (Q₁₂F₂₆L₆₂) that are dominated by volcanic lithic grains (Lv₉₃Ls₆Lm₁) and Mesozoic detrital zircons (94%), with minor Paleozoic (1%) and Precambrian (5%) zircons; and sandstones and mudstones have a chemical index of alteration similar to unweathered igneous rocks (CIA ≈ 50), Th/Sc ratios of ≈ 0.2, and display parallel, listric-shaped profiles with moderate enrichment of LREE and no Europium anomaly (Eu/Eu* ≈ 1.0) on chrondrite-normalized multi-element diagrams; and sandstones and mudstones have Nd-Sm systematics suggesting mixing of a depleted mantle source and an older crustal source (Ɛ_Nd⁽⁰⁾≈ 0 to +3.4).

The provenance data indicates that the Dezadeash Fm consists of first-cycle volcanogenic material derived from a young undifferentiated arc that was undissected to partly dissected (i.e., transitional), as well as older crustal material, all of which originated from the coeval Mesozoic arcs and the WCT. These results, combined with paleocurrent data that indicates only a westerly source, do not support a retroarc foreland basin interpretation for the Nutzotin-Dezadeash basin.