VOLCANICLASTIC GRAVITY FLOW DEPOSITS IN THE DEZADEASH FORMATION (JURA-CRETACEOUS), YUKON, CANADA: IMPLICATIONS REGARDING THE TECTONOMAGMATIC EVOLUTION OF THE CHITINA ARC IN THE NORTHERN CORDILLERA OF NORTH AMERICA

The Chitina arc in the northern Cordillera of North America evolved during the accretion of the Wrangellia composite terrane to the western margin of North America in the Jurassic and Cretaceous. The Dezadeash Formation, a 3000 m thick sequence of deepwater turbidites in southwestern Yukon, was deposited as a submarine fan in a backarc basin to the Chitina arc. Only limited geochemistry of altered volcanic rocks associated with the arc are available from southeastern Alaska. However, three thick (1.5-9.75 m) volcaniclastic beds occur in the Dezadeash Formation. They consist of fine- to medium-grained vitric, crystal-vitric and crystal tuffs that are massive, planar, wavy and convolute laminated, and are interpreted as resedimented syn-eruptive volcaniclastics. A U-Pb zircon age (149.4±0.3 Ma) indicates they are contemporaneous with the Chitina arc. Petrographic examination shows the volcaniclastic rocks are altered mainly by albite and locally calcite, and Harker diagrams indicate that K₂O, CaO and Rb were mobile. According to various trace element plots, the volcaniclastic rocks are calk-alkaline in composition (Zr/Y~12.23, La/Yb~16.96, and Th/Yb~8.25), and similar to Nb-enriched, low Mg, adakites. They plot in the active continental margin field on a variety of tectonic discriminant diagrams, and chondrite-normalized multi-element plots display parallel, listric-shaped profiles with significant light rare-earth element enrichment (x40-100) and minor heavy rare-earth element enrichment (x10). Sm-Nd systematics (εNd₍₁₄₉₎=+3.5) indicate the volcaniclastic rocks represent mixing of a depleted mantle source and an older crustal source. These data suggest mainly fractional control by hornblende and late feldspar of normal arc magma derived from partial melting of the mantle wedge, with possibly minor input by slab melting. The Chitina arc has previously been interpreted as an oceanic island arc, and the continental arc signature of volcaniclastic rocks in the Dezadeash Formation, together with a continental arc signature of the coeval altered volcanic rocks in southeastern Alaska (based on the Zr/Y vs. Zr tectonic discriminant diagram), is attributed to arc magmas erupting through thick Paleozoic sedimentary and plutonic rocks of the Wrangellia composite terrane proxying for continental crust.