THE KANUTI OPHIOLITE, INTERIOR ALASKA: GEOLOGY AND TECTONIC SETTING OF (POSSIBLE) SUPRA-SUBDUCTION OCEANIC LITHOSPHERE

New field observations, rock and mineral geochemistry, and isotope data are presented for the Kanuti ophiolite. Located in a remote area, its petrology and crustal significance is under-studied, despite hosting probable economic PGE concentrations. The ophiolite is part of the Angayucham terrane, occurring as NE-trending nappes within a contractional belt along the southern Yukon-Koyokuk basin (YKB), possibly correlative with the Brooks Range ophiolite north of the YKB. The Kanuti overlies the Pz Arctic Ruby terrane, and the SE-bounding thrust fault system between them localized strain and fluid flow, manifested by serpentinized ultramafic rocks (UM) overlying variably garnet- and scapolite-bearing amphibolite, and eclogite from at least one locality. Structurally higher UMs are comparatively pristine, apart from typical seafloor alteration. Kanuti lithostratigraphic groups include both residual mantle and cumulate UM units and intrusive mafic rocks (gabbro and diabase). Cumulate UMs locally grade to cumulate gabbro. Volcanics are rare and compositionally distinct from Kanuti gabbro, likely structurally juxtaposed to, rather than cogenetic with, Kanuti magmatism.

Geochronology, both of melting to produce Kanuti crust, and timing of obduction, is poorly constrained. Hornblende in dikes intruding UMs, and in metamorphic sole amphibolite have K-Ar ages between ca. 160-140 Ma, though datable U-bearing phases in Kanuti gabbros are elusive; efforts are ongoing.

Kanuti gabbros are subalkaline and N-MORB-like and gabbro cpx resembles mid-ocean ridge cumulates (TiO2 ~0.8, Mg# ~80). Small Th over Nb enrichment in gabbro point to weak crustal/subduction modification, yet Ti/V is not consistent with oxidized mantle. Gabbros have Pacific MORB-like Hf-Nd isotopes, with some samples extending to more enriched Nd along the mantle array. Lherzolite and dunite olivine has Fo ~90; Fo in chromitite is higher (93-96) and is lower in pyroxenite (86). Clinopyroxenes (cpx) in UMs have high Mg# (92-99) and low Ti (<0.2wt%), transitional between depleted abyssal and suprasubduction types. Spinels have high-Mg and low-Cr in mantle rocks, but are more depleted, and likely melt-reacted, in cumulates. Studies are ongoing, but Kanuti ophiolite shows a source with weak subduction modification, possibly at a forearc setting.