The Kanuti ophiolite, Alaska

Abstract

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.