A large alpine-type, mafic-ultramafic complex in the vicinity of Mount Stuart in Chelan County, Washington, consists chiefly of serpentinized harzburgite and uralitized augite gabbro with lesser amounts of pyroxenite and anorthositic gabbro. This complex was tectonically emplaced into metasedimentary and metavolcanic rocks of the Peshastin and Hawkins Formations, probably in Late Jurassic to Early Cretaceous time. The highly magnesian composition of the harzburgite, the high Ca and Al contsnts of the metagabbro, the tectonite fabric of these rocks, and the virtual absence of contact metamorphism are compatible with an origin at an active oceanic ridge or within a marginal basin. Primary textures and bulk modal compositions of volcanogenic sediments and flows in the Peshastin-Hawkins sequence are interpreted as evidence for an island-arc environment of deposition. Argillite, volcanic-derived epiclastic graywacke, sorted and unsorted breccia and tuff-breccia, and some pyroclastic rocks are associated with flows ranging in composition from basalt to quartz keratophyre. The mafic-ultramafic complex is interpreted as ophiolite that was thrust against older island-arc rocks in an arc-trench system that operated from late Paleozoic through Mesozoic time. The remains of the dismembered ophiolite are now found in intricate tectonic association with rocks of the former arc.

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