Abstract The Dun Mountain ophiolite, South Island, New Zealand, records complex overprinting of mantle fabrics. Using structural observations, microstructural analysis, geothermometry, geobarometry, geochronology and rheological constraints from the Red Hills and Dun Mountain massifs, we propose that three deformation events occurred during the early stages of subduction initiation along the Permian margin of Gondwana. During the first deformation event, the lineated Two Tarns Harzburgite from the Red Hills formed in a transtensional setting associated with subduction initiation. Deformation was pervasive, homogeneous and simultaneous with boninitic melt migration through the unit; it also occurred at very fast strain rates (10 −9 –10 −8 s −1 ). During the second deformation event, progressive exhumation to c. 5 kbar and cooling to 1000°C led to the localization of melt and deformation into distinct zones (Dun Mountain, and the Plateau Complex, Plagioclase Zone and Ellis Stream Complex of the Red Hills). The third deformation event resulted in continued cooling and exhumation along serpentinized faults. This history provides a rare glimpse of the coupled fabric development and melt migration that sequentially develop in the early mantle wedge during the initiation of a subduction zone.

The Ingalls ophiolite complex, central Cascades, Washington, mainly consists of mantle-derived ultramafic tectonite, with crustal rocks consisting of gabbro, diabase, basalt, and sedimentary rocks. The crustal rocks occur as faulted blocks within serpentinite mélange (Navaho Divide fault zone). Mafic rocks in most of these blocks comprise the Late Jurassic Esmeralda Peaks unit. Herein, we define an older, Early Jurassic unit within the Ingalls ophiolite complex, which we call the Iron Mountain unit. This unit occurs along the southern edge of the complex and consists dominantly of mafic volcanic rocks with minor sedimentary rocks. A rhyolite within the Iron Mountain unit yields a ca. 192 Ma U-Pb zircon age, consistent with an Early Jurassic age assignment based on radiolarians in cherts. The presence of volcanic rocks that have within-plate basalt magmatic affinities and oolitic limestone suggests that the Iron Mountain unit formed as a seamount. Magmatic affinities range from within-plate basalt to enriched mid-ocean-ridge basalt (E-MORB), which is compatible with a mantle plume close to a ridge. The Early Jurassic age of the Iron Mountain unit, which is ~30 m.y. older than the Esmeralda Peaks unit, indicates that the Ingalls ophiolite complex is polygenetic. The Iron Mountain unit most likely represents basement that was rifted in a suprasubduction-zone setting in the Late Jurassic during formation of the Esmeralda Peaks unit.