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.

A region of migmatitic felsic paragneiss and pelitic schist containing concordant pods of serpentinite, talc schist, talc-amphibole schist, amphibolite, and metagabbro—here called the Burks Mountain complex—occurs in the southeastern limb of the Kiokee antiform, an Alleghanian D 3 structure in the eastern Piedmont of Georgia and South Carolina. A similar region of migmatitic felsic paragneiss containing small pods of metamorphosed mafic and ultramafic rocks, which occurs in the northwestern limb of the Kiokee antiform, is inferred to be a continuation of the Burks Mountain complex across the crest of the antiform. The composition of the felsic paragneiss and pelitic schist of the Burks Mountain complex suggests derivation from graywacke and shale, respectively. Relict textures preserved in mafic and ultramafic rocks of the complex, along with preliminary whole-rock geochemistry, indicate derivation from ultramafic tectonite (harzburgite), olivine-pyroxene cumulate (wehrlite, olivine pyroxenite), pyroxene-plagioclase cumulate (gabbro, anorthositic gabbro), and mafic volcanic rock. These protoliths suggest an origin as part of a cumulate mafic and ultramafic intrusive and extrusive complex with some associated mantle tectonite. The complex was disrupted prior to or during amphibolite facies regional metamorphism. The mechanism of disruption is not clear. Possible modes of origin include accumulation as an olistostrome, accumulation in a subduction-related accretionary complex, and formation as extreme boudinage during a regional deformation event. The crystalline rocks in the core of the Kiokee belt (including the Burks Mountain complex) are contained in the footwall of a major ductile shear zone (the Modoc zone) that experienced oblique, down to the north-northeast displacement during the early part of the Alleghanian orogeny. Prior to the Alleghanian orogeny, the Burks Mountain complex was located beneath rocks of the Carolina slate belt that are presently exposed north of the Modoc zone. The Burks Mountain complex may have been derived from a unit within the Carolina slate belt or from a Precambrian basement(?) unit beneath the Carolina slate belt. Alternatively, in view of a unique lithostratigraphy, the Burks Mountain complex may be included in a terrane, exotic with respect to both North America and the Carolina slate belt, which was tectonically incorporated into the Appalachians prior to the Alleghanian orogeny.