In the Darrington and Sultan basin areas of the Cascade Mountains, mafic and ultramafic rocks occur as sheets and lenses that were tectonically emplaced along high-angle faults. The ultramafic rocks comprise pseudomorphic serpentinite (lizardite-chrysotile), antigorite serpentinite, and metamorphic peridotite (olivine-talc-tremolite). The peridotites are of two types: (1) subordinate black-weathering peridotite characterized by iron-poor olivine (Fo98−Fo94) and essential magnetite, and (2) dominant orange-weathering peridotite with more iron-rich olivine (Fo94−Fo85) and little magnetite. In contrast to upper-mantle peridotites, olivine composition in the orange-weathering peridotites is variable on all scales down to a few micrometres.

Field relations, textural and mineralogical relics, mineral composition, and oxygen isotope values all reflect formation of the Darrington peridotites by deserpentinization. Field relations demonstrate the prograde metamorphic sequence lizardite-chrysotile serpentinite → antigorite serpentinite → metamorphic peridotite. Incipiently dehydrated serpentinites exhibit veinlets and scattered porphyroblasts of new-formed high-Mn olivine. Altered chromite, Fe and Ni-Fe sulfides, magnetite, and Mg carbonates in the peridotites have been inherited from the serpentinite parent. In the black-weathering peridotites, relict textures such as bastite pseudomorphs after pyroxenes are inherited from the parent serpentinites and survive as ghosts marked by the distribution of finegrained magnetite.

In the black-weathering peridotites, iron is bound largely in magnetite inherited from the parent serpentinites; thus the olivine is iron poor. The variability of olivine composition in the orange-weathering rocks is thought to be related to the variable content of iron in antigorite of the parent serpentinites.

Progressive metamorphism of these rocks reached middle amphibolite facies (500 to 600 °C). Deserpentinization occurred prior to Tertiary tectonic transport and emplacement into fault contact with the unmetamorphosed rocks of the present structural setting.

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