Formation of peridotites by deserpentinization in the Darrington and Sultan areas, Cascade Mountains, Washington (1977)

Research Article| October 01, 1977 Formation of peridotites by deserpentinization in the Darrington and Sultan areas, Cascade Mountains, Washington JOSEPH A. VANCE; JOSEPH A. VANCE 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar MICHAEL A. DUNGAN MICHAEL A. DUNGAN 2Geology Branch, Code TN6, NASA Johnson Space Center, Houston, Texas 77058 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1977) 88 (10): 1497–1508. https://doi.org/10.1130/0016-7606(1977)88<1497:FOPBDI>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JOSEPH A. VANCE, MICHAEL A. DUNGAN; Formation of peridotites by deserpentinization in the Darrington and Sultan areas, Cascade Mountains, Washington. GSA Bulletin 1977;; 88 (10): 1497–1508. doi: https://doi.org/10.1130/0016-7606(1977)88<1497:FOPBDI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.