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Research Article| February 01, 1988 Petrology of flecked gneisses in the northern Wet Mountains, Fremont County, Colorado ROBERT B. TRUMBULL ROBERT B. TRUMBULL 1Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT B. TRUMBULL 1Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (2): 247–256. https://doi.org/10.1130/0016-7606(1988)100<0247:POFGIT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation ROBERT B. TRUMBULL; Petrology of flecked gneisses in the northern Wet Mountains, Fremont County, Colorado. GSA Bulletin 1988;; 100 (2): 247–256. doi: https://doi.org/10.1130/0016-7606(1988)100<0247:POFGIT>2.3.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 Flecks consisting of centimetre-sized quartz-K-feldspar-plagioclase segregations with cores of magnetite and/or hornblende occur locally in discrete zones within sillimanite-grade, migmatitic biotite gneiss and amphibolite in the Wet Mountains, south-central Colorado. The close field association of flecked gneisses with migmatites in this and other reported occurrences suggests a genetic relationship. This study evaluates the possibility that flecks formed by anatexis of the host rocks.Peak metamorphic conditions are estimated as 600-700 °C and 3-5 kbar from biotite-garnet-plagioclase-sillimanite equilibria, using garnet-core compositions. Plagioclase-K-feldspar and magnetite-ilmenite equilibria give apparent retrograde temperatures of 500-550°C.Although the metamorphic conditions may have permitted anatexis in the gneisses, the flecks show no tendency to cluster at a minimum melt field on a plot of An- Ab-Or-Q, and the REE patterns of separated flecks do not show the LREE enrichment relative to the matrix which would be expected in partial melting. It is suggested that the flecks formed by metamorphic segregation during diffusion- controlled dehydration reactions of biotite or hornblende. Mass-balance calculations indicate significant transport of Fe, Mg, and Ca between flecks and matrix; Si and A1 are constant; Na and K show slight and inconsistent changes. 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.
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