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TitleEstimates of garnet crystallization and rates of metamorphism for metapelites of the Snowcap assemblage, Yukon-Tanana terrane, Yukon
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorMorneau, Y E; Gaidies, F; Ryan, J J; Zagorevski, AORCID logo
SourceGeological Survey of Canada, Current Research (Online) 2017-2, 2017, 21 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Mediaon-line; digital
File formatpdf
NTS115J/13; 115J/14; 115K/16; 115N/01; 115N/08; 115O/03; 115O/04; 115O/05; 115O/06
AreaStewart River
Lat/Long WENS-140.5000 -139.0000 63.5000 62.7500
Subjectstectonics; igneous and metamorphic petrology; geochemistry; bedrock geology; lithology; metamorphic rocks; amphibolites; metapelites; garnet; crystallization; mineral assemblages; sedimentary rocks; shales; geological history; tectonic history; thermal history; metamorphism; deformation; deposition; crystallization; modelling; pressure-temperature conditions; whole rock geochemistry; electron probe analyses; phase equilibria; textural analyses; schistosity; foliation; crenulations; inclusions; quartz; ilmenite; mica; geochemical analyses; iron; magnesium; manganese; calcium; yttrium; zoning; crystal fractionation; crystallography; structural features; faults; zoning; thin section microscopy; Snowcap Assemblage; Yukon-Tanana Terrane; Phanerozoic; Cenozoic; Quaternary; Tertiary; Mesozoic; Cretaceous; Jurassic; Paleozoic; Permian; Carboniferous; Mississippian; Devonian
Illustrationsgeoscientific sketch maps; tables; photomicrographs; graphs; ternary diagrams; histograms; profiles; phase diagrams
ProgramGEM2: Geo-mapping for Energy and Minerals Western Cordillera, Redefinition of crustal blocks
Released2017 09 11
AbstractAmphibolite-facies Snowcap assemblage rocks in the Stewart River area (west-central Yukon) preserve significant information on the tectonometamorphic history of the Yukon-Tanana terrane. Garnet-bearing metapelites were analyzed to obtain detailed metamorphic pressure-temperature-time (P-T-t) information through the integration of whole-rock geochemistry, electron-microprobe analysis, X-ray micro-computed tomography, phase equilibria, and garnet-crystallization modelling. The bulk chemistry of the Snowcap assemblage rocks indicates that their protoliths are shale and wacke that were deposited in a passive-margin setting. Textural observations indicate at least three episodes of deformation, forming an early schistosity (SM-2) overprinted by a crenulation cleavage (SM-1) defined by quartz and ilmenite inclusions in garnet, and a dominant transposition foliation (SM) mainly recorded by micaceous layers. Compositional zoning of garnet with respect to its Fe, Mg, Mn, Ca, and Y contents indicates chemical disequilibrium across the garnet volume and between garnet and the rock matrix. The zoning is interpreted to result from chemical fractionation associated with garnet growth and relatively slow intracrystalline diffusion. Calculations on a key sample indicate garnet started to crystallize at approximately 510°C and 3.75 kbar and finished growing at about 670°C and 6.8 kbar. These conditions correspond to the peak P-T conditions experienced by the rock. The chemical composition of the garnet core indicates significant modification of the initial garnet chemistry by intracrystalline diffusion at temperatures of 600°C or higher for at least 20 Ma. There are no current age constraints on the samples analyzed, but previous studies indicate Permian and Jurassic diachronic and/or episodic regional metamorphism.
Summary(Plain Language Summary, not published)
This report presents preliminary results and models developed through GEM2 program on metamorphosed and deformed sedimentary rocks in Yukon. It details the distribution and characteristics of the sedimentary rocks on the basis of field and laboratory analyses. It models whole rock and mineral data to understand thermal history of the rocks. The results presented herein help progress GEM2 models to ultimately redefine the regional tectonic models of the Cordillera.

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