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TitleGeology and U-Pb geochronology of low-grade mafic rocks from the St. Cyr klippe and a marble from the footwall, Canadian Cordillera, Yukon
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorIsard, S J; Gilotti, J A; McClelland, W C; Petrie, M B; van Staal, C R
SourceGeological Survey of Canada, Current Research (Online) 2016-1, 2016, 26 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Mediaon-line; digital
File formatpdf
AreaCordillera; Mount St. Cyr
Lat/Long WENS-133.5000 -133.0000 61.5000 61.2500
Subjectsgeochronology; igneous and metamorphic petrology; structural geology; bedrock geology; uranium lead dates; uranium lead dating; igneous rocks; mafic rocks; ultramafic rocks; klippen; marbles; metamorphic rocks; island arcs; magmatic arcs; St. Cyr klippe; Mesozoic; Cretaceous
Illustrationslocation maps; ternary diagrams; photographs; plots
ProgramGSC Pacific Division
Released2016 04 08
AbstractA series of thin thrust slices form a klippe in the northwest part of the St. Cyr area, west of the South Canol Road, in south-central Yukon. The uppermost unit of the klippe, the Tower Peak unit, overlies a mafic-ultramafic unit, which in turn sits on a footwall of phyllite, marble, and limestone. The Tower Peak unit is a metabasalt the trace-element geochemistry of which suggests an oceanic or island-arc origin. The U-Pb ion microprobe ages of zircon from three samples of the Tower Peak unit define a xenocrystic zircon population with abundant Precambrian and lesser Paleozoic and Mesozoic magmatic components. Jurassic grains are euhedral, oscillatory zoned, and high-temperature, with Th/U ratios approximately 0.4, and positive Ce and negative Eu anomalies, pointing to an igneous origin. A similar population of five Mesozoic zircon crystals was found in a gabbro east of the Canol Road. All of the zircon crystals are interpreted as xenocrysts entrained in the mafic magmas during emplacement due to the large age spread and lack of a consistent magmatic age. The oldest possible intrusive age for the Tower Peak unit is Upper Jurassic, and Lower Cretaceous for the gabbro to the southeast. Detrital zircon samples from a prominent marble in the footwall of the klippe define a maximum depositional age of 368 ± 6 Ma. The marble likely correlates with the Finlayson assemblage of the Yukon-Tanana terrane composite arc. The mafic-ultramafic rocks at Tower Peak were structurally amalgamated with Mesozoic rocks during formation of the St. Cyr klippe.
Summary(Plain Language Summary, not published)
Geology and U-Pb geochronology of low grade mafic rocks from the St. Cyr klippe and a marble from the footwall, Yukon, Canadian Cordillera Sierra J. Isard, Jane A. Gilotti, William C. McClelland, Meredith B. Petrie, Cees R. van Staal This study describes the geological framework for a series of in the northwest part of the St. Cyr area, west of the South Canol Road, in south-central Yukon. The layers of rocks include metamorphic rocks including a higher temperature mafic-ultramafic package and lower temperature packages including phyllite, marble and limestone. Relationships between a number of rock units in the field suggest the rocks were originally formed in an oceanic setting. U-Pb geochronology using zircons confirm the ages for a number of the rock units. These rock units were subsequently faulted into their current location.

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