Record details

Title
    The origin of zircon and the significance of U-Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)
Author
    Cocherie, A.
    Skrzypek, Etienne
    Štípská, Pavla
Language
    anglicky
Publication type
    článek v periodiku
Source title - serial
    Contributions to Mineralogy and Petrology
Vol./nr.
    Roč. 164, č. 6
Pages
    23
Year
    2012
Thesaurus term
    U-Pb zircon dating, Mineral equilibria, modelling, HP granulite, Zircon petrogenesis, Variscan, Vosges Mountains
Keyword
    Ages
    Case
    France
    High-grade
    Metamorphic
    Mountains
    NE
    Origin
    Orogenic
    Rocks
    Root
    Significance
    Study
    U-Pb
    Variscan
    Vosges
    Zircon
Abstract (in english)
   U-Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700-500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340-335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of *16 kbar/850 C followed by isothermal decompression to 8-10 kbar/820-860 C. In felsic granulite samples, the assemblage K-feldspar- garnet-kyanite-Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of *13 kbar/925 C, and a subsequent P-T decrease to 6.5
   -8.5 kbar/800-820 C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P-T paths are used to discuss the significance of the U-Pb ages. In the monotonous unit, inherited zircon ages of 700-500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatoryzoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U-Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.
Contributor
    Česká geologická služba
Contributor code
    ČGS (RIV)
Source format
    R
Import date
    15. 10. 2014