Record details

Title
    Sm-Nd isotope tracer study of UHP metamorphic rocks : implications for continental subduction and collisional tectonics
Statement of responsibility
    Bor-Ming Jahn
Author
    Jahn, Borming, 1940-
Language
    anglicky
Source title - monograph
    Ultrahigh pressure metamorphism and geodynamics in collision-type orogenic belts
Pages
    s. 245-271
Notes
    11 diagr., 6 tab.
    il.
    Obsahuje bibliografické odkazy
Classfication no.
    550.9
    552
Conspectus category
    552
Subject group
    datování Sm/Nd
    eklogit
    krystalinikum Českého masivu
    metamorfóza vysokotlaká
    peridotit
    podmínky P-T
    subdukce
    vrásnění alpinské
    vrásnění kaledonské
    vrásnění panafrické
    vrásnění variské
Geographical name
    Česká republika
    Svět
Keyword
    Collisional
    Continental
    Implications
    Isotope
    Metamorphic
    Rocks
    Sm-Nd
    Study
    Subduction
    Tectonics
    Tracer
    UHP
Abstract (in english)
   Sm-Nd isotope tracer techniques are powerful tools in identification of the protolith nature of UHP and HP rocks and can be used to constrain modeling of tectonic processes of continental collision. UHP rocks may have diverse origins, and not all of them carry the same significance for subduction of continental blocks. In this paper, Sm-Nd isotopic data are compiled for UHP and HP rocks, mostly represented by eclogites and garnet peridotites, from the Alpine, Hercynian (Variscan), and Caledonian belts of western Europe; the Pan-African belts of northern Africa; and the Ross belt of Antarctica. These data then are compared with the isotopic characteristics of the UHP rocks from the Dabie orogen of central China.
   Except for the coesite-bearing quartzitic metasediments of Dora-Maira (Western Alps), which are clearly of continental origin, all HP and UHP rocks (eclogites and ultramafic rocks) from the Alpine, Hercynian, and Pan-African belts have oceanic affinities with the characteristic positive "epsilon"Nd/(T) values (= metamorphic initial 143Nd/144Nd ratios). They represent segments of oceanic lithosphere that were subducted, underwent eclogite-facies metamorphism, and later were tectonically transported into orogenic zones during continental collisions. By contrast, the majority of UHP rocks from the European Caledonide and the Dabie orogen have negative "epsilon"Nd/(T) values, indicating continental affinity.
   This suggests that these mafic and ultramafic rocks have had a long crustal residence time and that their UHP metamorphism is indicative of subduction of ancient and cold continental blocks, as represented by some Precambrian gneiss terranes containing mafic components including greenschists, amphibolites, or basic granulites. In the Dabie orogen, none of the UHP eclogites analyzed thus far have shown oceanic affinity; thus they do not represent subducted Tethys Ocean crust. The preservation of ultrahigh "epsilon"Nd/(0) values (+170 to +260) in eclogites of very low Nd concentrations (average 0.5 ppm) from the Weihai region and of the extraordinarily low "sigma"180 in many eclogites and gneisses, the general absence of syntec-tonic granites in the Dabie Shan, and the available age data obtained by different techniques all point to a rapid rate of exhumation and the absence of a pervasive aqueous fluid phase during the entire process of subduction and exhumation of the Dabie UHP terrane
Contributor
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
    U
Entered date
    26. 9. 2007
Import date
    8. 8. 2012