Record details

    Review of petrographic and mineralogical evidence for fluid induced dehydration of the mafic lower crust: could there be a relationship between granitoids and granulite facies xenoliths in the Variscan Belt?
Statement of responsibility
    Daniel E. Harlov
Other titles
    Petrographische und mineralogische Indizien für eine fluidinduzierte Dehydration der mafischen Unterkruste: Gibrt es eine Beziehung zwischen Granitoiden und granulitfaziellen Xenolithen im Variszischen Gürtel Europas?
    Harlov, Daniel E.
Source title - serial
    Zeitschrift für geologische Wissenschaften
    Vol. 30, no. 1-2
    p. 13-36
    6 obr., 4 fot.
    Bibliografie na s. 32-36
    Německé resumé
    Zkr. název ser.: Z. geol. Wiss.
Classfication no.
Conspectus category
Subject group
    Český masiv
    dynamika tekutin
    facie granulitová
    geologie regionální
    původ korový
    vrásnění variské
Geographical name
    Česká republika
    Itálie severní
Abstract (in english)
   Possible links between the fluid induced dehydration of the mafic lower crust and the subsequent generation of granitoid bodies in the mid to upper crust are explored specifically with respect to charnockite-enderbite xenoliths found in Tertiary and Mesozoic volcanics associated with granitoid bodies along the Variscan Belt. With regard to understanding these links, this paper compares and discusses the petrography as well as the silicate and phosphate mineral chemistry of metamorphosed mafic rocks from a series of terranes. A portion of each of these terranes has experienced a high-grade dehydration event transforming it from a H2O-rich, amphibolite facies zone to a H2O-poor, orthopyroxene-bearing granulite facies zone at approximately the same temperature and pressure, i.e. 700-800 °C and 7-8 kbar.
   These include the Seward Peninsula, Alaska (SP) traverse where dehydration took place over an 85-cm thick layer of tonalitic gneiss in contact with a marble during regional metamorphism and involved a CO2-rich fluid and the Val Strona di Omegna traverse, Ivrea-Verbano Zone, Northern Italy (IVZ), where dehydration involved an approximately 3-4 km thick sequence of interlayered metabasites and metapelites and is proposed to have involved a F- and Cl-bearing, CO2-poor fluid with a low H2O activity. In the latter case, both the heat and most of the fluids are hypothesised to have originated from mafic magmas intruded at the base of the sequence. Samples from either dehydration zone show micro-veins of partially ordered K-feldspar (originally sanidine) along quartz and plagioclase grain boundaries as well as replacement anti-perthite in a random scattering of the plagioclase grains.
   Modal mineral abundances suggest that the K+ came primarily from the breakdown of hornblende + quartz to orthopyroxene +/- clinopyroxene, feldspar and fluid. Close similarities between the two traverses also are observed with respect to mica chemistry. Biotite from the dehydration zones is distinctly higher in Ti and F relative to those from the amphibolite facies zone. High-grade fluid metasomatism was also responsible for the generation of Th-poor (< 0.5 wt per cent) monazite and/or xenotime in apatite from each dehydration zone. These inclusions are not seen in apatite from the amphibolite facies rocks. Depletion in Ce or Y in the immediate vicinity of the REE phosphate minerals and the positive correlations between Na and (Y+REE) as well as Si and (Y+REE) strongly suggest that these inclusions nucleated within the apatite itself from the REE budget available via the exchange vectors Na+ + (Y + REE)3+ = 2 Ca2+ and Si4+ + (Y + REE)3+ = P5+ + Ca2+.
   Apatite in the dehydration zone shows a relative increase in F compared to apatite in the amphibolite facies zone. If the fugacity ratio of HF relative to H2O for both biotite and fluorapatite is plotted as a function of the distance from the fluid/heat source for the SP and IVZ traverses, the same basic pattern,
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
Entered date
    4. 12. 2006
Import date
    8. 8. 2012