Record details

    Petrology, geochemical character and petrogenesis of a Variscan post-orogenic granite: case study from the Ševětín Massif, Moldanubian Batholith, Southern Bohemia
Statement of responsibility
    Vojtěch Janoušek, Stanislav Vrána, Vojtěch Urban
Other titles
    Petrologie, geochemický charakter a petrogeneze pozdně variské granitové intruze příklady ze ševětínského masivu, moldanubický pluton, jižní Čechy
    Janoušek, Vojtěch, 1968-
    Urban, Vojtěch
    Vrána, Stanislav
Source title - serial
    Journal of the Czech Geological Society
    Vol. 47, no. 1-2
    p. 1-22
    16 obr., 6 tab., 2 s. bibl.
    České resumé
    Zkr. název ser.: J. Czech geol. Soc.
Subject group
    analýza petrografická
    chemismus hornin
    granit biotitický
    granit dvojslídný
    izotopy radioaktivní
    moldanubický pluton
    vrásnění variské
Geographical name
    Deštná (Jindřichův Hradec)
    Ševětín (České Budějovice)
Abstract (in english)
   n the composite Ševětín Massif (Moldanubian Zone of Southern Bohemia) three main granite pulses can be distinguished: (1) the oldest, two-mica Deštná granite with cordierite +- andalusite (SE part of the massif), (2) biotite-muscovite Ševětín granite (BMG), constituting most of the granite pluton, and (3) fine-grained biotite Ševětín granite (BtG) forming only minor bodies. The Ševětín granites show transitional I/S type character. The whole-rock geochemical signature of the BtG is less evolved than that of the BMG. The former shows lower SiO2, Na2O, K2O and A/CNK accompanied by higher TiO2, FeOt, FeO, Al2O3 and CaO. The BtG is also characterized by higher content of Rb, Sr, Cr, Ni, La, LREE, Eu and Zr than the BMG. The initial Sr isotopic ratios for four of the samples are nearly uniform regardless their petrology (BtG or BGM), showwing fairly evolved character of the parental magmas (87Sr/86Sr300 = 0.70922-0.70950) but sample BR 484 is even more radiogenic (87Sr/86Sr300 = 0.71290).
   The initial Epsilon Nd values are all highly negative (Epsilon Nd300 = -7.4 to -8.0; BR 484: Epsilon Nd300 = 9.2) and this is reflected by high two-stage Nd model ages (T Nd DM = 1.60 - 1.75 Ga). Both the Ševětín granites (BtG and BGM) are coeval; their Sr-Nd isotopic compositions and whole-rock geochemistry correpsond to a quartz-feldspathic (?metapsammitic) parentage or, more likely, may reflect a mixing between: (1) a relatively primitive component (having low time-integrated Rb/Sr and Sm/Nd ratios, with 87Sr/86Sri<0.705 and E Ndi = -7: e.g., undepleted or slightly enriched mantle derived melts or metabasic rocks) and (2) a material geochemically matching the mature Moldanubian metasedimentary rocks or their melts (87Sr/86Sr1 > 0.713 and E Ndi < -9.5). Both BtG and BMG can be linked by up to c. 10% of (nearly) closed system biotite-plagioclase fractional crystallization.
   The observed minor Nd isotopic heterogeneity could be explained by an influx of slightly isotopically and geochemically different melt batch(es) into periodically tapped and replenished magma chamber (RTF). The Ševětín granites are probably fairly late, ith indirect evidnece suggesting their age with Mauthausen Group in Austria (-300 Ma?). This is in line with occurrence of Ševětín granites next to late regional fault forming a part of the late Variscan Blanice Graben as well as comparable whole-rock geochemistry and Sr isotopic compositions of the Ševětín and Mauthausen granites (even though the Nd isotopic signatures of the two do differ profoundly). Moreover, the shallow instrusion level and rapid cooling are indicated also by the morphology of minute, long-prismatic zircon and apatite crystals and the Ab-Qz-Or normative plot
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
Entered date
    30. 5. 2007
Import date
    8. 8. 2012