Petrology, geochemical character and petrogenesis of a Variscan post-orogenic granite: Case study from the Ševětín Massif, Moldanubian Batholith, Southern Bohemia
In 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, MgO, Al2O3 and CaO. The BtG is also characterized by higher contents 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 BMG), showing 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 (εNd300= -7.4 to -8.0; BR 484: εNd300= -9.2) and this is reflected by high two-stage Nd model ages (TNdDM=1.60-1.75 Ga).
Both the Ševětín granites (BtG and BMG) are coeval; their Sr-Nd isotopic compositions and whole-rock geochemistry correspond 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 εNdi = > -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/86Sri> 0.713 and εiNd< -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, with indirect evidence suggesting their age comparable with Mauthausen Group in Austria (~300 Ma?). This is in line with occurence 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 intrusion 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.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
Policy: Open Access