Early Carboniferous Rb-Sr mica-whole-rock ages of phases of medium- and low-pressure metamorphism in the Saxothuringian zone, western Bohemia

A Rb - Sr white mica - whole-rock age of 327 +/- 6 Ma for a muscovite - par agonite, chlorite schist in the eastern Fichtelgebirge, near Kraslice, is recorded and interpreted as the time of the D-2 - S-2 event at the peak of Barrovian-type dynamo-thermal metamorphism. A corresponding age of 327 +/- 5 Ma, obtained using the same isotopic system in another mica schist from the same district, is also recorded for a muscovite porphyroblast - whole-rock. The age is interpreted as the time of cooling through the closure temperature of muscovite following the overgrowth of randomly-disposed porphyroblasts and of paragonite-out reactions resulting from a low-pressure metamorphic overprint that had reset the Rb - Sr isotopic systems relatively late in the deformational sequence.

With the formation of three sets of thrusts that were major contributors to the tectonic stacking of the disparate crystalline units in the western part of the Bohemian Massif and of a number of sets of folds being between the two ca. 330-325 Ma metamorphic events in the deformational sequence, the timing of the Hercynian orogenic episode in the southern Fichtelgebirge tectonic domain is deduced to have straddled the Visean Namurian boundary. The new data together with the ca. 320 Ma age of emplacement of the earliest of the granites in the Fichtelgebirge indicate a relatively short time span for this late Early Carboniferous episode during which (1) compressional tectonism, expressed by the development of intrafolial folds acid schistosity during medium-pressure dynamothermal metamorphism, and then by sets of thrusts and upright folds, was followed by (2) a low-pressure metamorphic overprint and then by (3) further weak compressional (+/- transpressional) tectonism with the formation of upright folds and then a set of thrusts.