Metapegmatites in the western Bohemian massif: ages of crystallisation and metamorphic overprint, as constrained by U–Pb zircon, monazite, garnet, columbite and Rb–Sr muscovite data

Granitic metapegmatites of two crystalline units of the western Bohemian massif, the Zone of Erbendorf-Vohenstrauss in Germany (ZEV), and the Zone of Tepla-Domažlice in the Czech Republic (ZTD) have been dated by means of U–Pb and Rb–Sr methods. Ages interpreted to reflect emplacement and crystallisation of the pegmatites were found consistently to be approximately 480 Ma, as constrained by U–Pb analyses of primary magmatic zircon, monazite, garnet and columbite, and by Rb–Sr analyses of large pegmatitic muscovites. Later Devonian amphibolite-facies metamorphism caused ductile shearing of the pegmatites, leading to partial recrystallisation of pegmatitic material. A metamorphic, fine-grained generation of muscovite yielded consistent Rb–Sr ages of 371–376 Ma for both the ZEV and the ZTD, interpreted as dating the end of deformational activity. The Rb–Sr system of the large pegmatitic muscovites turned out to remain closed up to metamorphic temperatures of >600 °C. Deformation at elevated temperatures is identified as the dominant mechanism for opening of the Rb–Sr system of primary muscovites: apparent ages grade towards Devonian ages as the muscovites become more deformed and fragmented. The data derived from the metapegmatites point to a similar or common tectonometamorphic evolution for the ZEV and the ZTD since the Ordovician. Furthermore, the magmatic formation of granitic pegmatites implies an upper crustal position of the intruded rocks in Lower Ordovician times, clearly documenting the existence of two distinct metamorphic cycles. In contrast to this, in the neighbouring part of the Moldanubian, only unmetamorphosed granitic pegmatites with intrusion ages around 320 Ma are observed.