METAPEGMATITES IN THE WESTERN BOHEMIAN MASSIF - AGES OF CRYSTALLIZATION 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 Bohemi an massif, the Zone of Erbendorf-Vohenstrauss in Germany (ZEV), and th e Zone of Tepla-Domazlice in the Czech Republic (ZTD) have been dated by means of U-Pb and Rb-Sr methods. Ages interpreted to reflect emplac ement and crystallisation of the pegmatites were found consistently to be approximately 480 Ma, as constrained by U-Pb analyses of primary m agmatic zircon, monazite, garnet and columbite, and by Rb-Sr analyses of large pegmatitic muscovites. Later Devonian amphibolite-facies meta morphism 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 f or both the ZEV and the ZTD, interpreted as dating the end of deformat ional activity. The Rb-Sr system of the large pegmatitic muscovites tu rned out to remain closed up to metamorphic temperatures of >600 degre es C. Deformation at elevated temperatures is identified as the domina nt mechanism for opening of the Rb-Sr system of primary muscovites: ap parent ages grade towards Devonian ages as the muscovites become more deformed and fragmented. The data derived from the metapegmatites poin t to a similar or common tectonometamorphic evolution for the ZEV and the ZTD since the Ordovician. Furthermore, the magmatic formation of g ranitic pegmatites implies an upper crustal position of the intruded r ocks in Lower Ordovician times, clearly documenting the existence of t wo distinct metamorphic cycles. In contrast to this, in the neighbouri ng part of the Moldanubian, only unmetamorphosed granitic pegmatites w ith intrusion ages around 320 Ma are observed.