RB-SR AND U-PB GEOCHRONOLOGY OF MIGMATITIC GNEISSES FROM THE GORY SOWIE (WEST SUDETES, POLAND) - THE IMPORTANCE OF MID-LATE DEVONIAN METAMORPHISM

The Gory Sowie Block, West Sudetes, SW Poland consists of a gneiss-mig matite complex with minor amphibolites, calc-silicate rocks, ultrabasi c rocks and granulites. Migmatitic gneisses underwent a complex polyph ase history of deformation and metamorphism, each deformation phase (D 1 to D5) being associated with synkinematic amphibolite-facies metamor phism (M1 to M5). For a better understanding of the P-T-t-deformation path three types of gneisses, which represent distinct stages of defor mation and metamorphism (D2-M2, D3-M3 and D5-M5), have been studied by means of the Rb-Sr and U-Pb isotopic systems. U-Pb monazite and xenot ime ages of c. 380 Ma for rocks migmatized and deformed during the D3- M3 and D5-M5 stages constrain the timing of the last metamorphic-migma titic event in the Gory Sowie Block. Rb-Sr thin-slab and mica-whole-ro ck dating for samples with D2, D3 and D5 characteristics provide ages between 362 and 375 Ma which overlap within error (2 sigma). No correl ation between deformational characteristics and apparent Rb-Sr ages wa s recognized. The Rb-Sr data are interpreted as time constraints for p ost-peak metamorphic cooling. The new geochronological data indicate t he significance of Devonian high-temperature metamorphism in the Sudet es and support a similar interpretation of earlier studies. The isotop ic results are also consistent with the interpretation, based on field , structural and petrological data, that at least four (D2-D5) structu ral episodes occurred in Devonian times within a very short time inter val, the details of which cannot be further resolved with the methods used.