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-migmatite complex with minor amphibolites, calc-silicate rocks, ultrabasic rocks and granulites. Migmatitic gneisses underwent a complex polyphase history of deformation and metamorphism, each deformation phase (D1 to D5) being associated with synkinematic amphibolite-facies metamorphism (M1 to M5). For a better understanding of the P-T-t-deformation path three types of gneisses, which represent distinct stages of deformation 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 xenotime 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-migmatitic event in the Gory Sowie Block. Rb-Sr thin-slab and mica-whole-rock dating for samples with D2, D3 and D5 characteristics provide ages between 362 and 375 Ma which overlap within error (2 sigma). No correlation between deformational characteristics and apparent Rb-Sr ages was recognized. The Rb-Sr data are interpreted as time constraints for post-peak metamorphic cooling. The new geochronological data indicate the significance of Devonian high-temperature metamorphism in the Sudetes and support a similar interpretation of earlier studies. The isotopic results are also consistent with the interpretation, based on field, structural and petrological data, that at least four (D2-D5) structural episodes occurred in Devonian times within a very short time interval, the details of which cannot be further resolved with the methods used.