PETROLOGICAL AND ISOTOPIC STUDIES ON PALEOZOIC HIGH-PRESSURE GRANULITES, GORY-SOWIE MTS, POLISH SUDETES

Granulites of intermediate to felsic composition were investigated fro m the Gory Sowie (Owl Mts) Massif of the central Sudetes of SW Poland. The strongly deformed granulites are in tectonic contact with ortho- and paragneisses that underwent polyphase deformation and metamorphism before the late Devonian. The metamorphic evolution is best exhibited by intermediate granulites that once contained the primary assemblage ternary ldspar-garnet-clinopyroxene-quartz-kyanite-rutile, from which conditions of 900-1000 degrees C, 15-20 kbar were deduced. A medium-p ressure granulite facies overprint (775-910 degrees C, 6.5-8.5 kbar) i s documented with the growth of orthopyroxene at the expense of garnet and clinopyroxene and the unmixing of the ternary feldspar to produce antiperthite. U-Pb and Pb-Pb isotopic ratios were measured by the vap our digestion and single grain evaporation techniques for two distinct zircon populations recognized in these samples. Detrital grains yield ed Pb-207/Pb-206 ages between 452+/-22 and 2154.7+/-1.4 Ma, whereas 19 analyses of grain fractions and single grains of multi-faceted zircon s interpreted, from their characteristic spherical 'football' shape, a s newly crystallized metamorphic grains, provided a mean Pb-207/Pb-206 age of 401.5+/-0.9 Ma. Values epsilon(Nd) determined for the time of metamorphism range from -4 to -11, indicating highly heterogeneous pro toliths. Nd model ages of 1.4-1.8 Ga reflect a mixture of Archaean and Proterozoic crystal material in the granulite protoliths as for other rocks of the Variscan basement. The zircon age is identical to a rece ntly reported Sm-Nd mineral age of 402+/-3 Ma for garnet peridotite as sociated with the granulites, and suggests formation of the metamorphi c zircons at high-pressure granulite facies conditions. The age of sim ilar to 402 Ma is significantly older than ages determined for high-pr essure felsic granulites from other parts of the Bohemian Massif and s uggests (1) a long period of high temperature at different in a thicke ned crystal wedge, (2) growth of new zircon at different metamorphic s tages (high-, medium- or low-pressure granulite facies) in different a reas, or (3) spatially separated structural units that underwent the s ame anomalous high-pressure-high-temperature conditions at different t imes.