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Petrological and isotopic studies on palaeozoic high-pressure granulites, Gory Sowie Mts, Polish Sudetes

Abstract:

Granulites of intermediate to felsic composition were investigated from the Góry 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 feldspar-garnet-clinopymxene-quartz-kyanite-rutile, from which conditions of 900-1000°C, 15-20 kbar were deduced. A medium-pressure granulite fades overprint (775-910°C, 6·5-8·5 kbar) is documented with the growth of orthopymxene 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 vapour digestion and single grain evaporation techniques for two distinct zircon populations recognized in these samples. Detntal grains yielded 207Pb/206Pb ages between 452 ± 22 and 2154·7±1·4 Ma, whereas 19 analyses of grain fractions and single grains of multi-faceted zircons interpreted, from their characteristic spherical 'football' shape, as newly crystallized metamorphic grains, provided a mean 207Pb/296Pb age of 401·5± 0·9 Ma. Values of εNd determined for the time of metamorphism range fam -4 to -11, indicating highly heterogeneous protoliths. Nd model ages of 1·4-1·8 Ga reflect a mixture of Archaean and Proterozoic crustal material in the granulite protoliths as for other rocks of the Variscan basement. The zircon age is identical to a recently reported Sm-Nd mineral age of 402±3 Ma for garnet pmdotite associated with the granulites, and suggests formation of the metamorphic zircons at high-pressure granulite facies conditions. The age of ∼402 Ma is significantly older than ages determined for high-pressure felsic granulites from other parts of the Bohemian Massif and suggests (1) a long period of high temperatures at depth in a thickened crustal wedge, (2) growth of new zircon at different metamorphic stages (high-, medium- or low-pressure granulite facies) in different areas, or (3) spatially separated structural units that underwent the same anomalous high-pressure-high-temperature conditions at different times.