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Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon Granulite Massif, Germany

¹ Institut für Geowissenschaften, Universität Potsdam, Postfach 60 15 53, D-14415 Potsdam, Germany
² GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
³ Hildegrimstrasse 8, D-45239 Essen, Germany

^* Corresponding author, e-mail: roetzler{at}geo.uni-potsdam.de

The Saxon granulites, the type granulite locality, were deeply buried, extremely heated and then rapidly exhumed during the Variscan Orogeny; thus their evolution differs from many granulites elsewhere. The peak-metamorphic assemblages of layered felsic-mafic granulites from a 500 in deep borehole consist of garnet, kyanite, rutile, ternary feldspar and quartz in felsic granulite, and garnet, omphacite, titanite, ternary feldspar and quartz in mafic granulite. A minimum temperature of 1000–1020°C, calculated from reintegrated hypersolvus feldspar in felsic and mafic granulites, is consistent with the highest temperature estimates from garnet-clinopyroxene equilibria. Various equilibria in felsic and mafic granulites record a peak pressure of about 23 kbar. Diffusion zoning and local homogenisation of minerals reflect near-isothermal decompression that preceded cooling and partial hydration at medium- to low-pressure. U-Pb dating of titanite yields an age of peak metamorphism at 340.7±0.8 Ma (2). However, chemical inheritance from precursor rutile and post-peak Pb loss are also evident, suggesting a protolith age of 499±2 Ma (2) and partial resetting down to an age of 333±2 Ma (2). Rb-Sr mica ages of 333.2±3.3 Ma (2) are interpreted as dating cooling through about 620°C. Hence the Saxon granulites were exhumed to the upper crust during the short period of 6–11 Ma, which corresponds to average exhumation and cooling rates of 10 mm/year and 50°C/Ma, respectively. Such rapid exhumation is inconsistent with recent numerical models that assume foreland-directed transport of the Saxon granulites in the lower crust followed by extensional unroofing. Instead, high-pressure rocks of the Saxon Granulite Massif and the nearby Erzgebirge experienced a buoyant rise to the middle crust and subsequent juxtaposition with structurally higher units along a series of medium- to low-pressure detachment faults.

Key-words: high-pressure granulite, ultrahigh-temperature metamorphism, U-Pb titanite geochronology, exhumation, Saxon Granulite Massif.

This article has been cited by other articles:

				J. Rotzler, B. Hagen, and S. Hoernes Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part I: New evidence from key mineral assemblages and reaction textures European Journal of Mineralogy, December 1, 2008; 20(6): 1097 - 1115. [Abstract] [Full Text] [PDF]

				B. Hagen, S. Hoernes, and J. Rotzler Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part II: Thermal peak conditions and cooling rates inferred from oxygen-isotope fractionations European Journal of Mineralogy, December 1, 2008; 20(6): 1117 - 1133. [Abstract] [Full Text] [PDF]

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