Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon Granulite Massif, Germany: P-T-t path and geotectonic implications
Rötzler, Jochen; Romer, Rolf L.; Budzinski, Hubertus; Oberhänsli, Roland
European Journal of Mineralogy Volume 16 Number 6 (2004), p. 917 - 937
published: Dec 28, 2004
ArtNo. ESP147051606006, Price: 29.00 €
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 m 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 (2σ). However, chemical inheritance from precursor rutile and post-peak Pb loss are also evident, suggesting a protolith age of 499±2 Ma (2σ) and partial resetting down to an age of 333±2 Ma (2σ). Rb-Sr mica ages of 333.2±3.3 Ma (2σ) 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.