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Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon granulite massif, Germany; P-T-t path and geotectonic implications

Jochen Roetzler, Rolf L. Romer, Hubertus Budzinski and Roland Oberhansli
Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon granulite massif, Germany; P-T-t path and geotectonic implications
European Journal of Mineralogy (December 2004) 16 (6): 917-937

Abstract

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 degrees 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+ or -0.8 Ma (2sigma ). However, chemical inheritance from precursor rutile and post-peak Pb loss are also evident, suggesting a protolith age of 499+ or -2 Ma (2sigma ) and partial resetting down to an age of 333+ or -2 Ma (2sigma ). Rb-Sr mica ages of 333.2+ or -3.3 Ma (2sigma ) are interpreted as dating cooling through about 620 degrees 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 degrees C/Ma, respectively. Such rapid exhumation is inconsistent with recent numerical models that assume foreland-directed transport of the Saxon granulities 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 16
Serial Issue: 6
Title: Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon granulite massif, Germany; P-T-t path and geotectonic implications
Affiliation: Universitaet Potsdam, Institut fuer Geowissenschaften, Potsdam, Federal Republic of Germany
Pages: 917-937
Published: 200412
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 48
Accession Number: 2005-023462
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 11 tables, sketch map
N50°22'00" - N52°30'00", E11°00'00" - E15°04'00"
Secondary Affiliation: GeoForschungsZentrum Potsdam, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200512
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