Time of formation and peak of Variscan HP-HT metamorphism of quartz-feldspar rocks in the central Erzgebirge,Saxony, Germany |
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| Authors: | Kröner A Willner A P |
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| Institution: | (1) Institut für Geowissenschaften, Universit?t Mainz, D-55099 Mainz, Germany E-mail: kroener@mail.uni-mainz.de, DE;(2) Institut für Mineralogie, Ruhr-Universit?t, D-44780 Bochum, Germany E-mail: arne.willner@rz.ruhr-uni-bochum.de, DE |
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| Abstract: | The Variscan Erzgebirge represents an antiform with a core of gneisses and mica schists, surrounded by a phyllitic mantle.
The Gneiss-Eclogite Unit (GEU), in the central part, is a composite tectonometamorphic assemblage characterized by a HP-HT
imprint and comprises migmatitic para- and orthogneisses, HT mylonites, HP granulites, eclogites and garnet peridotites. It
is tectonically sandwiched between two major units with distinctly lower PT histories. The GEU experienced a characteristic “kinked” retrograde PT path after HP-HT equilibration with: (1) strong near-isothermal decompression at high temperatures; (2) extensive re-equilibration
at medium pressures, followed (3) by rapid cooling during continued uplift. We dated zircons (Pb-Pb evaporation) from granitoid
orthogneisses and metapelites of the GEU. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin that
provided protolith ages between 470 and 524 Ma. Metapelites retain well-preserved granulite-facies mineral assemblages and
contain spherical, multifaceted metamorphic zircons that grew near the peak of HP/HT metamorphism. Inclusions of prograde
HP phengite (∼15 kbar) and rutile are included in one such zircon. Metamorphic zircons of three samples from different localities
yielded identical 207Pb/206Pb ages of 340.5 ± 0.7 Ma, 341.2 ± 0.5 Ma and 341.6 ± 0.5 Ma respectively. Consideration of these zircon ages with published
39Ar/40Ar white mica ages suggests fast cooling and uplift rates in excess of 50 °C/Ma and 4 km/Ma. This is typical for large-scale
extensional tectonic unroofing of the ultra-deep part of a fossil, thickened Variscan continental crust (>60 km) during continuing
continental collision and orogenic collapse.
Received: 5 June 1997 / Accepted: 7 January 1998 |
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