P?-T-t history of the Lower Austroalpine Nappe Complex in the “Tarntaler Berge” NW of the Tauern Window: implications for the geotectonic evolution of the central Eastern Alps |
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Authors: | Christian Dingeldey R David Dallmeyer Friedrich Koller Hans-Joachim Massonne |
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Institution: | (1) Institut fuer Petrologie, Universitaet Wien, Althanstrasse 14, A-1090 Vienna, Austria, AT;(2) Department of Geology, University of Georgia, Athens, GA 30602, USA, GE;(3) Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany, DE |
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Abstract: | New petrologic and 40Ar/39Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The
P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed
above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence.
Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner
Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures
around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism
and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer
sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression.
Whole-rock 40Ar/39Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe,
and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely
the high-pressure metamorphism.
The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented
active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais)
Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the
Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner
and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area
were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to
final continental collision.
Received: 30 July 1996 / Accepted: 7 April 1997 |
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