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AGU: Tectonics

 

Keywords

  • ultrahigh-pressure metamorphism
  • eclogite
  • Alps
  • Pohorje

Index Terms

  • Planetology: Solid Surface Planets: Tectonics
  • Mineralogy and Petrology: Metamorphic petrology
  • Tectonophysics: Continental contractional orogenic belts
  • Information Related to Geographic Region: Europe
  • Information Related to Geologic Time: Mesozoic

Abstract

TECTONICS, VOL. 23, TC5014, 10 PP., 2004
doi:10.1029/2004TC001641

First evidence for ultrahigh-pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the Eastern Alps

Marian Janák

Geological Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic

Nikolaus Froitzheim

Geologisches Institut, Universität Bonn, Bonn, Germany

Branislav Lupták

Geological Institute, Slovak Academy of Sciences, Bratislava, Slovak Republic

Mirijam Vrabec

Department of Geology, University of Ljubljana, Ljubljana, Slovenia

Erling J. Krogh Ravna

Department of Geology, University of Tromsø, Tromsø, Norway

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Eastern Alps is reported from kyanite eclogites of the Pohorje Mountains in Slovenia. Polycrystalline quartz inclusions surrounded by radial fractures in garnet, omphacite, and kyanite are interpreted to be pseudomorphs after coesite. Abundant quartz rods and needles in omphacite indicate an exsolution from a preexisting supersilicic clinopyroxene that contained a Ca-Eskola component. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + phengite + quartz/or coesite yields peak pressure and temperature conditions of 3.0–3.1 GPa and 760°–825°C, well within the stability field of coesite, thus supporting the microtextural evidence for UHP metamorphism. This records the highest-pressure conditions of Eo-Alpine metamorphism during the Cretaceous orogeny in the Alps, implying a very deep subduction of the continental crust to at least 90–100 km depths. The new data are evidence for a regional southeastward increase of peak pressures in the Lower Central Austroalpine, indicating a south- to eastward dip of the subduction zone. Subduction was intracontinental; northwestern parts of the Austroalpine (Lower Central Austroalpine) were subducted under southeastern parts (Upper Central Austroalpine). The subduction zone formed in the Early Cretaceous in the northwestern foreland of the Meliata suture after Late Jurassic closure of the Meliata Ocean and the resulting collision, by a forward subduction shift to a Permian rift.

Received 25 February 2004; accepted 19 August 2004; published 20 October 2004.

Citation: Janák, M., N. Froitzheim, B. Lupták, M. Vrabec, and E. J. K. Ravna (2004), First evidence for ultrahigh-pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the Eastern Alps, Tectonics, 23, TC5014, doi:10.1029/2004TC001641.

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