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Inherited Fabric in an Omphacite Symplectite: Reconstruction of Plastic Deformation under Ultra-High Pressure Conditions

Published online by Cambridge University Press:  13 May 2013

Florian Heidelbach  and
Michael P. Terry
Florian Heidelbach*
Affiliation:
Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
Michael P. Terry
Affiliation:
Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD 57701, USA
*
*Corresponding author. E-mail: Florian.Heidelbach@uni-bayreuth.de
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Abstract

We investigated an eclogitic gneiss from the Western Gneiss Region in Norway, which underwent subduction as part of Baltica lithosphere beneath Laurentia during the Scandian orogeny. Petrologic data indicate that the eclogite was deformed plastically at about 4 GPa and 800°C producing a strong macroscopic foliation and lineation. Whereas garnet remained largely stable during the retrograde uplift, omphacite was transformed statically into a symplectite consisting of lamellar diopside and plagioclase with more equant grains of hornblende and orthopyroxene. Measurements of the crystallographic preferred orientation with electron backscatter diffraction show that diopside and hornblende, as well as orthopyroxene, have a systematic orientation relationship with the macroscopic fabric, as well as the (presumed) orientation of the host omphacite. The orientation relationship between the chain silicates is very sharp with the crystallographic forms {100}, {010}, and ⟨001⟩ being parallel. Their bulk texture shows a maximum of ⟨001⟩ parallel to the lineation and girdles of {010} and {110} perpendicular to the lineation with maxima subparallel to the foliation corresponding to an L-type texture of the original omphacite and indicating constrictional strain with an additional component of pure shear/simple shear component.

Keywords

symplectiteelectron backscatter diffractiontopotaxytexturesdeformationhigh pressure
Type
EBSD Special Section
Information
Microscopy and Microanalysis , Volume 19 , Issue 4 , August 2013 , pp. 942 - 949
Copyright
Copyright © Microscopy Society of America 2013 

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