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Journal of Petrology Advance Access published online on November 4, 2009
Journal of Petrology, doi:10.1093/petrology/egp059
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© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

The Role of Pyroxenites in Formation of Shear Instabilities in the Mantle: Evidence from an Ultramafic Ultramylonite, Twin Sisters Massif, Washington

Virginia G. Toy1,*, Julie Newman2, William Lamb2 and Basil Tikoff3

1Department of Geology, University of Otago, Po Box 56, Dunedin 9054, New Zealand
2Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115, USA
3Department of Geology and Geophysics, University of Wisconsin–Madison, 1215 W Dayton St., Madison, Wi 53706, USA

Received January 2, 2009; Revised typescript accepted August 11, 2009


  Abstract

In the Twin Sisters ultramafic massif, NW Washington, an ~0·5 cm thick, isolated ultramafic ultramylonitic shear zone displaces orthopyroxenite and clinopyroxenite dikes, by a minimum of 21 cm. The shear zone exists only adjacent to the orthopyroxenite and clinopyroxenite dikes, with deformation distributed along strike into the wall-rock less than 10 cm from the dikes in the outcrop face. Microstructurally, the shear zone contains domains of different grain sizes and phase proportions. A marginal domain of almost pure olivine displays a mean grain size of ~30–100 µm and an olivine lattice preferred orientation (LPO) indicating that glide occurred on (010)[100] and (010)[001]. A central domain of mixed olivine, pyroxene and amphibole displays a finer grain size, ranging down to ~5 µm. Aligned grain and phase boundaries and weak olivine and pyroxene LPOs indicate that this zone deformed by a grain-size-sensitive deformation mechanism (e.g. grain boundary sliding accompanied by diffusion creep). Geothermometry indicates that shearing occurred at temperatures in the range 650–750°C. We interpret the formation of this ultramylonitic shear zone as a shear instability caused by the presence of compositional heterogeneity. Localization was promoted by a deformation mechanism switch from dislocation creep to grain-size-sensitive creep as a result of syn-deformational grain-size reduction. Mineral compositions indicate that this grain-size reduction was associated with reaction. The fine grain size was stabilized by the presence of multiple phases, particularly pyroxene, within the central shear zone domain. The shear zone did not propagate beyond the vicinity of the pyroxenite dikes because the fine grain sizes necessary for the deformation mechanism switch to occur could not be maintained in the monophase olivine forming the surrounding dunite.

KEY WORDS: rheological heterogeneity; strain localization; grain-size reduction; grain-size-sensitive creep; pyroxene (010)-glide

*Corresponding author. Telephone: +64 3 479 7506. Fax: +64 3 479 7527. E-mail: virginia.toy{at}otago.ac.nz


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