Skip Navigation


Journal of Petrology Advance Access originally published online on July 7, 2009
Journal of Petrology 2009 50(8):1405-1442; doi:10.1093/petrology/egp035
This Article
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
50/8/1405    most recent
egp035v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Bucher, K.
Right arrow Articles by Grapes, R.
GeoRef
Right arrow GeoRef Citation
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

The Eclogite-facies Allalin Gabbro of the Zermatt–Saas Ophiolite, Western Alps: a Record of Subduction Zone Hydration

Kurt Bucher1,* and Rodney Grapes2

1Mineralogisch-Geochemisches Institut, University of Freiburg, D-79104, Freiburg, Germany
2Department of Earth and Environmental Sciences, Korea University, Seoul, South Korea

RECEIVED SEPTEMBER 3, 2007; ACCEPTED MAY 20, 2009


  Abstract

The Allalin gabbro is a 2 km x 0·5 km block of layered olivine-gabbro and troctolite included in the Zermatt–Saas ophiolite nappe of the Western Alps. Comprehensive texture, mineral and rock composition data together with a thermodynamic analysis of the complex phase associations permit a detailed reconstruction of the igneous and metamorphic reaction history recorded by the gabbroic rocks. Based on rock and mineral composition data, the Allalin gabbro represents part of a Middle Jurassic underplate of mafic magma at the base of the continental crust of the Apulian plate (Dent Blanche–Sesia Lanzo system). Granulite-facies recrystallization during cooling at ~825°C and 1·0 GPa, involving the formation of Opx–Grt coronas between Ol–Pl, can be related to crustal thickening. Eocene subduction of the Tethys oceanic lithosphere under the Apulian plate detached the gabbro from the base of the continental crust and incorporated it into the ophiolite. Increasing pressure in the descending slab had little effect on parts of the gabbro, which still locally contain unaltered igneous Ol, Aug and Pl and well-preserved magmatic textures. With increasing subduction depth an increasing amount of aqueous fluid accessed the gabbro and transformed Pl to Zo–Jd–Ky–Qtz. At about 2·5 GPa (93 km) at c. 610°C, a dramatic hydration process converted most of the rocks (>90 vol. %) into a fully hydrated eclogite-facies assemblage of Omp + Zo + Tlc + Cld ± Grt ± Ky + Rt. The full hydration under water-present conditions occurred at the greatest depth reached by the gabbro. After detachment from the downgoing slab (i.e. along the ascent path), Gln, Pg and Mrg formed as additional hydrates. This last phase of hydration desiccated the metagabbro at a depth of c. 78 km and from then on the rocks were essentially devoid of a free fluid phase. The Allalin gabbro provides strong evidence that the fundamental high-pressure transformation of mafic rocks involves the hydration of gabbro to form eclogite.

KEY WORDS: Allalin gabbro; gabbro–eclogite transformation; HP–LT metamorphism; mineral replacement textures; subduction

* Corresponding author. E-mail: bucher{at}uni-freiburg.de


Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?




Disclaimer: Please note that abstracts for content published before 1996 were created through digital scanning and may therefore not exactly replicate the text of the original print issues. All efforts have been made to ensure accuracy, but the Publisher will not be held responsible for any remaining inaccuracies. If you require any further clarification, please contact our Customer Services Department.