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Journal of Petrology Advance Access published online on January 7, 2005
Journal of Petrology, doi:10.1093/petrology/egh095
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Journal of Petrology © Oxford University Press 2005; all rights reserved
Received November 15, 2003
Accepted November 5, 2004

Article

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Ötztal-Stubai Crystalline Complex, Eastern Alps, Austria

JÜRGEN KONZETT 1*, CHRISTINE MILLER 1, RICHARD ARMSTRONG 2, and MARTIN THÖNI 3

1 INSTITUT FÜR MINERALOGIE UND PETROGRAPHIE, UNIVERSITÄT INNSBRUCK, INNRAIN 52, A-6020 INNSBRUCK, AUSTRIA
2 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA 0200, A.C.T., AUSTRALIA
3 INSTITUT FÜR GEOLOGISCHE WISSENSCHAFTEN, UNIVERSITÄT WIEN, GEOZENTRUM, ALTHANGASSE 14, A-1090 VIENNA, AUSTRIA

* To whom correspondence should be addressed.
JÜRGEN KONZETT, E-mail: juergen.konzett{at}uibk.ac.at


  Abstract

Olivine-rich rocks containing olivine + orthopyroxene + spinel + Ca-amphibole ± clinopyroxene ± garnet are present in the central Ötztal-Stubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet + clinopyroxene ± corundum. Protoliths of the olivine-rich rocks are thought to be olivine + orthopyroxene + spinel dominated cumulates generated from an already differentiated Fe-rich (XMgmelt = 0·57 - 0·65) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnet-rich rocks are interpreted as layers enriched in plagioclase and spinel intercalated in a cumulate rock sequence that is devoid of, or poor in, plagioclase. U-Pb sensitive high-resolution ion microprobe dating of zircons from a garnet layer indicates that emplacement of the cumulates took place no later than 517 ± 7 Myr ago. After their emplacement, the cumulates were subjected to progressive metamorphism, reaching eclogite-facies conditions around 800°C and >2 GPa during a Variscan metamorphic event between 350 and 360 Ma. Progressive high-P metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase + spinel to form garnet + clinopyroxene ± corundum in the garnet layers. Retrogressive metamorphism at T ≤ 650-680°C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, högbomite formed from corundum + spinel along with Al-rich spinel, Ca-amphibole, chlorite, aspidolite-preiswerkite, magnetite, ilmenite and apatite at the interface between olivine-rich rocks and garnet layers at P < 0·8 GPa. Progressive desiccation of retrogade fluids through crystallization of hydrous phases led to a local formation of saline brines in the garnet layers. The presence of these brines resulted in a late-stage formation of Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterized by extremely high Cl contents of up to 3·5 and 6·5 wt % Cl, respectively.

Keywords: cumulates; Variscan metamorphism; SHRIMP dating; högbomite; saline brines.
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