Contributions to Mineralogy and Petrology

, Volume 159, Issue 3, pp 293–314 | Cite as

Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths: mechanisms and rates

  • Júlia DégiEmail author
  • Rainer Abart
  • Kálmán Török
  • Enikő Bali
  • Richard Wirth
  • Dieter Rhede
Original Paper


The complex microstructure of kelyphitic rims around garnet in lower crustal garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field, Central Pannonian Basin has been studied in order to identify controls on garnet breakdown. Symplectites comprised of a vermicular intergrowth of submicron sized anorthite, orthopyroxene and spinel replace garnet at a sharp reaction front. Based on element distribution maps the transformation of garnet to symplectite is isochemical. Phase diagram calculations indicate that this reaction was induced by a pressure decrease and/or a temperature increase. In site-specific TEM foils prepared by focused ion beam technique and oriented parallel and perpendicular to the reaction front 200 nm wide rods of anorthite and 20 nm wide rods of spinel are identified. The rods are oriented approximately perpendicular to the replacement front and are embedded in an orthopyroxene matrix. The regular spacing of the symplectite phases along the reaction front suggests that their growth is controlled by diffusion. The kinetics of symplectite formation has been modelled based on irreversible thermodynamics. During interaction of the xenolith with the host basalt the microstructure and chemistry of the An–Opx–Spl symplectite was significantly modified and it was partially replaced by an olivine bearing symplectite. In contrast to primary symplectite formation, these processes were metasomatic in nature including addition of sodium, titanium and some trace elements from the basaltic melt and can clearly be discerned from the garnet breakdown. Based on these observations it is inferred that symplectite formation took place within the deep crust during the extension of the Pannonian Basin between 15 and 30 km depth at high temperature (850–1,050°C) prior to the volcanic transport to the surface.


Reaction Front Pannonian Basin Mafic Granulite Host Basalt Garnet Granulite 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



The authors would like to thank the careful reviews of Prof. S. Chakraborty and an anonymus reviewer and the editorial work of Prof. J. Hoefs. This study was financed by an MTA-DFG scientific cooperation (DFG/184, AB 341/4-1), the Hungarian Scientific Research Fund (OTKA K 61182), the Eötvös Scholarship of the Republic of Hungary and a research scholarship of the DAAD (A/07/91759) donated to Júlia Dégi. The Bolyai scholarship to Kálmán Török from the Hungarian Academy of Sciences is also acknowledged.


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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Júlia Dégi
    • 1
    • 2
    Email author
  • Rainer Abart
    • 3
  • Kálmán Török
    • 4
    • 2
  • Enikő Bali
    • 5
  • Richard Wirth
    • 6
  • Dieter Rhede
    • 6
  1. 1.Metals Research DepartmentResearch Institute of Solid State Physics and Optics of the Hungarian Academy of SciencesBudapestHungary
  2. 2.Lithosphere Fluid Research GroupEötvös UniversityBudapestHungary
  3. 3.Freie Universität BerlinBerlinGermany
  4. 4.Eötvös Loránd Geophysical Institute of HungaryBudapestHungary
  5. 5.Bayerisches GeoinstitutUniversity of BayreuthBayreuthGermany
  6. 6.GFZ German Research Centre for GeosciencesHelmholtz Centre PotsdamPotsdamGermany

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