Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths

Mechanisms and rates

Júlia Dégi, Rainer Abart, K. Török, Eniko Bali, Richard Wirth, Dieter Rhede

Research output: Contribution to journalArticle

35 Citations (Scopus)

Abstract

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 nmwide 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.

Original languageEnglish
Pages (from-to)293-314
Number of pages22
JournalContributions to Mineralogy and Petrology
Volume159
Issue number3
DOIs
Publication statusPublished - 2010

Fingerprint

symplectite
pressure reduction
Garnets
granulite
decompression
garnets
garnet
breakdown
rods
anorthite
spinel
volcanology
orthopyroxene
Bearings (structural)
microstructure
highlands
Microstructure
Focused ion beams
Trace Elements
Titanium

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths : Mechanisms and rates. / Dégi, Júlia; Abart, Rainer; Török, K.; Bali, Eniko; Wirth, Richard; Rhede, Dieter.

In: Contributions to Mineralogy and Petrology, Vol. 159, No. 3, 2010, p. 293-314.

Research output: Contribution to journalArticle

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