• Garnet Breakdown, Symplectite Formation and Melting in Basanite-hosted Peridotite Xenoliths from Zinst (Bavaria, Bohemian Massif)

    • P. Spacek
      Institute of Geophysics, Academy of Sciences of the Czech Republic, Boční II/1401, 14131 Prague, Czech Republic
    • L. Ackerman
      Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16500 Prague, Czech Republic; Institute of Earth Physics, Masaryk University, Tvrdého 12, 60200 Brno, Czech Republic
    • G. Habler
      Department für Lithosphärenforschung, Fakultät für Geowissenschaften, Geographie und Astronomie, Universität Wien
    • R. Abart
      Department für Lithosphärenforschung, Fakultät für Geowissenschaften, Geographie und Astronomie, Universität Wien
    • J. Ulrych
      Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16500 Prague, Czech Republic
  • Complex, symplectite-bearing pseudomorphs after garnet recently found in unique basanite-hosted peridotite xenoliths from Zinst, Bavaria, allow the study of the interaction between garnet peridotite and melts/fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorphs: I) coarse grained (≤ 1 mm) aggregate of orthopyroxene+clinopyroxene+spinel with a granular structure, II) fine- to medium grained (order of 10-100 µm) orthopyroxene+spinel symplectite, III) fine grained (5- 300 µm), radially fibrous orthopyroxene+spinel symplectite with interstitial anorthite, and, IV) ultrafine grained (≤ 1 µm) orthopyroxene+spinel+anorthite symplectite with an internal domain sub-structure. Zones III and IV have bulk compositions of pyrope rich garnet. All zones exhibit perfect inter-sample correlation and document the discontinuous evolution of peridotite under changing conditions with successively increasing rates of garnet breakdown. Based on thermometry and microstructural relations, a sequence of three pre- and syn-volcanic events is discerned: The first traceable event corresponds to regional heating in the uppermost mantle probably related to the early stages of Tertiary rifting, which triggered the reaction between garnet and olivine (Zone I) leading to a partial re-equilibration of the rock at 1040-1080°C within the spinel peridotite stability field. Subsequently a short period of heating by ~100-250°C lead to largely isochemical, fluid-mediated in situ melting of garnet and to the formation of kelyphite by crystallization from the melt (Zone III). The subsequent metasomatic alteration by external, Na-rich, K-poor, carbonate-bearing melts/fluids suggests that this phase of garnet breakdown occured largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with the rapid, isochemical, decompression during ascent, the garnet relics were transformed into microsymplectite (Zone IV). The positive volume change associated with this reaction caused fracturing, producing radial cracks that emanate from Zone IV and extend into the adjacent peridotite allowing infiltration of basanite-derived melt components. The well developed and clearly separated symplectite zones indicating the isochemical breakdown of garnet are uncommon to garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, Na-carbonate metasomatism and the final garnet breakdown on the basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.

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  • http://phaidra.univie.ac.at/o:406675

  • Wissenschaftlicher Artikel

  • Angenommene Version

  • Journal of Petrology

  • 2013

  • 54

  • 8

  • 1691-1723

  • Oxford University Press (OUP)

  • Englisch

  • Embargo

  • 01.06.2014

  • I471-N19 – Fonds zur Förderung der wissenschaftlichen Forschung (FWF)

  • 0022-3530

  • garnet; symplectite; kelyphite; xenolith; melting; metasomatism; lithospheric mantle; Eger Rift; Bohemian Massif

  • Dewey Dezimal Klassifikation → Naturwissenschaften und Mathematik → Geowissenschaften, Geologie → Petrology