| Description |
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 or fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, and major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorph: Zone I, coarse-grained (1 mm) aggregate of orthopyroxene + clinopyroxene + spinel with a granular structure; Zone II, fine- to medium-grained (order of 10-100 mu m) orthopyroxene + spinel symplectite; Zone III, fine-grained (5-300 mu m), radially fibrous orthopyroxene + spinel symplectite with interstitial anorthite; Zone IV, ultrafine-grained (1 mu m) orthopyroxene + spinel + anorthite symplectite with an internal domain substructure. 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 degrees C within the spinel peridotite stability field. Subsequently a short period of heating by similar to 100-250 degrees C led 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 or fluids suggests that this phase of garnet breakdown occurred largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with rapid, isochemical, decompression during exhumation, 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 in garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, metasomatism by Na- and carbonate-rich agents and the final garnet breakdown during the host basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.
|
