Deformation, mass transfer and mineral reactions in an eclogite facies shear zone in a polymetamorphic metapelite (Monte Rosa nappe, western Alps)

AbstractThe sizes, distributions and shapes of zircon grains within variably deformed granite gneiss from the western Alps have been studied. Zircon shows numerous indicators of a metamorphic response in both the host gneiss and a 5 cm wide continuous ductile shear zone, within which the zircon grain sizes range from <1 urn to >50 μm. However, the very fine grain sizes are virtually absent from grain boundaries. Within this zone, zircons consistently have more rounded and embayed margins, which are interpreted as evidence of dissolution in response to fluid influx during shearing. Zircons are preferentially located near metamorphic muscovite in both the host gneiss and the shear zone and tend to show the poorest crystal shape, indicating that fluids linked to the formation and presence of muscovite may enhance both the crystallization of zircon and its subsequent dissolution. Larger zircon crystals typically show a brittle response to deformation when adjacent to phyllosilicates, with fractures consistently perpendicular to the (001) mica cleavage. The variety of metamorphic behaviour observed for zircon indicates that it may be highly reactive in sub-solidus mid-crustal metamorphic environments.

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Coupling of oceanic and continental crust during Eocene eclogite-facies metamorphism: evidence from the Monte Rosa nappe, western Alps

Contributions to Mineralogy and Petrology ◽

10.1007/s00410-006-0144-x ◽

2006 ◽

Vol 153 (2) ◽

pp. 139-157 ◽

Cited By ~ 54

Author(s):

Thomas J. Lapen ◽

Clark M. Johnson ◽

Lukas P. Baumgartner ◽

Giorgio V. Dal Piaz ◽

Susanne Skora ◽

...

Keyword(s):

Continental Crust ◽

Western Alps ◽

Facies Metamorphism ◽

Oceanic And Continental Crust ◽

Eclogite Facies ◽

Monte Rosa

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Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Contributions to Mineralogy and Petrology ◽

10.1007/s00410-021-01806-4 ◽

2021 ◽

Vol 176 (7) ◽

Author(s):

Thomas Bovay ◽

Daniela Rubatto ◽

Pierre Lanari

Keyword(s):

Oxygen Isotope ◽

Western Alps ◽

Large Contribution ◽

Chemical Mapping ◽

Rock Interaction ◽

Rock Density ◽

Chemical Zoning ◽

Dehydration Reactions ◽

Subducted Oceanic Crust ◽

Eclogite Facies

AbstractDehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

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