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Metasomatic horizon sealing serpentinite-metasediments pair in the Zermatt-Saas metaophiolite (Northwestern Alps): record of a channel for focussed fluid flow during subduction

Metasomatic horizon sealing serpentinite-metasediments pair in the Zermatt-Saas metaophiolite (Northwestern Alps): record of a channel for focussed fluid flow during subduction
Metasomatic horizon sealing serpentinite-metasediments pair in the Zermatt-Saas metaophiolite (Northwestern Alps): record of a channel for focussed fluid flow during subduction
A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T=550-630 °C, P=1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e. serpentinite and carbonate-bearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg- H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling.
Italy, Jurassic Tethys, Northwestern Alps, Zermatt-Saas Zone, metaophiolites and metaophicarbonates, metasomatic rocks, subduction
0391-2612
1-25
Rotondo, Francesca Claudia
7e05d184-fd35-44db-9b29-b093aeabe708
Tartarotti, Paola
7b14ab6a-465b-4572-9990-23edacb4e308
Guerini, Sara
f2b10b34-a201-4392-9e8a-9619aa10b818
Della Porta, Giovanna
8270af96-cffa-45e0-9576-b3dad8086b01
Campomenosi, Nicola
e3e657bd-b1a7-4ae5-862e-65601990864b
Rotondo, Francesca Claudia
7e05d184-fd35-44db-9b29-b093aeabe708
Tartarotti, Paola
7b14ab6a-465b-4572-9990-23edacb4e308
Guerini, Sara
f2b10b34-a201-4392-9e8a-9619aa10b818
Della Porta, Giovanna
8270af96-cffa-45e0-9576-b3dad8086b01
Campomenosi, Nicola
e3e657bd-b1a7-4ae5-862e-65601990864b

Rotondo, Francesca Claudia, Tartarotti, Paola, Guerini, Sara, Della Porta, Giovanna and Campomenosi, Nicola (2021) Metasomatic horizon sealing serpentinite-metasediments pair in the Zermatt-Saas metaophiolite (Northwestern Alps): record of a channel for focussed fluid flow during subduction. Ofioliti, 46 (1), 1-25.

Record type: Article

Abstract

A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T=550-630 °C, P=1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e. serpentinite and carbonate-bearing metabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg- H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different fluids circulating, mixing and interacting with lithologies along slab-parallel discontinuities, which act as major fluid conduits for deep volatile recycling.

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More information

Accepted/In Press date: 17 September 2020
e-pub ahead of print date: 26 September 2020
Published date: 2021
Keywords: Italy, Jurassic Tethys, Northwestern Alps, Zermatt-Saas Zone, metaophiolites and metaophicarbonates, metasomatic rocks, subduction

Identifiers

Local EPrints ID: 444524
URI: http://eprints.soton.ac.uk/id/eprint/444524
ISSN: 0391-2612
PURE UUID: 0c441cb4-8eb5-4401-8915-e60019a5ae7a
ORCID for Francesca Claudia Rotondo: ORCID iD orcid.org/0000-0002-3922-9254

Catalogue record

Date deposited: 22 Oct 2020 16:33
Last modified: 26 Nov 2021 03:21

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Contributors

Author: Paola Tartarotti
Author: Sara Guerini
Author: Giovanna Della Porta
Author: Nicola Campomenosi

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