Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

Bovay, Thomas; Lanari, Pierre; Rubatto, Daniela; Smit, Matthijs; Piccoli, Francesca (2021). Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland). Journal of metamorphic geology, 40(2), pp. 175-206. Wiley 10.1111/jmg.12623

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Collisional orogens commonly include mono-metamorphic and poly-metamorphic units, and their different evolution can be difficult to recognise and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt-Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels and chloritoid-schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling and Lu-Hf garnet dating. The data reveal a coherent mono-metamorphic history with a β-shape Pressure-Temperature (P–T) path characteristic of oceanic subduction. Garnet Lu-Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (± 0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15 °C and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15 °C were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Myr. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid-rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K-white mica. Initial exhumation was followed by re-heating of ~30°C at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction-zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Bovay, Thomas; Lanari, Pierre; Rubatto, Daniela and Piccoli, Francesca

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

1525-1314

Publisher:

Wiley

Funders:

[4] Swiss National Science Foundation

Projects:

Projects 166280 not found.

Language:

English

Submitter:

Daniela Rubatto

Date Deposited:

12 Jan 2022 10:21

Last Modified:

27 Feb 2022 01:52

Publisher DOI:

10.1111/jmg.12623

BORIS DOI:

10.48350/162184

URI:

https://boris.unibe.ch/id/eprint/162184

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