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

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License: CC BY-NC 4.0
Serval ID
serval:BIB_CE7B945A67F0
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)
Journal
Journal of Metamorphic Geology
Author(s)
Bovay Thomas, Lanari Pierre, Rubatto Daniela, Smit Matthijs, Piccoli Francesca, Baldwin Julia
ISSN
0263-4929
1525-1314
Publication state
Published
Issued date
17/07/2021
Language
english
Abstract
Collisional orogens commonly include mono-metamorphic and poly-metamorphic units, and their different evolution can be difficult to recognize 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-shaped 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℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ 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 Ma. 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℃ 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.
Keywords
Geology, Geochemistry and Petrology
Web of science
Open Access
Yes
Funding(s)
Swiss National Science Foundation
Swiss National Science Foundation
Create date
30/07/2021 11:08
Last modification date
05/08/2021 7:12
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