Whiteschist genesis through metasomatism and metamorphism in the Monte Rosa nappe (Western Alps)

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License: CC BY 4.0
Serval ID
serval:BIB_455CA1BEAA12
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Whiteschist genesis through metasomatism and metamorphism in the Monte Rosa nappe (Western Alps)
Journal
Contributions to Mineralogy and Petrology
Author(s)
Luisier Cindy, Baumgartner Lukas P., Putlitz Benita, Vennemann Torsten
ISSN
0010-7999
1432-0967
Publication state
Published
Issued date
01/2021
Peer-reviewed
Oui
Volume
176
Number
1
Language
english
Abstract
Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H2O and depletion of Na2O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K2O and SiO2, were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ18O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ18O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87Sr/86Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87Sr/86Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.
Keywords
Whiteschist, Metasomatism, Hydrothermal alteration, High-pressure metamorphism
Web of science
Open Access
Yes
Funding(s)
Swiss National Science Foundation / 206021_163991
University of Lausanne
Create date
18/11/2021 15:46
Last modification date
10/01/2023 6:50
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