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

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.

Interplay between fluid circulation and Alpine metamorphism in the Monte Rosa whiteschist from white mica and quartz in situ oxygen isotope analysis by SIMS

“…The whiteschists are metasomatic alteration products of the Monte Rosa granite. The metasomatism occurred pre-Alpine, likely during late hydrothermal alteration of the cooling granite intrusion (Luisier et al, 2021;Pawlig and Baumgartner, 2001). Subsequently, the mineralogy was produced during Alpine high-pressure metamorphism (Luisier et al, 2019;Marger et al, 2019).…”

“…Deformation is concentrated in high strain areas, such as thin shear zones as observed for example in the metagranite around the studied whiteschist body (Vaughan-Hammon et al, 2021). Here, we focus on the whiteschist body described in detail by Luisier et al (2021), which is ellipsoidal. It is a cross-section through a whiteschist pipe (Luisier et al, 2021(Luisier et al, , 2019.…”

“…The white mica standards are applied to the study of whiteschists from the Monte Rosa nappe (Western European Alps), which have been the focus of numerous studies (Chopin and Monié, 1984;Le Bayon et al, 2006;Luisier et al, 2021Luisier et al, , 2019Marger et al, 2019). Whiteschists have been defined by Schreyer (1973) as high-pressure, Mg and Al-rich rocks, in which the mineral association talc -kyanite or talc -chloritoid is commonly found (Chopin, 1985;Schreyer, 1974).…”

“…The origin of whiteschists in the Alps is still debated (Ferrando, 2012). While alteration by ultramafic derived fluids during prograde to peak Alpine conditions were proposed for the Dora-Maira whiteschist (see Ferrando, 2012 for a review), the genesis of the Monte Rosa whiteschists has been explained by metasomatic alteration of the granite by late magmatic hydrothermal fluids (Luisier et al, 2021;Pawlig and Baumgartner, 2001). Luisier et al (2021) have demonstrated that the chemistry of the whiteschist was acquired before the onset of Alpine metamorphism.…”

“…While alteration by ultramafic derived fluids during prograde to peak Alpine conditions were proposed for the Dora-Maira whiteschist (see Ferrando, 2012 for a review), the genesis of the Monte Rosa whiteschists has been explained by metasomatic alteration of the granite by late magmatic hydrothermal fluids (Luisier et al, 2021;Pawlig and Baumgartner, 2001). Luisier et al (2021) have demonstrated that the chemistry of the whiteschist was acquired before the onset of Alpine metamorphism. The mineral assemblages now observable in the whiteschist and metagranite result from a combination of Alpine metamorphism and the deformation relative to the nappe emplacement and exhumation.…”