Peak Alpine metamorphic conditions from staurolite‐bearing metapelites in the Monte Rosa nappe (Central European Alps) and geodynamic implications

Abstract

The tectono‐metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectono‐metamorphic evolution. However, the reported peak Alpine pressure and temperature estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640°C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite–chloritoid‐bearing metapelitic assemblages. These assemblages exhibit an Alpine high‐P metamorphic overprint of a former contact‐metamorphic mineral assemblage generated by post‐Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe‐ and Mg‐staurolite end‐members, not considered in any thermodynamic database. We employ two end‐member mixing models for Zn in staurolite, site mixing, and molecular mixing. Both models enlarge the pressure and temperature stability range for the observed assemblage, where site mixing has the largest influence of ±0.2 GPa and ±20°C. Our results for three metapelite assemblages, with and without staurolite, indicate peak Alpine pressure of 1.6 ± 0.2 GPa and peak temperature of 585 ± 20°C. These peak pressure estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak pressure obtained from talc‐, chloritoid‐, phengite‐, and quartz‐bearing lithologies termed ‘whiteschists’ (>2.2 GPa). Our results confirm a variation of peak Alpine pressure of 0.6 ± 0.2 GPa between metagranite/metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic mélange. We suggest that the consistent peak pressure for metapelite and metagranite assemblages represents the regional peak pressure and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction‐induced stresses. If the calculated pressure of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak pressure estimates > 2.2 GPa for whiteschist lithologies.

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

“…DOI: https://doi.org/10.2138/am-2020-7523. http://www.minsocam.org/ conditions from the literature (500-575 °C; Chopin and Monié, 1984;Le Bayon et al, 2006;Luisier et al, 2019;Vaughan-Hammon et al, 2021). Slightly lower oxygen isotope fractionation temperatures compared to the peak temperature conditions from the literature could be explained by re-equilibration by diffusion during post-peak, retrograde cooling.…”

“…http://www.minsocam.org/ the antigorite breakdown reaction in a P-T diagram (Kerrick and Connolly, 2001;Ulmer and Trommsdorff, 1995). Nevertheless, this reaction occurs at significantly higher temperatures than the proposed temperature-pressure path for the Monte Rosa nappe (Luisier et al, 2019;Vaughan-Hammon et al, 2021), so that the fluids would either have traveled a large distance through the nappe stack from hotter parts of the orogen, or that extraneous fluids (surface fluids for example) would have equilibrated with the local serpentinite stack during exhumation (e.g., Barnes et al, 2004;Philippot and Selverstone, 1991). Hence, while it is not possible to exclude this model, it seems not very likely.…”

“…Large volumes of rocks were very little deformed during the nappe formation and exhumation. 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.…”

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy

“…The Monte Rosa basement exposed at the cirque du Véraz documents variable metamorphic conditions, specifically during peak Alpine high-pressure metamorphism. These variable metamorphic conditions were highlighted by discussions on peak pressure (P) estimates (Luisier et al, 2019 ; Vaughan-Hammon et al, 2021 ), but also published peak temperature (T) estimates are highly variable. A wide range of calculated peak metamorphic conditions has prompted the discussion concerning variations in estimated peak pressures in the Monte Rosa nappe.…”

“…In the cirque du Véraz, recent work has highlighted large disparities in peak Alpine P between whiteschist (ca. 2.2 GPa) and Monte Rosa metagranite (1.4 to 1.6 GPa) (Luisier et al ( 2019 ) as well as metapelitic (1.6 ± 0.2 GPa) assemblages (Vaughan-Hammon et al, 2021 ), resulting in a potential P difference, ∆P, of 0.6 ± 0.2 GPa for the same peak Alpine metamorphic event.…”