Peak Alpine metamorphic conditions from staurolite bearing metapelites in the Monte Rosa nappe (Central European Alps) and geodynamic implications
Résumé
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 tectonometamorphic evolution. However, the reported peak Alpine P and T 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 P and T 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 P of 1.6 ± 0.2 GPa and peak T of 585 20 C. These peak P estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak P obtained from talc, chloritoid, phengite and quartz bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine P 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 P for metapelite and metagranite assemblages represents the regional peak P 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 P 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 P estimates >2.2 GPa for whiteschist lithologies.
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