Metamorphic peak estimates of the Marguareis Unit (Briançonnais Domain): New constrains for the tectonic evolution of the south‐western Alps

Abstract

It was widely recognized that quantitative pressure-temperaturetime (P-T-t) paths extracted from metamorphic units portray the thermo-mechanical evolutions of orogens (e.g. England and England & Thompson, 1984;England & Richardson, 1977). The latter are usually reconstructed by means of the P-T conditions of highgrade units. However, the reconstruction of P-T paths, especially high-pressure metamorphism, in low-to very low-grade metamorphic rocks, is of paramount importance to better understand the geodynamic evolution of orogens. Indeed, under low-temperature condition reaction, kinetics is sluggish and consequently chemical equilibrium is rarely achieved at the whole-rock scale impeding to unambiguously constrain P-T conditions of metamorphism (Frey & Robinson, 1999;Lanari & Engi, 2017). It is all the more difficult to determine P-T conditions in meta-sediments in which detrital minerals remain as relict phases, so making the so-called reactive rock composition (Lanari & Engi, 2017 and reference therein), to be taken into account in order to model the chemical equilibrium, still questionable. This is the case of the very low-grade Marguareis Unit (Briançonnais Domain), a European fragment of continental crust (Decarlis et al., 2013), located at the boundary between Maritime and Ligurian Alps. This unit was involved in the Alpine collision since middle Eocene age (Seno et al., 2005). The deformation history of the Marguareis Unit has been deeply investigated (Brizio et al., 1983;Sanità et al., 2021), whereas the reconstruction of metamorphic conditions remains partly unconstrained and interpreted as the result of very shallow tectonic events (anchizone, Messiga et al., 1981).

Subduction signature in the Internal Ligurian units (Northern Apennine, Italy): Evidence from P–T metamorphic peak estimate

“…Black circles along the colored lines indicate the activity of the water (aH 2 O). Given the presence of calcite, water activity was set to 0.8 for all the samples (e.g., Di Rosa et al., 2020; Sanità et al., 2022b). Crosses indicate the P – T equilibria conditions of a single representative chlorite–phengite couple estimated with the Chlorite‐Phengite‐quartz–water multiequilibrium approach (Vidal & Parra, 2000).…”

“…The IL units as part of a pre-Oligocene subduction system allows us to discuss their evolution in the bigger context of the Alps-Apennine system and geodynamic evolution. The Western Alps, located west of the IL units, feature several ophiolite-bearing units, from the westernmost Moglio-Testico unit (Sanità et al, 2022a), through the Montenotte unit (Federico et al, 2015;Seno et al, 2005) Given the presence of calcite, water activity was set to 0.8 for all the samples (e.g., Di Rosa et al, 2020;Sanità et al, 2022b). Crosses indicate the P-T equilibria conditions of a single representative chlorite-phengite couple estimated with the Chlorite-Phengite-quartzwater multiequilibrium approach (Vidal & Parra, 2000).…”