A window into an older orogenic cycle: P–T conditions and timing of the pre‐Alpine history of the Dora‐Maira Massif (Western Alps)

Abstract

Deciphering the pre-orogenic evolution of subducted continental basement is challenging due to pervasive reworking of crust during subduction and exhumation. Survival of such polycyclic basement may occur locally in low strain domains bounded by intensely overprinted rocks. The Palaeozoic history of basement involved in Alpine continental subduction is investigated in the northern Dora-Maira Massif where a kilometre-scale domain of low strain preserves a pre-Alpine amphibolite-facies foliation in garnet-biotite orthogneiss and garnet-staurolite micaschist. By contrast, a first generation garnet is the only pre-Alpine relict in pervasively reworked domains surrounding the lowstrain domain. Thermodynamic modelling based on garnet isopleths in micaschist constrains the pre-Alpine pressure-temperature (P-T) evolution from 4 to 5 kbar and $500 C to 6-7 kbar and $650 C, which is consistent with Barrovian metamorphism up to the staurolite zone. In this micaschist, monazite included in garnet rims provide an age of 324 AE 6 Ma (95% confidence interval; c.i.). On the basis of textural and chemical data, this is interpreted as recording peak Barrovian metamorphic conditions. Low Th/U metamorphic zircon overgrowths and crystals yield an age of 304 AE 2 Ma (95% confidence interval). On the basis of the trace element concentrations and rare earth element (REE) patterns measured in garnet and metamorphic zircon, the latter is tentatively interpreted as having grown during early exhumation or cooling, involving garnet consumption and fluid infiltration. The reconstructed Variscan Barrovian metamorphism of the northern Dora-Maira basement is consistent with that documented in the External Crystalline Massifs and in the Austroalpine domain of the Alps. The Palaeozoic basement of the Dora-Maira Massif likely represents upper crustal material, later involved in Alpine continental subduction under high-to ultra-high-pressure conditions.

Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

“…A number of inherited ages have also been reported in the literature: Caledonian (457 ± 2 and 442 ± 2 Ma) and late Variscan (mainly Permian ∼304, 304 ± 3, 290 ± 2, 288 ± 2, 283–268, 280–267, and 268 ± 5 Ma) magmatic ages have been reported for orthogneisses and metagranites throughout the massif (Bussy & Cadoppi, 1996; Nosenzo et al., 2022; Paquette et al., 1999; Tilton et al., 1989; Xiong et al., 2021). In pyrope whiteschists, Tilton et al.…”

“…Assuming pressures of 0.3 GPa, Zr thermometry in Permian rutile and titanite yields temperatures of ∼660°C (Sanfront-Pinerolo unit), ∼640°C (RSU), and 510°C (RU)-whose variations might be a function of the distance to intrusion(s). These results are synthetized on Table 2 and Figure 9b. A consequence is that the continental crust exposed in Dora-Maira likely consists of relatively shallow portions of the Variscan orogen (there is no evidence of metamorphism deeper than 20 km, Ballèvre et al, 2020;Nosenzo et al, 2022), thinned and stretched by Permian post-collisional collapse and Jurassic rifting. The occurrence of mantle rocks, mafic magmatic rocks and overlying sediments within the stack (the OB) suggests that stretching was locally intense enough to exhume subcontinental mantle and generate limited amounts of magmatism.…”

“…Relics of HT minerals indicate that the Dora‐Maira massif experienced several stages of pre‐Alpine polymetamorphic evolution, largely overprinted by Alpine HP metamorphism. These stages imply Variscan regional metamorphism at amphibolite‐facies conditions, and Late Variscan contact metamorphism (e.g., Chopin et al., 1991; Compagnoni et al., 1995; Compagnoni & Rolfo, 2003; Groppo, Castelli, & Rolfo, 2007; Nosenzo et al., 2022; Sandrone et al., 1993; Vialon, 1966).…”

“…(2006); 15, Rubatto and Hermann (2001); 16, Nosenzo et al. (2022). A significant number of Ar‐Ar dates affected by excess argon were dismissed.…”

“…Detrital or metamorphic titanites as old as 253 Ma were found in the calcsilicates of the coesite‐bearing unit (Rubatto & Hermann, 2001). In Sanfront‐Pinerolo unit metasediments, the youngest detrital zircons have Carboniferous ages of 330 Ma (Manzotti et al., 2016), while this method yielded Ediacaran and Ordovician sedimentation ages (598 ± 2 and 453 ± 4 Ma) in the Muret unit (Nosenzo et al., 2022).…”