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

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 low-strain domain. Thermodynamic modelling based on garnet isopleths in micaschist constrains the pre-Alpine pressure-temperature (P-T) evolution from 4 to 5 kbar and similar to 500 degrees C to 6-7 kbar and similar to 650 degrees 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 +/- 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 +/- 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.