The Dora Maira Massif in the Western Alps is a key example of continental-affinity rocks metamorphosed under (ultra-)high-pressure/low-temperature ((U)HP-LT) conditions. It consists of a stack of tectono-metamorphic units formed by basal accretion during the subduction of the former Eurasian passive margin in the late Eocene–early Oligocene. Some units are characterized by an unusual composition, comprising decametric- to hectometric-scale lenses of continental (micaschists, quartzites) and oceanic (metabasalts, serpentinites) lithologies embedded in a calcschist matrix. The origin of these atypical units remains debated. They are interpreted either as tectonic mélanges formed at the subduction interface or as the result of protolith heterogeneity within the sedimentary sequences deposited along the distal Eurasian margin.

This study combines petrological and structural observations, geochemical analyses, Raman spectroscopy on carbonaceous material and thermodynamic modeling to reconstruct the pressure-temperature (P-T) history of both the lenses and matrix, as well as adjacent units. Field evidence and a similar metamorphic record suggest a common history between the metabasic lenses and the calcschist host, supporting the hypothesis of intercalated volcanic and volcano-sedimentary sequences within the sedimentary protolith. Conversely, notable differences in the P-T records of the micaschist lenses, sharing, instead, similarities with the adjacent Dora Maira units, suggest that these lenses were likely incorporated through localized deformation involving isoclinal folding, boudinage, and transposition involving limited displacement.

Excluding the micaschist lenses, the lithological and petro-metamorphic similarities between these heterogeneous units and the neighboring Queyras unit, reinforced by a similar geochemical signature in metabasic lenses, suggest a common Liguro-Piemontese origin. Their tectonic intercalation with the continental units of the Dora Maira massif may thus have occurred only later, during the intense deformation associated with the rapid exhumation of the whole HP-LT nappe stack.