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High- to ultrahigh-pressure (HP–UHP) rocks recording high-temperature (HT) > 700 °C are well exposed in the Central Alps, making it an ideal region to study the timing of metamorphic stages and the mechanisms of deep-seated rocks exhumation. Here, we report an integrated dataset of petrological and U–(Th–)Pb dating of metapelites surrounding ultramafic lenses from the Cima Lunga unit. At the interface with ultramafics preserving (U)HP–HT assemblages (1.5–3.1 GPa, 650–850 °C), metapelites record higher P‒T values (1.3–2.7 GPa, 700–850 °C) and traces of partial melting, whereas the rest of the unit is dominated by amphibolite-facies conditions. U–Th–Pb dating on zircon and monazite from migmatites indicates that partial melting was episodic involving at least two stages at ~38 to 35 Ma and 33–30 Ma, respectively. While the 38–35 Ma stage matches the HP conditions (> 1.5 GPa) and it is recorded around only one lens with scarce volumes of melt, partial melting at 33–30 Ma is witnessed at lower pressure (~1 GPa) and more widely distributed around the lenses, as within the major shear zones. Far from the ultramafics, zircon from the amphibolite-facies metasedimentary rocks record inherited pre-Variscan ages, while monazite ages at ~22 Ma document mineral growth during the Barrovian cooling. Field and petro-chronological evidence highlight that multiple episodes of partial melting locally developed at the rheological interface promoted by the interplay of fluids extracted from the ultramafic lenses associated with shear heating. New evidence suggests that local variation of P‒T equilibria play a significant role during the exhumation history. [ABSTRACT FROM AUTHOR]

The record of metamorphic conditions is commonly highly heterogeneous in spatially close rocks of different composition and rheology. The well-studied Cima di Gagnone area in the Central Alps represents such an example, as relatively small, 1 – to 100 s-meter scale, ultrahigh–pressure and high–temperature ultramafic lenses are enveloped within amphibolite–facies metasediments. We present new field observations, microstructural and petrological analyses, and thermodynamic modelling results on these metasediments, showing that these rocks generally experienced medium pressure and medium temperature conditions of 1.0 – 1.2 GPa and 640 – 700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, a few samples from the immediate proximity of the ultramafic lenses record significantly higher P–T conditions of 1.3–1.7 GPa and 750–850 °C, approaching the high pressure and high temperature conditions of the ultramafic bodies (1.5–3.1 GPa, 650–850 °C). Mineral/bulk chemistry changes during growth of new mineral phases hint to local melt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. Preliminary U-Pb LA–ICP–MS dating suggests that zircon grains from the metasomatic reaction zone have been fully re–equilibrated during the early stage of Alpine exhumation (~36 Ma), while the large part of the metasediments records only pre–Alpine ages. We finally recast these new data into the regional P–T–t–D paths and discuss the consequences of these findings for understanding the exhumation processes of HP rocks. We found different local equilibria (Δ P up to 2 GPa; ΔT up to 160 °C) that cannot be explained simply by retrograde metamorphism or by tectonic amalgamation. Rather, our work suggests a significant role of deformation in producing variable pressure and temperature record within the rocks. [Display omitted] • Metapelites enveloping ultramafic lenses record heterogeneous metamorphism. • Alpine HP–HT relicts occur close to the rheological boundaries. • Melt/fluid–assisted processes promote metasomatism at HP–HT conditions. • HP–HT signature is locally developed rather than locally preserved. • Exhumation processes must account the deformation control on the P–T record. [ABSTRACT FROM AUTHOR]