Exhumation dynamics of high-pressure metamorphic rocks from the Voltri Unit, Western Alps: constraints from phengite Rb–Sr geochronology

Fluid‐Mediated Mass Transfer Between Mafic and Ultramafic Rocks in Subduction Zones

“…Based on these and previous constraints, we infer a clockwise P‐T path, that exhibits a phase of relatively isothermal decompression at ∼500–520°C from the peak pressure of ∼2.5 GPa down to ∼1.0 GPa (Figure 9f). Lastly, these results are consistent with previous regional thermobarometric constraints for the Voltri Massif (Malatesta et al., 2012a; Smye et al., 2021; Starr et al., 2020).…”

“…A recent study investigated the P-T-time evolution and bulk Sm-Nd garnet ages of different high-pressure (HP) mafic rocks in the Voltri Massif to determine whether these mafic rocks were indeed exhumed as blocks within a chaotic mélange or as coherent sections of oceanic lithosphere (Starr et al, 2020). Different from previous studies (e.g., Federico et al, 2007;Malatesta et al, 2012aMalatesta et al, , 2012b it was suggested that a significant portion of the Voltri Massif was exhumed as large kilometric-scale coherent sections of ultramafic and mafic slab material (Cannaò et al, 2016;Smye et al, 2021;Starr et al, 2020). A more detailed assessment of the structural aspects of the entire Voltri Massif is discussed elsewhere (Federico et al, 2007(Federico et al, , 2015Malatesta et al, 2012a).…”

“…Previous studies of high‐pressure serpentinite at Voltri have documented a foliation in serpentinite that is generally parallel to the garnet‐pyroxene foliation observed in adjacent eclogitic blocks (Cannaò et al., 2016; Federico et al., 2007; Haws et al., 2021; Malatesta et al., 2012a). Furthermore, a range of prograde to peak metamorphic conditions from 1.8 to 2.4 GPa and 500–600°C occurring at ∼50–33 Ma, followed by partial retrogression through blueschist‐ (∼1.2 GPa and 550°C) and greenschist‐facies conditions (∼0.8 GPa and 480°C) occurring at ∼34–24 Ma have been proposed for the Voltri Massif metagabbros (Messiga et al., 1983; Messiga & Scambelluri, 1991; Smye et al., 2021; Starr et al., 2020; Vignaroli et al., 2005).…”

Metamorphism and Deformation on Subduction Interfaces: 1. Physical Framework

“…Such a framework draws empirical support from a number of regional-scale structural and metamorphic observations from exhumed HPLT terranes. Particular examples include the Cycladic Blueschist Unit (e.g., Kotowski et al, 2022;Ring et al, 2020), the Tavşanli zone (western Turkey) (e.g., Plunder et al, 2015), the southern Tianshan metamorphic belt (e.g., Bayet et al, 2018), the HPLT massifs of the Western Alps (e.g., Angiboust et al, 2011Angiboust et al, , 2012Reinecke, 1998;Smye et al, 2021;Starr et al, 2020) and the Raspas Complex of Ecuador (e.g., John et al, 2009). In each of these examples, stacking of thrust sheets occurred at ∼2-3 GPa and 500-600°C over several millions of years.…”

Metamorphism and Deformation on Subduction Interfaces: 2. Petrological and Tectonic Implications

“…It seems probable, for the reasons just discussed, that the majority of sediments are lost from the interface before or as the slab passes beneath the arcs. Small volumes of sediments may however be entrained between large volumes of mafic lithologies-as observed in some HPLT terrains (e.g., the Zermatt-Saas, Monviso and Voltri ophiolites of the Western Alps; Angiboust et al, 2009Angiboust et al, , 2012Smye et al, 2021)-and be subducted along with the stronger units (e.g., Kelemen & Manning, 2015). Consideration of the subducted flux of C provides a demonstration of the viability of this mechanism.…”