Metaserpentinite carbonation and decarbonation reactions during subduction metamorphism and subsequent tectonic exhumation

In subduction zones, serpentinite-hosted ophicarbonates and their main dehydration and decarbonation reactions linked to prograde metamorphism are relatively well understood. On the contrary, the geological conditions and processes leading to the carbonation of subduction-zone metaserpentinites by fluid–rock interactions remain poorly constrained. At different arc depths, the reaction of decarbonation fluids derived from marble and carbonate‐bearing sediment with slab and mantle wedge serpentinites, as well as tectonic mixing and deformation along subduction zone interface, may produce magnesite-bearing rocks with a bulk composition similar to that of ophicarbonates. Subsequently, these hybrid metasomatic lithologies will undergo decarbonation reactions at prograde or retrograde conditions which may influence the cycling of C and other volatiles from the slab to the mantle wedge and the global estimates of C fluxes at convergent margins. This can be evaluated through the study of exposed paleo-subduction metamorphic suites.

Here we investigate the tectonic, textural and mineralogical evolution of marble layers and magnesite-rich lenses hosted in chlorite harzburgite (Chl-harzburgite) in the Cerro Blanco ultramafic massif (Nevado-Filábride Complex, Betic Cordillera, S. Spain), which records high-pressure alpine subduction metamorphism as evidenced by the transition from antigorite serpentinite (top of the body) to Chl-harzburgite (bottom) due to high-pressure deserpentinization. Chl-harzburgite is separated from a gneiss and mica schist crustal sequence by a footwall of strongly heterogenous mylonite (around 20 m thick) encompassing: transposed, foliated and brecciated marble layers, foliated Chl-harzburgite lenses (in some cases completely transformed to retrograde serpentinite), centimetre to several decimetre thick boudins of idiomorphic coarse to very coarse magnesite aggregates, associated to chlorite and magnetite and enveloped by the mylonitic foliation, and, finally, abundant almost monomineralic amphibole aggregates. We interpret this mylonite zone as a detachment leading to the exhumation of the Cerro Blanco massif after reaching peak subduction metamorphic conditions that formed the Chl-harzburgite assemblage.

Combined field, EDS-SEM, and EPMA data obtained from a detailed cross-section sampling of this mylonite zone reveal that, locally, metamorphic equilibrium was reached between Chl-harzburgite and the transformation products of the magnesite boudins during the mylonite foliation development. Thermodynamic modelling of these assemblages allows inferring the relationship between deformation and metamorphic conditions during exhumation, including possible decarbonation reactions.

We thank the Universidad de Jaen 1263042 FEDER-UJA grant, funded by the European Social Fund and the European Regional Development Fund.