The interplay of deformation and devolatilization – texture evolution during subduction of meta-ophicarbonates

A key characteristic of subduction zones is that lithologies in slabs undergo prograde devolatilization while simultaneously being subject to deformation. A growing number of studies suggest that dissolution-precipitation creep may be the dominant deformation mechanism when fluid is present [1] and that the related fabric development and weakening are most pronounced during metamorphic reaction [2]. Here we investigate the microstructural and textural evolution of meta-ophicarbonate lenses hosted in Atg-serpentinite and Chl-harzburgite of the Milagrosa and Almirez ultramafic massifs in the Nevado-Filábride Complex (Betic Cordillera, S. Spain), which record high-pressure alpine subduction metamorphism. Similar rocks typically occur along tectonized lithological contacts between peridotite and mafic or sedimentary units, thus they can be ideal archives of focused deformation, prograde devolatilization and large scale fluxing by fluid from high-P serpentinite dehydration in subduction zones. In Milagrosa, serpentinite-hosted ophicarbonates —formed after variable mixtures of Ca-carbonate and serpentine— underwent prograde metamorphism to foliated antigorite-diopside-dolomite rocks and Ti-clinohumite-bearing diopside marbles (550 – 600 °C, 1.0 – 1.4 GPa). In Almirez, ophicarbonates were transformed to high-grade assemblages of Ti-clinohumite, olivine, diopside, chlorite, aragonite and dolomite (650 – 680 °C, 1.7 – 1.9 GPa) [3]. We combine microstructural and textural data obtained from electron backscatter diffraction (EBSD) and optical cathodoluminescence (CL) imaging to investigate the relationship between deformation and metamorphic devolatilization during prograde metamorphism. In Milagrosa, Atg-serpentinites show a fabric with a crystallographic preferred orientation (CPO) of antigorite typical of high-P Atg-serpentinites in subduction zones. Antigorite in antigorite-diopside-dolomite rocks displays the same fabric, with a c-axes maximum coinciding with the pole to the foliation, and diopside c-axes parallel to lineation. Dolomite and calcite in the Milagrosa meta-ophicarbonate lenses have a consistent CPO with their c-axes orientated subparallel to that of antigorite, which correlates with the poles to (100) of diopside. In foliated meta-ophicarbonates at Almirez, diopside, dolomite and calcite show a very similar distribution of their crystallographic axes with respect to foliation. CL imaging reveals concentric core-rim zoning of diopside and dolomite in both localities. Clusters of aragonite inclusions indicate that all zoning generations in diopside formed at high-P by growth and/or dissolution-reprecipitation.

These zoning patterns and low intra-grain misorientations point to diffusion creep by dissolution-precipitation as the dominant deformation mechanism. The strong correlation of diopside, carbonate and antigorite CPOs may have been caused by oriented crystal growth under differential stress and anisotropic fluid flow in the reacting ophicarbonate. We infer that antigorite dehydration in meta-ophicarbonate at 580 – 600 °C is related to transient ductile deformation enhancing fluid drainage, followed by compaction and an increased bulk rock strength once all antigorite devolatilizes. This causes deformation and fluid flow during dehydration of the host serpentinite at 650 °C to focus around the meta-ophicarbonate lenses, shielding carbonate from dissolution at subarc conditions.

 

[1] Malvoisin & Baumgartner (2021) G³; https://doi.org/10.1029/2021GC009633

[2] Stünitz et al. (2020) JSG; https://doi.org/10.1016/j.jsg.2020.104129

[3] Menzel et al. (2019) JMG; https://doi.org/10.1111/jmg.12481

M.D.M acknowledges funding of Junta de Andalucía (Postdoc_21_00791)