Metamorphic Record of High-pressure Dehydration of Antigorite Serpentinite to Chlorite Harzburgite in a Subduction Setting (Cerro del Almirez, Nevado-Filábride Complex, Southern Spain)

The antigorite dehydration front preserved in the Cerro del Almirez (Nevado-Filábride Complex, Betic Cordillera, SE Spain) offers a unique opportunity to investigate the high-pressure prograde breakdown of antigorite serpentinite (Atg-serpentinite) to chlorite harzburgite (Chl-harzburgite; olivine + orthopyroxene + chlorite + tremolite) in a subduction setting. Our detailed mapping of the antigorite dehydration front shows that in the transition between Atg-serpentinite and prograde Chl-harzburgite a narrow band of transitional lithologies occurs that consists of chlorite-antigorite-olivine-serpentinite (Chl-serpentinite), grading through antigorite-chlorite-orthopyroxene-olivine (Atg-Chl-Opx-Ol) rocks and, subsequently, to Chl-harzburgite. Just beneath the reaction front, Chl-harzburgite shows everywhere a granofelsic texture; however, down section, it is interspersed in decameter-sized domains with spinifex-like Chl-harzburgite. Field and textural relationships, mineral compositions and chemographic phase relations indicate that the prograde breakdown of Atg-serpentinite to Chl-harzburgite occurred through a series of continuous metamorphic reactions forming transitional lithologies. Chl-harzburgites with granofelsic and spinifex-like textures were formed under similar P-T conditions (∼ 1 · 6 - 1 · 9 GPa and 680-710°C). We ascribe their differing textures to shifts in the growth rate of the products caused by temporal and spatial fluctuations in the affinity of the antigorite-breakdown reaction driven by cyclic variations of the fluid pressure. Crystallization at a low affinity of the antigorite-breakdown reaction, producing granofelsic Chl-harzburgite, was attained by a slow drain of fluids from the dehydrating system. With the advancement of the dehydration front overpressured domains were left behind, preserving serpentinite that was highly metastable under lithostatic pressure. Brittle failure of rocks surrounding these overpressured domains resulted in an increase in their permeability, a fast drop in fluid pressure towards hydrostatic pressure, and sudden stabilization of the prograde reaction products. This process resulted in the crystallization of the prograde assemblage under a high affinity of the antigorite-breakdown reaction, accounting for disequilibrium fast growth of spinifex-like Chl-harzburgite. Prograde textures in the Cerro del Almirez hence witness the feedback between the rate of metamorphic reactions and the cyclic dynamics of meta