LOCALIZED INCIPIENT GRANULITE FACIES ASSEMBLAGE FORMED VIA SUBSOLIDUS METASOMATISM BY HYPERSALINE FLUIDS IN A LOW ANGLE SUBDUCTION SETTING, CEMETERY RIDGE, SW ARIZONA, USA

The amphibolite to granulite facies transition is marked by stabilization of a dominantly anhydrous mineral assemblage, and owes to increasing temperatures and/or pronounced decrease in activity of H₂O (aH₂O) caused by interaction with a water undersaturated melt, CO₂-rich fluid, or hypersaline (NaCl, KCl) brine. We present thermodynamic, mineralogic, and bulk chemical evidence of transitional amphibolite to granulite facies metamorphism in a low-angle subduction channel exposed within the Orocopia Schist at Cemetery Ridge, SW Arizona. Given the subduction zone setting, record of extensive metasomatism, and preservation of mineralogic reactions that track the formation of low aH₂O fluids this represents a highly novel exhumed system.

At Cemetery Ridge, actinolite-rich replacement veins and metasomatic lenses crosscut and envelop blocks of serpentinized peridotite enclosed within quartzofeldspathic schist that reached peak metamorphic conditions of ~ 0.8 GPa and 650 °C. Outflow of silica-depleted fluids from metasomatic veins in peridotite into enclosing schist produced meter-scale lens or pipe-like bodies of ultrabasic aluminous gneiss (UAG; SiO₂ 36, Al₂O₃ 25 wt. %). UAG comprises epidote-clinozoisite, aluminous hornblende (pargasite), strontian anorthite (>An₉₉; SrO as much as 1.7 wt.%), aluminous diopside (“fassaite”), ferroan spinel, and numerous accessory phases. Metamorphic replacement textures support progression of an amphibolite to granulite facies reaction: 2 epidote + 1 pargasite → 2 anorthite + 4 fassaite + 2 H₂O + (Na, Fe, Al, Si)_aq, which can proceed only at aH₂O ≲ 0.5. This transition was preceded and/or accompanied by extensive volume loss corresponding to ≈ 50% removal of SiO_2, as well as loss of LILE, Zr, and LREE; and gain of Ti, Ca, and Sr.

Low aH₂O of fluid, extent of metasomatism, and the presence of sylvite (observed by SEM) indicate that generation of a transitional granulite facies assemblage owes to influx of highly saline brine, perhaps with involvement of a miscible CO₂-rich fluid. Production of the saline fluid was controlled by the availability of Cl which was provided by metasomatic transformation of serpentine to tremolite (Cl in serpentinite ~ 0.1 wt. %). These findings provide insights into the nature of low-angle (flat slab) subduction fluids, and suggest potential fluid sources during production of subsolidus granulites.