Jadeitite formed during subduction: Zircon geochronology constraints from two different tectonic events on the Guatemala Suture Zone

Jadeitite is a rare rock type associated with high-pressure—low-temperature blocks from serpentinite matrix mélange. Evolving models of formation involve precipitation from subduction zone aqueous fluids in veins cutting the overlying mantle wedge or metasomatism of channel blocks emplaced into it [1]. Age determination of the mélanges is based on dating included rocks for "peak metamorphism" or constituent minerals, such as micas by Ar-Ar, which may only provide exhumation constraints. New SHRIMP-RG 238U-206Pb dates from metasomatic/solution-precipitate zircons in jadeitites and mica-albite rocks from Guatemala combined with other recently reported dates show (a) older mean ages that are a clear evidence for jadeitite crystallization 10-30 Ma prior to peak subduction zone metamorphism (e.g., crystallization of eclogite), and (b) a second group of younger mean ages slightly younger or similar to exhumation ages given by Ar-Ar dates from micas. These age relationships are also observed at other jadeitite localities, such as Sym-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite mélanges of the Río San Juan complex (Dominican Republic). The data argue for formation of jadeitite in the non-subducting mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation. Reference: [1] Tsujimori & Harlow 2012, Eur. J. Mineral. 24, 371-390.