Jadeitite formed during subduction: In situ zircon geochronology constraints from two different tectonic events within the Guatemala Suture Zone

Jadeitite is a rare rock type associated with high-pressure-low-temperature blocks within serpentinite matrix melanges. Models of formation involve precipitation from subduction-zone aqueous fluids veining the overlying mantle wedge (P-type), or metasomatism of igneous and/or sedimentary protoliths previously emplaced into the melange (R-type). Age determinations of melange lithologies provide constraints on the timing of "peak metamorphism" and subsequent exhumation. The timing of jadeitite formation, particularly in the rich source of the Guatemala Suture Zone (GSZ), is a controversial subject needing further attention. Over 80 in situ zircon crystals from three jadeitites and two mica-albite rocks from the North Motagua Melange and one phengite jadeitite from the South Motagua Melange of the GSZ were studied for age and trace-element determination. Most of these zircons are characterized by low Th/U ratios, depleted chondrite-normalized REE patterns relative to zircons from oceanic gabbros, and contain fluid and mineral inclusions that reflect the primary mineralogy (i.e., jadeite) and context (i.e., crystallization from an aqueous fluid) of the host rock, and thus formed during jadeitite crystallization. The SHRIMP-RG and LAM-ICP-MS U-Pb dates from zircon indicate that jadeitites and mica-albite rocks from the GSZ were formed through vein precipitation at similar to 98-80 and similar to 154-158 Ma, respectively. These data show (a) older ages that indicate jadeitite crystallization occurred similar to 10-30 Ma before the preserved subduction-zone peak metamorphism (e.g., exhumed eclogite), and (b) a second group of ages slightly younger than, or similar to, exhumation ages given by Ar-Ar dates from micas. Similar relationships occur at other jadeitite occurrences, such as the Syum-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite melanges of the Rio San Juan complex (Dominican Republic). The data argue for formation of jadeitite within the mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation. (c) 2013 Elsevier B.V. All rights reserved.