PETROLOGIC CHARACTERIZATION OF GUATEMALAN LAWSONITE ECLOGITE: ECLOGITIZATION OF SUBDUCTED OCEANIC CRUST IN A COLD SUBDUCTION ZONE

Abstract

Early Cretaceous lawsonite eclogites and related high-pressure rocks occur as tectonic inclusions within serpentinite mélange south of the Motagua fault zone, Guatemala. Petrologic and microtextural analyses of mafic high-pressure rocks reveal three metamorphic stages linked to several deformational textures. The prograde stage represents an incipient eclogitization and is preserved in prograde garnet, along with an older S 1-S2 foliation. The prograde assemblage is garnet (X Mg = ̃0.22) + omphacite (̃52 mol% jadeite) or jadeite (̃83 mol % jadeite) + lawsonite + chlorite + rutile + quartz ± phengite (3.6 Si p.f.u.); some rocks also have ilmenite and rare ferroglaucophane. Lawsonite in garnet of some lawsonite eclogites contains rare pumpellyite inclusions. The presence of synmetamorphic brittle deformation, inclusions of pumpellyite, Fe2+-Mg distribution coefficients between omphacite inclusions and adjacent garnet with Ln(KD) = 2.7-4.5, and garnet-clinopyroxene-phengite thermobarometry suggest that eclogitization initiated at temperature (T) = ̃300 °C and pressure (P) > 1.1 GPa, and continued to T = ̃480 °C and P = ̃2.6 GPa. In contrast, the retrograde eclogite-facies assemblage is characterized by reversely zoned garnet rims and omphacite ± glaucophane + lawsonite + rutile + quartz ± phengite (3.5 Si p.f.u.) along the S3 foliation. Garnet-phengite- clinopyroxene thermobarometry yields P = ̃1.8 GPa and T = ̃400 °C. The youngest, blueschist-facies assemblage (glaucophane + lawsonite + chlorite + titanite + quartz ± phengite) locally replaces earlier mineral assemblages along S4 crenulations. The inferred prograde P-T trajectory lies near a geotherm of ̃5 °C km-1, comparable to the calculated thermal and petrologic structure of the NE Japan subduction zone. These petrologic characteristics indicate: (1) the basalt-eclogite transformation may occur at T = ̃300 °C in cold subduction zones, (2) glaucophane-bearing prograde assemblages are rare during incipient eclogitization in cold subduction zones, and (3) the chlorite-consuming reactions that form Fe-Mg-Mn garnet are more effective than the lawsonite-consuming reaction that forms a grossular component. At depths of ̃100 km in cold subduction zones, dehydration embrittlement may be caused by such chlorite-consuming reactions. © 2006 Geological Society of America.