MULTIPLE STAGES OF FLUID INFILTRATIONS IN HP-LT LAWSONITITE (LAWSONITE-RICH ROCK) FROM THE MOTAGUA FAULT ZONE, GUATEMALA

Lawsonitite—a very uncommon rock that consists predominantly of lawsonite—occurs as tectonic inclusions within a serpentinite mélange south of the Motagua fault zone, Guatemala, and records multiple fluid events in a cold subduction zone. The investigated samples consist mainly of idiomorphic lawsonite (~90 vol%), with minor amounts of jadeitite, omphacite, phengite (3.5 Si p.f.u. for O=11), pumpellyite and titanite. Randomly-oriented lawsonites are coarse-grained (maximum 5 cm long); some lawsonite crystals showed sector- and oscillatory-growth zoning on millimeter to submicronmeter scale revealed by electron microprobe (EMP) X-ray imagining. Lawsonite averages ~0.2 wt% Fe₂O₃ and grain cores contain abundant tiny inclusions of jadeite (jd_84-95), omphacite (jd_37-51) and titanite whereas rims are inclusion-poor and enriched in Sr (~0.88 wt% SrO) and Ce (~0.14 wt% Ce₂O₅). X-ray images reveal micro flow paths (< 3 mm wide) for Cr-rich fluids through crystal fractures, indicating the infiltration occurred after lawsonite crystallization. The flow paths follow the track of chromian minerals including Cr-rich (kosmochloric) pyroxene with 22 wt% Cr₂O₃ (ko_25-63) and Cr-rich phengite with up to 6 wt% Cr₂O₃. Adjacent to the flow paths, Cr-bearing pyroxene occurs as inclusions in lawsonite. These microtextures and mineral compositions indicate a chronological sequence of fluid infiltration: (1) lawsonite grains together with jadeitic pyroxene crystallized from fluids in cavities in a cold and wet subduction slab, possibly synchronous with jadeitite formation (P> 1.0 GPa, T< 300 °C); (2) Sr and LREE increased during the growth of zoned lawsonite crystals; and (3) subsequently a Cr-rich fluid infiltrated into the lawsonitite and replaced the pre-existing pyroxene. Lawsonitite may have formed by Ca-metasomatism of a plagioclase-rich protolith during peridotite serpentinization and the later Cr-bearing fluid was derived from the ultramafics after serpentinization was completed.