ECLOGITIZATION OF A COLD SUBDUCTING SLAB: PROGRADE EVOLUTION OF LAWSONITE-ECLOGITES FROM THE MOTAGUA FAULT ZONE, GUATEMALA

Lawsonite-eclogite and jadeitite occur as tectonic blocks within antigorite-dominant serpentinites in the Motagua Fault Zone, Guatemala (e.g. McBirny et al., 1967); black-wall assemblage of actinolite-chlorite rock is well developed in between the block and host serpentinite. Some mafic eclogite lenses are interlayered with minor pelitic schists that contain quartz, phengite, garnet, lawsonite and glaucophane. Such association of metabasalt and metapelite in serpentinites suggests oceanic protoliths with trench-fill sediments, typical for the ePacific-typef subduction. The eclogites are foliated and composed mainly of lawsonite, prograde-zoned garnet (Alm_50-71Pyr_2-14Sps_0-14Grs_19-31), omphacite (Jd_29-50Ae_4-19Aug_35-57), glaucophane with minor rutile, quartz, rare pumpellyite, chlorite and retrograde minerals (titanite, chlorite, phengite and stilpnomelane). Garnet porphyroblasts (up to 20 mm) in the foliated matrix contain numerous inclusions of lawsonite, omphacite (Jd_39-46Ae_8-20Aug_40-46), rutile and rare chlorite. Lawsonite occurs as either coarse-grained neoblasts in matrix or fine-grained inclusions in garnet, omphacite and glaucophane (Gln_85-90Act_2-3Rbk_2-9); matrix omphacite contains inclusions of lawsonite and glaucophane. This textural evidence indicates synchronous growth of lawsonite and other eclogite-facies minerals. Lawsonite inclusions in garnet contain tiny pumpellyite. Matrix omphacite contains oriented quartz rods similar to eclogitic omphacite from several ultrahigh-pressure terranes. The Fe-Mg partitioning between garnet and adjacent omphacite inclusions and the lawsonite + omphacite + quartz assemblage gives T=410 ‹C at P > 1.2 GPa for the core and T=550 ‹C at P > 2.0 GPa for the rim. Such prograde path is also consistent with the occurrence of pumpellyite in lawsonite inclusions in garnet indicating that lawsonite-eclogite was formed from pumpellyite-glaucophane assemblage. The pumpellyite-consuming reaction to form lawsonite-bearing assemblage requires hydration. Eclogitization generally accompanies various dehydration reactions. In cold subduction zone, however, the infiltration H₂O-rich fluid may play an important role for the formation of lawsonite-eclogite.