UNRAVELING THE HISTORY OF COMPLEX ZONED GARNETS IN RETROGRADE ECLOGITES: INSIGHTS FROM THE NORTH MOTAGUA MÉLANGE IN CENTRAL GUATEMALA

In the Guatemala Suture Zone (Central Guatemala), eclogites are found as relict within mafic bands in a coherent metamorphic belt or as enclose blocks in two serpentinite mélanges (South Motagua Mélange and North Motagua Mélange). Among the eclogites from the North Motagua Mélange analyzed to unravel their P-T paths, four samples show peculiar features in garnet zoning. Garnet grains are euhedral to sub-euhedral and the omphacite is replaced by glaucophane, phengite, epidote, titanite, magnesiohornblende, albite, and late biotite and chlorite. Inclusions phases in garnet include glaucophane, epidote and albite from the blueschist facies, whereas omphacite, rutile, epidote and phengite record the eclogite facies. X-ray maps acquired by EMPA on garnet grains reveal two types of zoning: (i) a spessartine-rich core, almandine-rich mantle, and pyrope+grossular-rich rim, attributed to garnet growth, and (ii) grossular-rich patches distributed in the core, the mantle, and the rim. Trace elements were analyzed by LA-ICP-MS in the different garnet zones (both concentric and patchy zoning). Finally, EBSD images on the garnets grains were acquired to determine the crystallographic orientations among the different zones.

In all four samples, the REE patterns and spider-diagrams acquired in the inclusions and similar species in the matrix (epidote, phengite, omphacite) show a systematic enrichment in HREE in the inclusions in comparison to the matrix, while the growth zones of garnet and the grossular-enriched patches show similar REE patterns. The lack of variation in LILE of minerals present as inclusions and in the matrix indicates that no major fluid influx/outflux occurred during the metamorphism, arguing for a nearly closed system at the sample scale. Therefore, hypotheses that Ca-rich patches formed by dissolution/reprecipitation or incongruent dissolution of garnet are not supported. EBSD measurements show a consistent orientation between the host garnet and the Ca-rich patches, indicating that the main garnet did not incorporate earlier Ca-rich relics. Interestingly, garnet patches are often found in association with other Ca-rich minerals, like epidote, and sometimes in a somewhat lozenge-shape area. We hypothesize that these patches grew at the expense of lawsonite, upon increasing temperature.