Complex Garnet Zoning from Lawsonite Eclogite from South of the Motagua Suture Zone, Guatemala

Serpentinite mélange of the Guatemalan suture zone hosts the exhumed remains of one of the coldest subduction zones in the world. The mélange contains several different metamorphic rocks, including lawsonite eclogite. Well-preserved lawsonite eclogite is particularly rare, and is preserved only in high-pressure, low-temperature metamorphism wherein exhumation is rapid. Though lawsonite eclogite is well-preserved, the full extent of the P-T-t path this HP-LT rock experienced is contentious, with two competing proposed P-T paths: one with a prograde path with garnet growth from 300 to 480 °C at 1.1-2.6 GPa, and another with a prograde path from 470 to 520 °C at 2-2.5 GPa. Combining major, trace, and rare earth element (REE) garnet concentrations with crystal size distribution analysis, X-ray element maps, REE modeling, and Sm-Nd age data, constraints are placed on the prograde P-T-t path of these HP-LT metabasites. My data reveal a complex zoning in major, trace, and rare earth elements. Core to rim concentration profiles define normal prograde zoning, but deviated from this zoning towards the rim, with restricted regions of complex zoning in garnet compatible elements (e.g., Mn and HREEs), with a decrease of these elements at the rim. This complex zoning is related to changes in bulk composition, fluid availability, and garnet growth rate, which indicates that garnet experienced different stages of growth over a protracted period, perhaps as long as 33 Ma. The overall variability of chemical zoning between eclogite samples in the mélange, it is also apparent that different P-T histories may be recorded in different eclogite blocks.Earth and Atmospheric Sciences, Department o