Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite melange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slabmantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde PT path that evolves from 20 kbar, 470 degrees C (M1) to 25 kbar, 520 degrees C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak-T of similar to 520 degrees C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to similar to 10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca-rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsoniteblueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyiteactinolite facies (35 kbar, 230-250 degrees C) is associated with dehydration and veining (M5). The presence of eclogite as m- to 10 m-sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde PT path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate-T subduction zone.