GARNET-CHLORITOID-PARAGONITE METAPELITE FROM THE CHUACúS COMPLEX (CENTRAL GUATEMALA): NEW EVIDENCE FOR CONTINENTAL SUBDUCTION IN THE NORTH AMERICA-CARIBBEAN PLATE BOUNDARY

We describe the occurrence of high-pressure metapelites in the northern Chuacús Complex of Central Guatemala. Garnet-chloritoid-paragonite-bearing pelitic schist are part of a predominantly metasedimentary sequence consisting of intercalated garnet paragneisses, pelitic schists, marbles, granitic orthogneisses and minor garnet amphibolites. This metapelite is mainly composed of almandine-rich garnet porphyroblasts in a schistose matrix consisting of phengite, paragonite, quartz, chloritoid and rutile, with minor amount of chlorite, epidote-clinozoisite, Fe-Ti oxides and sporadic kyanite. The metapelite also includes fine-grained paragonite+quartz and paragonite+epidote-clinozoisite aggregates that resemble jadeite and lawsonite pseudomorphs, respectively. A late staurolite-bearing assemblage overprints the high-pressure paragenesis. We have used a phase-equilibria modeling approach to constrain the P-T conditions of metamorphism. Equilibrium phase diagrams were calculated in the system MnNCKFMASHTO (MnO-Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃) for three hypothetical effective bulk-compositions. The compositions of garnet, chloritoid and phengite cores indicate they crystallized during an early stage of prograde high-pressure metamorphism at 20-21 kbar and 500-540 °C. The composition of garnet rims constrains the P-T peak conditions to ~19.5-20 kbar and 580-600 °C. This data suggest that the studied metapelite could be formed during a single progressive metamorphic event along a subduction geothermal gradient of 7-9 °C /km. The retrograde path is less well constrained, although decreasing pressure coupled with a slight drop of temperature are indicated by the absence of biotite and the growth of post-peak chlorite and epidote-clinozoisite. Moreover, a late-stage heating event is required, in order for the metapelite to reach the stability field of staurolite at 7-8 kbar and 590-620 °C. The occurrence of this high-pressure metapelite implies that high-pressure metamorphism in the Chuacús Complex must have extended at least 10 km further to the north than previously thought. Our results contribute to the knowledge of the geodynamic evolution of the North America-Caribbean plate boundary and the circum-Caribbean tectonics.