Garnet-chloritoid-paragonite metapelite from the Chuacus Complex (Central Guatemala): new evidence for continental subduction in the North America-Caribbean plate boundary

We describe for the first time the presence of high-pressure metapelites in the northern Chuacus Complex of Central Guatemala. Garnet-chloritoid-paragonite-bearing pelitic schist occurs in a predominantly metasedimentary sequence consisting of intercalated garnet paragneisses, pelitic schists, impure marbles, granitic orthogneisses and minor garnet amphibolites, an association that denotes a passive continental margin origin. The metapelite we have studied is mainly composed of almandine-rich garnet porphyroblasts in a schistose matrix consisting of phengite, paragonite, quartz, chloritoid and rutile, with minor amounts of chlorite, epidote, Fe-Ti oxides and sporadic kyanite. The metapelite also includes fine-grained paragonite + quartz and paragonite + epidote aggregates, which 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 degrees C. The composition of garnet rims constrains the P-T peak conditions to similar to 19.5-20 kbar and 580-600 degrees C. Both the zoning pattern of the garnet and the calculated P-T path indicate that the studied metapelite could be formed during a single progressive metamorphic event along a subduction geothermal gradient of 7-9 degrees 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. 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 degrees C. The occurrence of this high-pressure metapelite implies that high-pressure metamorphism in the Chuacus Complex must have extended at least 10 km further to the north than previously thought. We propose that both Chuacus Complex and Rabinal Granite show a succession of decreasing P-T peak conditions towards the north, which may preserve in part the original thermal structure of the subduction zone in the southern margin of North America during the Cretaceous.