- © 2016 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
We describe for the first time the presence of high-pressure metapelites in the northern Chuacús 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 °C. The composition of garnet rims constrains the P–T peak conditions to ~19.5–20 kbar and 580–600 °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 °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 °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. We propose that both Chuacús 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.