Garnet–chloritoid–paragonite metapelite from the Chuac us Complex (Central Guatemala): new evidence for continental subduction in the North America–Caribbean plate boundary
Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Hernández-Uribe, David
European Journal of Mineralogy Volume 28 Number 6 (2016), p. 1169 - 1186
published: Dec 1, 2016
ArtNo. ESP147052806009, Price: 29.00 €
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–Na2 OCaO–K2 O–FeO–MgO–Al2 O3 –SiO2 – H2 O– TiO2 – Fe2 O3) 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 stabilityfield of staurolite at 7–8 kbar and 590–620°C. The occurrence of this high-pressure metapelite implies that high-pressure metamorphism in the Chuacúus 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.