23 Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism (Posters)

We welcome diverse submissions on the petrology, structure, geochemistry, and geophysics of subduction zones from all approaches. Example topics include initiation, evolution, volatile cycling, ophiolites, arc tempos, high-P metamorphism, mélanges, seismicity, and accretionary wedges.

Thursday, 23 April 2026
8:00 AM - 5:30 PM
LMH, 5th Floor Chapel

Thermodynamic modeling and P-T paths of high-grade metamorphic rocks from the Piaxtla Suite (Acatlán Complex, southern México)

Paper number: 23-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Piaxtla Suite, is part of the Paleozoic Acatlán Complex in southern México; contains high-pressure mafic rocks recording different petrological features and deformation stages coupled to subduction-related metamorphic evolution. The assemblage consists of polydeformed metamorphic belts in different metamorphic grades in eclogite, amphibolite, and blueschist facies, commonly retrogressed to greenschist facies.

The present study, use the phase equilibria modeling performed via bulk and effective rock compositions, in the MnNCFMAST chemical system for eclogites and the MnNCFKMAST(H) system for blueschist and amphibolites. This approach allowed for the estimation of the peak metamorphic conditions and suggest the reconstruction of subsequent retrograde evolution of the eclogites, including P-T re-equilibration during retrogression toward amphibolite and blueschist facies conditions, culminating in the greenschist facies.

Petrographic observation integrated with thermodynamic modeling reveal at least three metamorphic events.

1. Peak Metamorphism, corresponds to eclogite facies metamorphism, defined by the mineral assemblage Grt + Omp ± Qz + Rt ± Ttn. Peak conditions are constrained at ~22 kbar and ~710°C, consistent with the results obtained using the Zr-in-rutile thermometer and garnet-omphacite geothermometer.

2. Exhumation and Divergent Retrogression: Eclogites followed two distinct retrograde P-T paths:

i) One trajectory records decompression and hydration toward amphibolite facies, characterized by a mineral assemblage of Amp+Grt+Ep ±Pl± Chl ± Qz, at ~10-14 kbar and ~450-550°C.

ii) The second trajectory involves cooling prior to decompression, leading to blueschist facies conditions. This path is inferred from relict garnet pseudomorphs in equilibrium with Gln + Amp + Ep and a stable mineral assemblage of Gln + Ep + Ph ± Amp (Ca; Ca-Na) ± Pl +Qz at ~13.8-16 kbar and ~450-580°C.

3. The late metamorphic event corresponds to retrogression to greenschist facies, represented by the appearance of Act+Chl+Ms in both amphibolites and blueschists.

Overall, these results suggest divergent P-T retrograde trajectories for the Piaxtla eclogites within a subduction channel, consistent with the structure described in the Piaxtla Suite and with geochronological data indicating continuous Carboniferous exhumation.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410843

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