305 Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology (Posters)

We invite contributions that explore the geologic processes of subduction zones from new field observations, analytical methodologies, and numerical modeling of metamorphic rocks along the subduction interface or try to integrate or reconcile the rocks record with geophysical observations.

Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

The Significance of Blueschist Metaconglomerates in the External Hellenides on Rheology and Metasomatism in the Subduction Interface From Deposition to Exhumation.

Paper number: 305-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Studying subduction zones is essential for understanding the formation and geodynamics of plate tectonics, geochemical cycling, and geologic hazards. High-pressure units exhumed from subduction zones preserve pressure-temperature-time (P-T-t) records that provide critical information about fluids, deformation, and rheology of the subduction interface. Although metaconglomerates are often a significant input in modern subduction zones (e.g., Japan), little work has been done to understand their metamorphism or effect on subduction interface rheology due to their limited exposures and poor preservation. Crete Island in Greece lies in the forearc of the Hellenic subduction zone and exposes high-pressure metamorphic rocks that were once part of a subduction interface and experienced peak metamorphic conditions in the Miocene. Included within the high-pressure rocks in Crete is a metasedimentary sequence that ranges from blueschist to greenschist facies and from metaconglomerate to fine-grained metasandstones. Here we present petrographic, geochemical, and geochronologic analyses to constrain the origin, deformation, and metamorphic history of these rocks. 

In clasts from the blueschist facies metaconglomerate, the quartz exhibit microstructures that suggest deformation accommodated by dislocation creep, and mica (phengite and muscovite) preserve a relict foliation. Detrital zircon U-Pb geochronology shows that these clasts with the inherited deformation textures were sourced from Triassic to Proterozoic-aged rocks. The blueschist matrix is composed of phengite, epidote, and glaucophane. Glaucophane compositions (collected by electron microprobe) are homogeneous both inside the clasts and in the matrix, suggesting growth during subduction. The greenschist facies metaconglomerate contains albitized and chloritized clasts that preserve volcanic textures which is in agreement with the Triassic zircon population associated with volcanism during the opening of Neotethys. 

The structurally higher blueschists (metaconglomerates and metasandstones) preserve high-pressure paragenesis while the underlying metaconglomerates were retrogressed to greenschist-facies during exhumation. In contrast with the matrix-supported blueschist facies samples, the greenschists are clast-supported, matrix-poor, and hence more permeable. The stronger, less permeable, fine-grained metasandstones may have acted as an impermeable barrier for the blueschists while promoting retrogression of greenschist facies metaconglomerates. This study highlights the importance of studying subduction zone metaconglomerates by showing that clasts preserve relic textures and mineralogy, and provide valuable evidence of their provenance and age, as well as processes during subduction and exhumation (e.g., underplating, fluid flow).

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412668

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