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

Garnet-albitite formed during subduction – A window into fluid-rock interaction in the subduction interface

Paper number: 305-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Exhumed slab-derived metamorphic rocks and fluid-related rocks formed in the subduction interface at depths ~30-100 km represent the only direct samples that record fluid production and subsequent fluid-rock interactions in subduction zones. The discovery of P-type jadeitites and other associated vein-rocks linked to the crystallization of aqueous fluids are critical to understanding the generation of volcanism and critical element cycling.

Recently, we discovered enigmatic meter-sized, non-to-highly deformed slivers/blocks of garnet-bearing albitite rocks within the North Motagua Mélange (NMM) in central Guatemala. They occur within a serpentinite matrix that also encloses retrogressed eclogite, garnet-bearing restite, and jadeitite-phengitite blocks, or as slivers juxtaposed to serpentinite, amphibolite, garnet-schist, and marble sequences. Mineral chemistry and mapping of NMM garnet-albitites revealed modal abundances of matrix albite (53-29%) and quartz (43-27%), associated with white mica (13-8%), epidote (12-8%), garnet (2-0.5%), and secondary chlorite (1-0.7%). Accessory minerals include titanite, rutile, zircon, and apatite. Chemistry of key minerals yields phengite (TSi 3.1-3.4), clinozoisite (Czo 98-85), and almandine (Alm 69-48) compositions. Quartz and plagioclase display dynamic recrystallization while rotated garnet porphyroblasts and subparallel bands of phengite and clinozoisite reveal incipient to well-developed foliation.

A total of 399 zircon crystals from seven garnet-albitites were analyzed for U-Pb ages, Ti-in-zircon concentrations, and trace-element determination. LASS analysis yielded mean ages ranging from 101 Ma ± 0.61 to 87 ± 0.27 Ma. These zircons are characterized by Ti concentrations ranging from an average of 3.31 – 7.91 ppm, low Th/U ratios (0.001 to 0.900), and have REE chondrite normalized patterns that are slightly depleted in light REE and enriched in heavy REE, resembling zircons within jadeitites-phengitites from the same complex. These data show that garnet-albitite crystallization occurred contemporaneously with jadeitite-phengitite crystallization (~95-87 Ma). The latter linked to slab-derived fluid precipitation in the mantle wedge during active subduction, suggesting a similar origin. Radiogenic Sr and Nd isotopes reveal that the garnet-albitites are genetically related to slab relics (eclogite and amphibolite) from the same complex while the jadeitites-phengitites display a more evolved Sr signature. While the jadeitites–phengitites reflect fluid precipitation within the subduction interface, the garnet-albitites exhibit abundant, well-preserved euhedral albite + quartz crystals more consistent with silicic supercritical fluid precipitation. Further work is ongoing to determine the origin of these strange, fluid-related rocks.

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

doi: 10.1130/abs/2025AM-413479

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