305-6 Garnet-albitite formed during subduction – A window into fluid-rock interaction in the subduction interface

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

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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 Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8953

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