244-6 Spatial relationships between lawsonite pseudomorphs and matrix compositions, and potential significance for major element metasomatism.

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

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Abstract:

We present results of a study examining the spatial relationship between pseudomorphs after lawsonite and matrix composition in blueschist facies rocks from the Cycladic Blueschist Unit (CBU) in Syros, Greece. The CBU is an example of exhumed rocks that reached conditions of at least 1.5 GPa and 500 ˚C during subduction (Trotet et al., 2001). Pseudomorphs that are thought to record lawsonite breakdown into epidote-group minerals during or just before attainment of peak temperatures (Hamelin et al., 2018) have been frequently described in CBU mafic and sedimentary rocks. Lawsonite is hydrous, so fluid pulses should be released during these breakdown reactions. Thermodynamic modeling indicates that these fluids should be rich in Mg and Si if equilibrated with mineral assemblages present in MORB or average subducted sediments at these PT conditions. We examine the spatial relationship between lawsonite pseudomorphs and matrix composition in blueschist facies metavolcanics to identify any preserved evidence for these Mg and Si-rich fluids using an approach combining CT scans and electron microprobe mapping of pseudomorph-bearing samples. Preliminary data demonstrate that CT scans can effectively differentiate lawsonite pseudomorphs from surrounding matrix material and garnet crystals, and microprobe maps from a CBU sample show 0.5 mm thick rims of Mg and Si enrichment surrounding lawsonite pseudomorphs. Principal component analyses and k-means clustering are used to quantitatively examine the significance of these haloes and more subtle compositional variations at greater distances from pseudomorphs after lawsonite. These reveal the extent of local metasomatism during lawsonite breakdown.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7179

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