Trace element mobility and mineral-scale redistribution during retrograde metamorphism of lawsonite-bearing high-pressure rocks from Elekdağ, Turkey

Abstract

Subduction-zone metamorphism significantly influences trace element redistribution during burial and exhumation of oceanic lithosphere. We investigate high-pressure metamorphic rocks from the Elekdağ ophiolite (Turkey) using petrography, mineral chemistry, phase-equilibrium modelling, and rutile U–Pb geochronology. Phase-equilibrium modelling indicates peak conditions of 2.3–2.6 GPa and 530–600 °C, with rutile U–Pb dating constraining near-peak metamorphism at 102 ± 14 Ma, followed by blueschist-facies retrogression. Whole-rock geochemistry reveals that while some samples retain MORB-like compositions, others exhibit high field strength element (HFSE) depletion and large-ion lithophile element (LILE; e.g., K, Rb, Ba, B) enrichment. Phengite-hosted LILE enrichment points to open-system fluid infiltration, likely reflecting a combination of protolith seafloor alteration and retrograde metasomatism. Semi-quantitative mass-balance calculations and mineral-scale data indicate that trace element features reflect scale-dependent processes. Locally, light rare earth elements (LREEs), Sr, and HFSEs were predominantly redistributed during the breakdown of lawsonite to clinozoisite/epidote via interface-coupled dissolution–reprecipitation. Although LREEs and Sr are traditionally fluid-mobile, their high sequestration into these newly formed phases suggests restricted, localized mobility, though minor matrix-scale external fluid exchange cannot be entirely ruled out. These findings highlight a scale-dependent decoupling of element mobility and demonstrate that whole-rock geochemical signatures in retrogressed high-pressure rocks integrate protolith inheritance, open-system fluid overprinting, and mineral-scale redistribution. Relying solely on bulk geochemistry may thus lead to the misinterpretation of subduction-zone fluid-rock dynamics without integrating detailed mineral-scale budgets.