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Fluid Migration above a Subducted Slab-Constraints on Amount, Pathways and Major Element Mobility from Partially Overprinted Eclogite-facies Rocks (Sesia Zone, Western Alps)

Journal article
Authors Matthias Konrad-Schmolke
P. J. O'Brien
Thomas Zack
Published in Journal of Petrology
Volume 52
Issue 3
Pages 457-486
ISSN 0022-3530
Publication year 2011
Published at Department of Earth Sciences
Pages 457-486
Language en
Keywords fluid migration, subduction, fluid-rock interaction, sesia zone, high-pressure rocks, fore-arc mantle, lanzo zone, regional metamorphism, garnet growth, shear zones, prograde metamorphism, porphyroblast growth, grain-boundaries, re-equilibration
Subject categories Earth and Related Environmental Sciences, Geology, Solid earth geology and petrology, Geochemistry

Abstract

The Western Alpine Sesia-Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0 center dot 1-0 center dot 5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature-high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body.

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