Fluid-controlled crustal metasomatism within a high-pressure subducted mélange (Mt. Hochwart, Eastern Italian Alps)

The Nonsberg–Ultental Region of northern Italy contains a Palaeozoic mélange that was partially subducted during the Variscan orogeny. This mélange is constituted mainly by metapelites characterized by shale-type REE-patterns, displaying partial melting which began under high-pressure conditions. The resulting migmatites enclose minor slivers of mantle-wedge peridotites that have been incorporated into the mélange during subduction. Peridotites display important large ion lithophile elements (LILE) enrichment consequent to amphibole recrystallization contemporaneously with metapelite migmatization at P ≈ 2.7 GPa and T ≈ 850 °C in the garnet–peridotite field. Crustal and mantle (ultramafic) rocks of the mélange display the same Sm–Nd ages of about 330 ± 6 Ma, which dates both the metamorphic peak and the migmatization event. The zircon U–Pb age of the metasomatic amphibolitic contact between garnet peridotite and migmatite is identical (333.3 ± 2.4 Ma) within analytical errors. Therefore, metasomatism, migmatization and peak metamorphism are constrained to the same event. The presence of Cl-rich apatite and ferrokinoshitalite in the contact amphibolite, together with the trace-element patterns of peridotites, suggest that metasomatism was driven by Cl- and LILE-rich fluids derived from ocean water transported into the subduction zone by sediments and crustal rocks. These fluids interacted with the crust, prompting partial melting under water oversaturated conditions and partitioning LILE from the crust itself. Peridotites, which were well below their wet solidus temperature, could not melt but they recrystallized in the crustal mélange under garnet-facies conditions. Crustal fluids caused extensive hydration and LILE-enrichment in peridotites and severe Sm–Nd isotope disequilibrium between minerals, especially in the recrystallized peridotites. The proposed scenario suggests massive entrapment of crustal aqueous fluids at high-pressure conditions within subduction zones.