Zoisite- and clinozoisite-segregations in metabasites (Tauern Window, Austria) as evidence for high-pressure fluid-rock interaction

High-pressure zoisite- and clinozoisite-bearing segregations are common in garnet- and albite-bearing amphibolites of the Palaeozoic part of the Lower Schieferhulle, south-central Tauern Window, Austria. The zoisite segregati ons (primary assemblage: Zo+Qtz+Cal) formed during an early to pre-Hercynia n high-pressure event (P > 0.6 GPa, T =500-550 degrees C) by hydrofracturin g as a result of protolith dehydration. Zoisite is growth zoned from Fe3+-p oor cores (Al2Fe=9 mol%) to Fe3+-rich rims (17 mol%), and has high Sr, Pb a nd Ga contents and LREE-enriched REE patterns, controlling the trace elemen t budget of the segregations. Hercynian deformation at c. 0.7 GPa/600 degre es C kinked and cracked primary zoisite and enhanced breakdown into seconda ry zoisite (13 mol% Al2Fe), clinozoisite (40-55 mol% Al2Fe), albite (an(< 2 0)), calcite and white mica during an Eoalpine high-pressure event at 0.9-1 .2 GPa/400-500 degrees C. The clinozoisite segregations (primary assemblage : Czo+Qtz+Omp+Ttn+Chl+Cal) are mm- to cm-wide, vein-like bodies, cross-cutt ing fabric elements of the host garnet amphibolite. They formed during the Eoalpine high-pressure event at 0.9-1.2 GPa/400-500 degrees C. During Alpin e exhumation, omphacite was pseudomorphed by amphibole, albite, quartz and clinozoisite. Oxygen isotope data suggest equilibrium between host metabasi te and zoisite segregations and indicate an internal fluid source and fluid buffering by the protolith. Mobility of P, Nb and LREE changed the protoli th's trace element composition in the vicinity of the zoisite segregations: Mobilization of LREE is evidenced by decreasing modal amounts of LREE-rich epidote and decreasing LREE contents in LREE-rich epidote towards the segr egations, changing the REE patterns of the host metabasite from LREE-enrich ed to LREE-depleted. Tectonic discrimination diagrams, based on the trace e lement content of metabasites, should be applied with extreme caution.