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

Abstract

High‐pressure zoisite‐ and clinozoisite‐bearing segregations are common in garnet‐ and albite‐bearing amphibolites of the Palaeozoic part of the Lower Schieferhülle, south‐central Tauern Window, Austria. The zoisite segregations (primary assemblage: Zo+Qtz+Cal) formed during an early to pre‐Hercynian high‐pressure event (P≫0.6 GPa, T =500–550 °C) by hydrofracturing as a result of protolith dehydration. Zoisite is growth zoned from Fe3+‐poor cores (Al2Fe=9 mol%) to Fe3+‐rich rims (17 mol%), and has high Sr, Pb and Ga contents and LREE‐enriched REE patterns, controlling the trace element budget of the segregations. Hercynian deformation at c. 0.7 GPa/600 °C kinked and cracked primary zoisite and enhanced breakdown into secondary zoisite (13 mol% Al2Fe), clinozoisite (40–55 mol% Al2Fe), albite (an<20), calcite and white mica during an Eoalpine high‐pressure event at 0.9–1.2 GPa/400–500 °C. The clinozoisite segregations (primary assemblage: Czo+Qtz+Omp+Ttn+Chl+Cal) are mm‐ to cm‐wide, vein‐like bodies, cross‐cutting fabric elements of the host garnet amphibolite. They formed during the Eoalpine high‐pressure event at 0.9–1.2 GPa/400–500 °C. During Alpine exhumation, omphacite was pseudomorphed by amphibole, albite, quartz and clinozoisite. Oxygen isotope data suggest equilibrium between host metabasite and zoisite segregations and indicate an internal fluid source and fluid buffering by the protolith. Mobility of P, Nb and LREE changed the protolith’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 segregations, changing the REE patterns of the host metabasite from LREE‐enriched to LREE‐depleted. Tectonic discrimination diagrams, based on the trace element content of metabasites, should be applied with extreme caution.