Jadeite-gneiss from the Eclogite Zone, Tauern Window, Eastern Alps, Austria: Metamorphic, geochemical and zircon record of a sedimentary protolith

In the Eclogite Zone of the Tauern Window, a layer of strongly retrogressed leucocratic jadeite-bearing gneiss is intercalated between mafic kyanite-eclogites. The jadeite-gneiss consists of garnet + quartz + paragonite ± phengite ± zoisite + zircon + rutile + apatite + pyrite. Kyanite, jadeite or omphacite are exclusively present as inclusions in garnet. Retrogressive hydration during exhumation led to a breakdown of matrix jadeite to form pseudomorphs of calcic amphibole + albite. Peak metamorphic conditions derived from the primary gneiss assemblage are 2.0-2.4 GPa at approximately 640 °C. Major, trace element and isotopic compositions of the jadeite-gneiss are consistent with a siliciclastic sedimentary protolith. Zircon morphology and zonation patterns reveal a complex history. The presence of fracture-truncated zircons suggests a detrital origin, whereas most internal structures and Th/U ratios are characteristic of zircons from magmatic rocks. In situ LA-ICP-MS and SHRIMP U-Pb geochronology and zircon geochemisty provide evidence of at least three magmatic events in the provenance area. These were dated at 466 ± 2 Ma, 437 ± 2 Ma and 288 ± 9 Ma. Older ages ranging from 503 to 691 Ma are preserved in the cores of some zircon grains, suggesting derivation from peri-Gondwanan sources. Surprisingly, no firm evidence of the Tertiary high-pressure metamorphic event and subsequent retrograde overprint was seen in any of the studied zircons. However, some zircons show resorbed surfaces suggesting corrosion by a superficial fluid phase undersaturated in zirconium and one extensively altered porous zircon yielded highly discordant 206Pb/238U ages in the range 325-109 Ma documenting partial recrystallization by dissolution-reprecipitation of a highly reactive grain.