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

摘要

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 deg 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 ~(206)Pb/~(238)U ages in the range 325-109 Ma documenting partial recrystallization by dissolution-reprecipitation of a highly reactive grain.