Dissolution-reprecipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)

Group GSI.IR
Location International Geological Congress,oslo 2008
Author Rubatto, Daniela۱; Müntener, Othmar۲; Barnhoorn, Auke۳; Gregory, Courtney۱
Holding Date 08 October 2008

In recent years, studies of low-temperature high-pressure rocks and metasediments showed that zircon may form and react at relatively low metamorphic temperatures (250-600 °C). At these conditions, zircon recrystallization requires a sub-solidus mechanism such as coupled dissolution-reprecipitation (Geisler et al., 2007). We investigate here a case of zircon that preserves unusual recrystallization textures from a rock that suffered relatively low temperature and high-pressure metamorphism. An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral and magmatic zircon to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral micro-zircons. Fractures and voids in the recrystallized micro-zircons are mainly filled by high pressure Na-pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic crystals and micro-zircons, with less than 2° misorientation between neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircons. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas micro-zircons have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being texturally associated with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y.
Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as agent for coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low strain environment and was not significantly enhanced by metamictization. The source of fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite. ~