Zircon from two high-pressure, low-temperature orthogneisses (1–1.5 GPa and ∼ 550 °C) from the Sesia Zone, Western Alps were examined by means of scanning electron, back-scattered and cathodoluminescence imaging and isotope dilution thermal ionization mass spectrometry to investigate U-Pb systematics of zircon in high-pressure, low-temperature terranes.
The results of the U-Pb geochronology show a simple, two-stage mixing trend between crystallization of the leucogranitic protolith and eo-Alpine high-pressure metamorphism. The upper intercept ages of 435 ± 8 and 396 ± 21 Ma constrain the age of the protoliths and can be correlated to the zircon interiors that display strong cathodoluminescence. The lower intercept age of 68 ± 6.6 Ma dates the eo-Alpine high-pressure metamorphism and can be correlated with weakly luminescent, slightly zoned overgrowth seen in CL- and BSE-images, respectively. Two air-abraded single-grain analyses of apparently inherited zircons give a concordant age of 3.7 Ga and a discordant age of 2.4 Ga.
Eo-Alpine zircon dissolution and overgrowth are characterized by internal and external embayed surfaces and multi-faceted crystal forms, respectively. A notable lack of evidence for pervasive fluid-rock interactions supports the view that dissolution of zircon was aided by local pore fluids of low salinity produced during metamorphic dehydration reactions of biotite and zoisite to form garnet. Precipitation of zircon overgrowths may have accompanied the H2O-consuming reaction of phengite formation from garnet.