U–Pb dating of serpentinization: hydrothermal zircon from a metasomatic rodingite shell (Sudetic ophiolite, SW Poland)

Abundant zircon grains were found within a metasomatic chlorite–corrensite (trioctahedral regularly interstratified chlorite/smectite) shell that surrounds rodingite from the Jordanów–Gogołów (JG) serpentinite massif (Sudetic ophiolite, Bohemian Massif). The zircon grains bear two types of primary fluid inclusions containing either liquid CO2 or aqueous solutions. The inclusions were used to assign the P–T conditions of the zircon formation (270–300 °C, ca. 1 kbar), using the crossed-isochore method. Abraded single zircon grains give a U–Pb age of 400+4/−3 Ma, which is interpreted as the age of zircon crystallization. This zircon formed during rodingitization, i.e., a metasomatic process directly connected with the serpentinization and thus provides the opportunity of direct dating of the early serpentinization process. rcon age from the Jordanów–Gogołów massif is consistent with the crystallization age of metagabbro protoliths from the Sudetic ophiolite as determined by Oliver et al. [J. Geol. Soc. (Lond.) 150 (1993) 355]. A Ce anomaly, combined with the mineralogical record of greenschist facies metamorphism of the mafic rocks from Sudetic ophiolite, suggests that the Sm–Nd whole-rock isotopic data of Pin et al. [Lithos 21 (1988) 195] (353±21 Ma) possibly characterize the age of later metamorphic event.