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Dating of zircon from Ti-clinohumite–bearing garnet peridotite: Implication for timing of mantle metasomatism

¹ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
² Department of Earth Sciences, Waseda University, Tokyo 169-8050, Japan
³ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
⁴ Department of Earth and Planetary Sciences, Hiroshima University, Hiroshima 739-8526, Japan
⁵ Ocean Research Institute, Tokyo University, Tokyo 164-8639, Japan
⁶ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

Garnet peridotites from the Kokchetav ultrahigh-pressure (UHP) massif contain abundant volatile and high field strength element (HFSE) bearing minerals, such as Ti-clinohumite and zircon. These characteristics are interpreted to be related to fluid-infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area. The zircons from the peridotites were dated by using sensitive high-resolution ion microprobe (SHRIMP) and yielded apparent U-Pb ages of 554–494 Ma (weighted mean age, 528 Ma) that are mostly consistent with the timing of UHP metamorphism deduced from diamond-bearing country rocks in this massif. These zircons have an almost flat rare earth element (REE) pattern and very low REE concentrations; these characteristics are similar to those observed in kimberlitic zircons. Inherited zircon cores, although only rarely preserved, yielded apparently Proterozoic ages and have different trace element characteristics compared to the overgrowth rims. These features indicate that the mantle metasomatic events and the recrystallization of Ti-clinohumite and zircon were due to HFSE-enriched fluid infiltration during the UHP metamorphism at great depths. The metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts, because these rocks have characteristically high volatile and HFSE concentrations such as those of the Ti-clinohumite–bearing garnet peridotites.

Key Words: mantle metasomatism • Ti-clinohumite • zircon • U-Pb geochronology • ultrahigh-pressure metamorphism

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