Age of zircon from apoharzburgite serpentinite representing mantle of the Uralian paleoocean

Аннотация

Over 60 zircon grains from apoharzburgite serpentinite were dated using SHRIMP–IIe/mc at the Laboratory IBERSIMS of the Granada University (Spain). The apoharzburgite serpentinite represents an oceanic mantle of the Uralian paleoocean, which was exhumed in the crustal structures of the Paleozoic Ural Mobile Belt during obduction. Individual grains span a huge ²⁰⁶Pb/²³⁸U age range from 2740 to 250 Ma and are clustered into six discrete age groups (in Ma): (I) > 2500, (II) 2500–1950, (III) 1260–1210, (IV) 480–400, (V) 370–330, and (VI) < 280. Two last groups were formed under the effect of granitoids on serpentinites. The traces of this effect were studied in outcrops and confirmed by age of zircon from contact talc–carbonate rock. The morphologies of zircon crystals from serpentinite bear signs typical of both magmatic and metamorphic varieties, which indicate their polygenetic–polychronous nature. No striking morphological features and peculiar U and Th contents were found in the studied zircons to discriminate unambiguously between different age groups. Pre-Paleozoic events with ages of groups I–III were found in zircons from many oceanic mantle rocks. The similarity of age groups of zircons from Paleozoic and modern oceanic lithosphere is caused by global mantle reworkings, which provoke magma generation and metasomatism probably accompanied by zircon crystallization.