U-Pb Age of Zircon from Mantle Peridotite Nodules in Cenozoic Alkali Basalts of the Vitim Plateau, Transbaikal Region

The U-Pb (SHRIMP-II) age of zircons from garnet-spinel peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau, Transbaikal region were determined. Most of the zircons are euhedral and subhedral prismatic crystals with an elongation of 1.5-2.0. Fragments of crystals and nearly equant crystals with rounded edges are present as well. Rounded or irregular cores are observed in some grains. None of the zircons yielded an age that would correspond to the time of basalt eruption (21-2.35 Ma or younger). The youngest dates range from 135.2 +/- 2.7 Ma to 141 +/- 3 Ma (Early Cretaceous). Both concordant values and the lower intersection of discordia with concordia (138.8 +/- 5.7 Ma) are within this age interval. The upper intersection corresponds to 1891 +/- 26 Ma. A considerable part of the concordant values are grouped within the intervals (164.6 +/- 1.6)-(183.4 +/- 2.0) and (264.0 +/- 7.3)-(295.7 +/- 0.76) Ma (Early-Middle Jurassic and Early Permian, respectively). The older concordant values fall in the interval 1462 +/- 19 to 1506 +/- 4 Ma (Mesoproterozoic). Proterozoic age was obtained for cores of composite zircon grains. Zircons pertaining to all age intervals are enriched in REE relative to chondrite (except La). The chondrite-normalized REE patterns are positively sloped with an increase in contents from LREE to HREE. The LREE and HREE contents and the depth of the Eu minimum tend to increase with age. In composite zircons of Proterozoic age, cores are somewhat enriched in REE. It has been suggested that crystallization of zircon as a separate phase in peridotites extremely depleted in Zr was related to a low degree of partial melting. The melt that formed in the intergranular space and that was repeatedly enriched in Zr was not extracted from the solid framework of rock and crystallized in situ under the changed thermodynamic conditions in the upper mantle. The occurrence of zircons of several age intervals in peridotites testifies to the multistage evolution of the upper mantle and recurrent partial melting under various physicochemical conditions.