Archean U-Pb isotope age of zircon from dunite of the Nizhny Tagil massif (the Uralian Platinum Belt)

Abstract

Material identity of dunite from zoned-type massifs of the Aldan shield (Kondyor, Chad and others) and dunite “cores” from the Uralian Platinum Belt (i.e., UPB) allows considering them as blocks of ancient material, which have the platform nature, integrated into complex structure of the UPB. In order to provide more clarity in the proposed hypothesis, grains of zircon from dunite of the Nizhny Tagil massif, an undisputable example of a zoned Uralian-type clinopyroxenite-dunite complex, genetically not connected to gabbro, have been studied by methods of isotope geology. Grains of zircon are represented by crystals and their fragments in the size range from 70 up to 170 microns long with length to width ratios from 1.3 up to 1.6. Parameters of U-Th-Pb isotope system were measured using sensitive high-resolution ion microprobe SHRIMP-II at 11 local areas of 8 crystals. Results for studied zircons in concordia diagram ²⁰⁷Pb/²³⁵U-²⁰⁶Pb/²³⁸U define three statistically different age clusters, i.e., 585 ± 29, 1608 ± 56 and about 2655 million years, accordingly. We presume that the Late Archean age cluster characterize time of dunite generation in a subcontinental mantle. Presence of three discrete age clusters, along with morphological variety of zircons and ²³²Th/²³⁸U values in an interval 0.071–1.692, together with already existing geological facts, reflect prolonged evolution of the dunite in complex geochemical and geodynamic conditions, supporting the conclusion about autonomy of the dunite “cores” in the structure of the UPB.