Литосфера (May 2020)
Ancient age of zircons and issues associated with the genesis of dunites from gabbro-ultrabasite complexes of orogenic areas and central-type platform massifs
Abstract
The determination of ancient ages of zircons in dunites from the orogenic regions and central-type platform massifs raised a number of issues: 1) the equilibrium of zircon with dunite material and, as a result, the possibility of determining the age of dunite using zircon; 2) polychronic zircons in dunites and the mechanism for the formation of zoned zircon crystals; 3) the origin of the oldest dunite material dated at more than 2500 Ma; 4) mechanism for the formation of zoned zircon crystals in dunite. The article presents results of studying phase equilibrium in the system MgO–SiO2–ZrO2, which confirmed the possibility of zircon crystallisation in equilibrium with olivine and pyroxene. It was found that zircon is stable in dunite up to 1450°C, whereas at higher temperatures zircon is replaced by baddeleyite. It is shown that zoned zircon crystals can be formed in dunite as a result of the gradual transformation of zircon into baddeleyite and vice versa. Drawing on the experimental data, the authors proposed a mechanism for the accumulation of dunite material in the form of restite, which forms during the partial melting of mantle peridotite, as well as the possible way for dunite resitite to raise to the surface in the form of diapir. The difference between the Ural alpine-type ultrabasites and the ultrabasites of the Platiniferous Belt is discussed. It is proposed that the alpine-type ultrabasite occur сloser to the surface where they actively interact with water.
