Lu-Hf isotope systematization of relict zircons from restitogenic ultramafic rocks of Shaman massif (East Transbaikalia)

The work presents the results of a study on Lu-Hf systematization of zircons from rocks of the Shamanic ultramafic massif, which is part of the Baikal-Muya ophiolite belt. It is composed to varying degrees of serpentinized and dynamometamorphized harzburgites and dunites subordinate to them, which have a restitogenic nature. From a composite sample of these rocks weighing about 4 kg, 31 zircon grains 100-150 μm in size were extracted and sold by U-Pb. All these grains had a rounded shape and a rough surface. Most of them are characterized by a very low to complete absence of cathodoluminescent light intensity. According to the values of the isotopic age, the entire collection of zircons was divided into three clusters: a) “ancient” (3,049-1,189 Ma); b) “intermediate” (827-812 Ma); c) “young” (630-502 Ma). In representative mineral grains from these clusters, the parameters of their Lu-Hf isotopic systems were determined. Zircons from the “ancient” cluster are characterized by increased values of the parameter ²⁰⁷Pb ^{/ 206}Pb, as well as lower values of the parameters ²³²Th/²³⁸U, ¹⁷⁶Yb/¹⁷⁷Hf, ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf. Zircons from the “intermediate” cluster are characterized by increased values of the parameter ²³²Th/ ²³⁸U, lower values of the parameter ¹⁷⁶Yb/¹⁷⁷Hf, as well as intermediate values of the parameters ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf. Zircons from the “young” cluster are characterized by intermediate values of the parameter 232Th/ 238U, approximately the same as in the previous cluster, by the values of the parameter ¹⁷⁶Yb/¹⁷⁷Hf, and also by increased average values of the parameters ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf. The studied zircons are considered as a relic phase. It is assumed that initially the few grains of this mineral were in the form of a juvenile phase in the composition of the upper mantle protolith, whose age exceeded 3,000 million years. It is also assumed that subsequently, during the heating and partial melting of protolith, which initiated the diffusion of Pb and U ions in the structure of juvenile zircons, disturbances occurred in their U-Pb and Lu-Hf isotopic systems, which caused the observed uneven “rejuvenation” of their age.

Keywords

Authors

References