Age and genesis of the Myanmar jadeite: Constraints from U-Pb ages and Hf isotopes of zircon inclusions

Myanmar jadeite (jadeitite) is well known for its economical value and distinctive tectonic locality within the collisional belt between India and Eurasian plates. However, it is less studied for its genesis and geodynamic implications due to precipitous topography, adverse weather and local military conflicts in the area. By means of combined ICP-MS and LA-MC-ICPMS techniques, we have carried out in-situ trace elements, U-Pb and Lu-Hf isotopes for zircon inclusions in a piece of jadeite gem sample. CL imaging suggests that the zircons are metasomatic in origin, and contain mineral inclusions of jadeite and omphacite. Seventy-five analyses on 16 grains of the zircons yield a U-Pb age of 158 ± 2 Ma. The Myanmar zircons differ from other types in that they have no significant Eu anomalies despite high HREE concentrations. Measured ¹⁷⁶Hf/¹⁷⁷Hf ratios range from 0.282976 to 0.283122, with an average value of 0.283066 ± 7; ε_Hf(t) value of 13.8 ± 0.3 (n=75). These results indicate that the Myanmar jadeite was formed in the Late Jurassic, probably by interaction of fluid released from subducted oceanic slab with mantle wedge. Therefore, its formation has no genetic relationship to the continental collision between Indian and Euroasian plates. Supported by National Natural Science Foundation of China (Grant No. 40673039) and the Science Plan Foundation of Guangdong (Grant No. 2007B031200005)