Mineralogy and Geochemistry of “Laoshan Jade” from Shandong Province, China: Implications for Petrogenesis

Abstract

“Laoshan Jade” is located in the Yangkou area of the Sulu UHP metamorphic belt. This is the first time a systematic research study is performed on its conventional gemological characteristics, mineral compositions, geochemical characteristics, and petrogenesis using a polarized light microscope, an electron probe microanalyzer (EMPA), an X-ray fluorescence spectrometer (XRF), and a Raman spectrometer. The gemological characteristics of “Laoshan Jade” are consistent with serpentine jade. It can be named serpentinite, mainly composed of serpentine (antigorite), chlorite (clinochlore), and magnetite. Some samples contain magnesite and chromite. Only small amounts of unaltered forsterite, chrysolite, and diopside can be found in poor-quality serpentine jade. The protolith of “Laoshan Jade” is metamorphic peridotite. Antigorite is altered from olivine. Clinochlorite is altered from the diopside. Disseminated magnetite is the metamorphic product. Unaltered olivine and diopside belong to protolith. Chromite with zonal texture also belongs to protolith. The average temperature and pressure of its core are calculated to be 1348 °C and 3.1 GPa, which indicates that the protolith comes from the mantle asthenosphere. The formation temperature of its edge is lower than 500 °C, with metamorphic origin. XRF experiments show that “Laoshan Jade” samples are relatively rich in Mg and more depleted in fusible elements (Al, Ca) than the original mantle peridotite, indicating that serpentinite protolith has undergone a certain degree of partial melting. Combined with EPMA results, the findings of this study reveal that protolith belongs to the depleted mantle peridotite in the subduction zone, partially melted in the mantle wedge, and reacted with subduction zone fluid. Pyroxene is altered into chlorite, olivine is altered into serpentine, and magnet is generated. The edge of chromite in the protolith is altered by the iron-rich fluid into magnetite. Finally, serpentinite is formed.