Study of the UHP mineral silicon spinel in the Yarlung Zangbo ophiolite

Abstract：A mantle mineral assemblage consisting of 70－80 unusual varieties of minerals has been found in chromitite of the Luobusa ophiolite in the Yarlung Zangbo ophiolite zone, Tibet. This mineral assemblage includes a spinel-group mineral with a special composition. The mineral occurs as inclusions in arsenopyrite. The average chemical composition for 28 grains of the spinels is as follows：Na2O 1.58%, MgO 7.52%, Al2O3 36.59%, SiO2 44.45% and FeO 8.74 (%), with minor CaO and TiO2. Laser Raman spectrometry shows that several grains have the Raman spectrum of franclinite (zinc-iron spinel Zn Fe2O4). According to the crystal structure and chemical composition, the spinel ( franclinite) has two formulas, i.e.：(Mg0.52 Na0.14 Fｅ0.32 Al0.74)1.72 (Si2.00 Al1.20)3.20 O8 and ( Mg0.52 Na0.14Fe0.32 Si0.50)1.48(Si1.50 Al1.94)3.44 O8. The two formulas both show that the six-coordinated Si cations occupy octahedral sites of the crystal structure of the spinel. Experiments demonstrate that silicate with six-coordinated Si has the UHP nature and should be derived from the 400－670 km depth equivalent to the transition zone of the Earth. This indicates that the magmatism of the Yarlung Zangbo ophiolite (paleo-oceanic lithosphere) may have reached the transition zone. It is possible that the mantle plume might transport UHP minerals such as silicon spinels from the transition zone to the shallow level of the upper mantle.